July 2, 2020

The A-B-C’s of cucurbit powdery mildew control

Cucurbit powdery mildew (CPM), caused by Podosphaera xanthii, is one the most important diseases of cucurbit crops throughout the world. The pathogen is an obligate parasite, just like cucurbit downy mildew, meaning it needs a living host in order to survive. In northern regions that have a killing frost in the fall the pathogen will die out when the crop freezes. Not being able to overwinter, the pathogen must be re-introduced each spring or summer in the mid-Atlantic region. The pathogen accomplishes this by re-infecting cucurbit crops in the spring as they are planted up the east coast starting in Florida, then the Carolina’s, Virginia, and so forth. By late May, as soon as cucurbit crops begin to germinate in the mid-Atlantic region, the potential threat for potential powdery mildew infections begin.

The first step in mitigating CPM begins with planting powdery mildew tolerant (PMT) or resistant (PMR) cultivars if they meet your needs. It is important to remember that these cultivars are not “immune” to CPM; they will become infected at some point in the growing season depending on disease pressure. Hopefully, this will occur later in the season when compared to CPM susceptible cultivars. Organic growers hoping to mitigate losses to powdery mildew should always chose CPM tolerant or resistant cucurbit cultivars first. There are a number of OMRI-approved fungicides labeled to help suppress CPM development, these should always be used in concert with CPM tolerant or resistant cultivars and a preventative fungicide program. Cultural practices such as increasing in-row plant spacing to improve air flow and cultivation to keep weeds to a minimum will also be advantageous. Avoiding the use of overhead irrigation will help reduce disease pressure from another important pathogen, cucurbit downy mildew. Thus, growing cucurbits on a mulch with drip irrigation has its advantages, but also increases costs.

In the past, a typical conventional fungicide program consisted of rotating two different FRAC group fungicides every other week, such that the pattern looked like:

A – B – A – B – A – B

Often a protectant fungicide such as chlorothalonil or mancozeb is added to the tank mix on a weekly basis to 1) help control other important fungal diseases, such as anthracnose or gummy stem blight and 2) to help reduce selection pressure on the high-risk fungicide that was being applied. This type of preventative program was used for many years, because, in most cases there were just a few effective fungicides available for CPM control depending on the crop. An example of this would be:

A = (azoxystrobin [FRAC group 11] + chlorothalonil (MO5) rotated weekly with B = (myclobutanil [FRAC group 3] + chlorothalonil (MO5)

This type of control strategy worked extremely well as long as the pathogen didn’t develop resistance to either the FRAC group 11 (azoxystrobin) or FRAC group 3 (myclobutanil) fungicide. To better understand modes of action and how fungicide resistance develops in FRAC group 11 and FRAC group 3 fungicides please click here. Unfortunately, because of fungicide resistance development this type of program is no longer effective and is no longer recommended for CPM control.

Over the past 10 years, there have been a number of new fungicides released with new modes of action (i.e., new FRAC groups) for CPM control in cucurbit crops. Unfortunately, all have a moderate to high-risk for resistance development because of their specific modes of action. The good news are these new fungicide chemistries have less effects on humans, non-target organisms, and the environment.

These fungicides include:

  • FRAC group 13 (quinoxyfen)
  • FRAC group 39 (fenazaquin)
  • FRAC group 50 (metrafenone)
  • FRAC group U06 (cyflufenamid)
  • FRAC group U013 (flutianil)

Not all of the fungicides listed above are labeled for all cucurbit crops. Growers will need to refer to local recommendations and the label for crop specifics. Remember, the label is the law.

These fungicides offer new strategies when it comes to controlling and mitigating losses to CPM. Instead of rotating two fungicides with a moderate to high-risk for resistance development every other week ( A – B – A – B), growers now have option to reduce the total number of times any single fungicide might be applied during the production season; further reducing the risk for resistance development to any one mode of action. For example, in pumpkin, a new CPM preventative fungicide program may look like this:

A – B – C – D – E – A – B – C – D – E

Where A=(FRAC group 3);B=(FRAC group 13); C=(FRAC group 50); D=(FRAC group U013); E=(FRAC group 11)

A protectant fungicide such as chlorothalonil or mancozeb should be added to the tank mix with each high-risk fungicide to reduce selection pressure and to help control other important diseases such as anthracnose and plectosporium blight.

In this type of CPM preventative program any one high-risk fungicide would only be applied twice per growing season and 5 weeks apart greatly reducing the risk for fungicide resistance development. Importantly, for cucurbit growers, the easiest method to mitigate the potential for fungicide resistance development are to reduce the total number of applications of any one high-risk fungicide during the production season.

When to start spraying for CPM

Initiating a preventative spray programs begins with paying attention to Extension reports, scouting, and when the crop was seeded. If the crop is seeded the early-spring (i.e., early to late May) there is a very good chance CPM is not present in the mid-Atlantic region. If CPM is not present, there is no need to initiate a spray program using high-risk fungicides. In this instance, general protectant fungicides such as chlorothalonil will help mitigate other foliar diseases. As cucurbit crops are seeded into early to mid-June (and afterward) the risk for CPM development will rise in the mid-Atlantic region. This is when scouting and paying close attention to Extension reports becomes important. The first application should be done when CPM has been detected in the immediate region or when it is detected by scouting (e.g., with one lesion found on the underside of 45 mature leaves per acre). This will help reduce the use of unwarranted high-risk fungicide applications early in the production season. Importantly, the use of PMR or PMT cucurbit varieties will also help delay the onset of CPM development as well. Once CPM preventative fungicide programs are initiated, applications need to occur at every 7 to 10 days (at the latest) for as long as you expect to harvest (e.g., summer squash) or hold the crop (e.g., pumpkin and winter squash). During harvest, growers need to pay careful attention to pre-harvest intervals because they may vary significantly between different FRAC groups or fungicides within the same FRAC group (a good example are fungicides in FRAC group 3). Once harvest is complete, those blocks or fields need to be destroyed immediately to help reduce the spread of CPM to other blocks or fields that are scheduled to be harvested later in the production season. This is especially important for other diseases such as cucurbit downy mildew.

In some instances, rotating between many different FRAC group fungicides are not an option because the chemistries aren’t available for use. An example would be leaf spot control in spinach, where FRAC groups (7, 11, 7 + 11, 7 + 12, and 9 + 12) are available. In this example, options for control might look like this:

A – B – C – D

Where A=(FRAC group 7); B=(FRAC group 9 + 12); C=(FRAC group 11); D=(FRAC group 7 + 12)

Here, we have maximized the use of as many different FRAC groups as possible and spread their use as far apart as we can during the production season. Its important to remember that fungicides with more than one active ingredient (e.g., 7 + 11) should also be rotated as far apart as possible with fungicides that contain the single active ingredient (e.g., FRAC group 7 or FRAC group 11).

Monitoring fungicide efficacy

With the use of high-risk fungicides, all growers need to monitor fungicide efficacy accordingly. Once the lack of efficacy is detected there is a chance that fungicide resistance might be present. Importantly, the lack of efficacy should not be misconstrued with poor applications or waiting too long between fungicide applications. Reports of poor efficacy from Extension personnel from one region may not reflect fungicide efficacy in another region. Therefore, fungicide efficacy needs to be done at the farm level and the only way to accomplish this is to scout your fields and know what is and isn’t working for you.

The principles mentioned above also extend to other important diseases in vegetable production where there are multiple FRAC groups with high-risk fungicides available to control specific diseases. As a general rule, growers need to rotate as many different modes-of-action (i.e., fungicides from different FRAC groups) as possible during the production season to help mitigate fungicide resistance development in conjunction with best management practices.

For more information on fungicide use, FRAC groups, and specific control recommendations please see the 2020/2021 Mid-Atlantic Commercial Vegetable Production Recommendation Guide.

 

Filed Under: Commercial Ag Updates, Vegetable Crops Tagged With: ,
April 22, 2020

Understanding the SDHI (FRAC group 7) Fungicides

The SDHI (succinate dehydrogenase inhibitor) fungicides belong to FRAC group 7 which have been on the market since the late 1960s. Third generation SDHIs have been available since 2003 with release of boscalid (penthiopyrad). Examples of FRAC group 7 fungicides used to control important vegetable diseases include: flutolanil ( year introduced,1986), boscalid (2003), penthiopyrad (2008), fluxapyroxad (2011), fluopyram (2013), and pydiflumetofen (2016). All fungicides in FRAC group 7 inhibit complex II of the fungal mitochondrial respiration by binding and blocking SDH-mediated electron transfer from succinate to ubiquinone. The SDHI fungicides work much like the FRAC group 11 fungicides, just at a different site in mitochondrial respiration. Much like FRAC group 11 fungicides, they are also at-risk for fungicide resistance development because of their specific modes of action. Research has shown there are numerous single point mutations that can lead to resistance development to FRAC group 7 fungicides. Although all SDHI fungicides share the same target site, sensitivity to the different fungicides within the FRAC group may vary. Thus, this variation in sensitivity among different SDHI fungicides leads to confusion on what the term “cross-resistance” means. With cross-resistance, once a pathogen develops resistance to one fungicide within the FRAC group, it becomes resistant to all others (e.g., strobilurin resistance in FRAC group 11 fungicides). However, in FRAC group 7 fungicides, there seems to be differences in sensitivity between fungicides within the group after resistance has been detected in one particular fungicide. The good news is that other FRAC group 7 fungicides may retain there efficacy even if resistance is detected in one particular fungicide. “The practical implications for resistance management would be the recommendation of mixtures of SDHIs, alternations of SDHIs, or even the substitution of members of the SDHI fungicide class. However, this would be counterproductive in protecting this mode of action.” (Klappach and Stammler, 2019). The important point, switching exclusively to another SDHI fungicide after resistance has been found in one is not a good resistance management strategy. As with all fungicide resistance management strategies, growers should focus on rotating as many different FRAC groups into their fungicide programs as possible, and limit the total amount of any one FRAC group during the production season.

Additional resources:

Theories on tank mixing and rotating fungicides in different FRAC groups

Understanding protectant fungicides

Understanding FRAC group 3 and 11 fungicides

Filed Under: Vegetable Crops Tagged With: , , ,
February 15, 2020

Theories on managing fungicide resistance development by tank mixing or rotating fungicides

The question of whether to tank mix high-risk (HR) fungicides with low-risk (LR) protectant fungicides or the rotation of HR fungicides with LR fungicides remains an open debate. The tank mixing or alternation of fungicides has been widely advocated as a means to delay or minimize the risks of resistance development (Genet et al., 2006; McGrath 2011; Van der Bosch and Gilligan, 2008; van den Bosch et al., 2014; Elderfield et al, 2018), although differences in opinion on whether one is better than the other exist (Genet et al., 2006), or that either method may be an effective means at reducing resistance development (van den Bosch and Gilligan, 2008). The theories behind the rotation or tank mixing of different fungicides follows strategies analogous with managing antibiotic resistance, using methods known as complementary therapy or cycling therapy (van den Bosch and Gilligan, 2008). Fungicide resistance studies with tank mixes or alternations use similar density-independent models as antibiotic resistance and assumes the sensitive and resistant strains to be at low initial densities. Resistance management studies incorporate what is often referred to as takeover time as the evaluation criterion (Van der Bosch and Gilligan, 2008). Take-over time is defined as the time-period in which the fraction of the resistant population passes a critical threshold level, thereby reducing the value of the fungicide for disease control (van den Bosch and Gilligan, 2008).

The concept behind the alternation of fungicides with different modes-of-action is that cyclic selection pressure placed on the fungus should help reduce the buildup of resistant populations, however, this idea has been criticized by numerous authors (van den Bosch and Gilligan, 2008). The argument against the alternation of fungicide chemistries is that this method would only work if it comes with a fitness cost (e.g., the ability to reproduce) associated with the resistant population in absence of selection pressure against the target fungicide (van den Bosch and Gilligan, 2008). Thus, without a fitness cost, the fraction of the resistant pathogen population would not change during the time period when the target fungicide is not used (van den Bosch and Gilligan, 2008). This suggests that resistance development would continue as if there had been no alternation at all, and it would take exactly the same number of fungicide applications of the target fungicide to build up a given level of resistance to that fungicide, although the time for resistance buildup (i.e., take-over time) would be potentially delayed (van den Bosch and Gilligan, 2008). Birch and Shaw (1997) state that one of the advantages to alternation is the possibility of stabilizing selection pressure, if only one of the fungicides were applied at a time.

The concepts behind the tank mixing of fungicides closely follows the concept behind the alternation of fungicides with different modes-of-action. Van den Bosch and Gilligan (2008) using density-dependent models, showed that if no fitness costs exist, mixtures are no different from alternation strategies when comparable doses are used. Tank mixes can be useful if fitness costs exist, but is questionable whether fitness costs would ever be large enough to make mixtures a useful resistance management strategy. Van den Bosch and Gilligan (2008) suggested that tank mixtures deserve attention for their ability to act as insurance in the sense that large scale losses could be avoided if one component of the tank mixture (i.e., the HR fungicide) suddenly fails, and that this is especially important in pathogens where large-scale epidemics (e.g., cucurbit downy mildew) may occur in one year, but not others. Van den Bosch et al. (2014) using empirical and theoretical modeling suggested the following conclusions with using mixtures as a fungicide resistance tactic: 1) adding a multi-site (i.e., LR fungicide) or a specific site (another HR) fungicide to a high-risk fungicide helps reduce the rate of selection against the fungicide(s) with the specific mode-of-action, 2) adding a partner fungicide while reducing the dose of the high-risk fungicide reduces the selection pressure for resistance development without compromising effective control; and 3) while there were few studies done, evidence suggests that mixing two high-risk fungicides is also a useful resistance management strategy. The authors also pointed out that due to the limited research in this area of tank mixes, the lack of these studies should be a warning against over interpreting the findings in their review (van den Bosch et al., 2014). Elderfield et al. (2018) in exploring the alternation or tank mixing of low- and high-risk fungicide programs on lifetime yield (e.g., use) of the high-risk fungicide, in other words, the time period before the high-risk fungicide was no longer economically effective, showed through empirical and theoretical modeling that lifetime yield may be different for different fungicide-pathosystems and that alternation or tank mixing may lead to longer lifetime yields (i.e., use). The authors, based on their evidence, suggest that mixtures of low and high risk fungicides will always be the best resistance management tactic when the objective is optimizing the lifetime yield (i.e., use) of the high-risk fungicide (Elderfield et al., 2018). Gisi et al. (2006) determined in the testing of resistance development in P. viticola (down mildew of grape) using a QoI (FRAC group 11) and protectant (LR) fungicide tank mix that increasing the dose of the non-QoI partner (LR) fungicide in the mixture resulted in reduced selection pressure. The authors also suggested that the choice of non-QoI (LR) fungicide tank-mix partner and its dosage can significantly affect the success of QoI resistance management strategies under practical conditions.

Parnell et al. (2007) suggested that in-field strategies, such as the alternation or tank mixing of fungicides, used to combat fungicide resistance development may be more useful through the restricted deployment of fungicides over large areas. Restrictions on fungicide use in this manner may be extremely beneficial in controlling and managing fungicide resistance development in pathogens such as Podosphaera xanthii (cucurbit powdery mildew) and Pseudoperonospora cubensis (cucurbit downy mildew) which spread over vast geographic areas (i.e., the east coast of the U.S.) each year. Research in the mid-Atlantic region of the U.S. has confirmed the presence of cucurbit powdery mildew populations resistant to FRAC codes 3 and 11 fungicides in recent years. This suggests that QoI- and/or DMI-resistant cucurbit powdery mildew populations could be disseminating up the east coast from the southeast region of the U.S. each production season. Importantly, fungicides in FRAC code 11 are still widely recommended and used in some southern tier states, whereas recommendations and use of FRAC code 11 fungicides for cucurbit powdery mildew control in the mid-Atlantic region have been mostly discontinued in recent years. In order to help combat fungicide resistance development issues such as this in the future, more collaboration between extension personnel from different regions must be done to help establish more defined fungicide resistance management guidelines for large geographic areas such as the south- and northeast regions of the US.

 Importance of risk management.

Because certain pesticide chemistries have specific MOA’s there is always a much greater chance for pests (e.g., pathogens, weeds, or insects) to develop resistance. For example, fungi which produce a vast amount of asexual inoculum (i.e. conidia), undergo multiple diseases cycles during a given production season (e.g., powdery and downy mildews), or fungi which have a high probability for sexual reproduction in a field population (e.g., Phytophthora capsici) often have a much greater chance for fungicide resistance development. Importantly, in controlling pathogens where there are but a few, HR fungicide chemistries available for use, selection pressure put on the pathogen may be increased through their overuse. Therefore, the lack of proper chemical rotations (i.e., pesticides with different modes-of-action) or improper tank mixes or rotations may have a dramatic effect on resistance development, especially if these high-risk pesticides are over used or used improperly according to the label.

The grouping of similar chemistries together by their modes-of-action (e.g., FRAC group) and the inclusion of resistance management guidelines on pesticide labels are designed to i) reduce the chances for resistance development and ii) help agricultural producers develop and follow resistance management programs. Although application restrictions and resistance management guidelines have been widely adopted by the chemical industry, the follow-through effects of such guidelines have been left solely to the individual applicator; or extension personnel or crop specialists who help train those applying agricultural pesticides. Jutsum et al. (1998) pointed out that the challenge was to develop fungicide resistance management strategies which were relevant to local production practices. In recent years, the use of FRAC, HRAC and IRAC codes has been widely included in state and regional vegetable commercial production recommendations and promoted and used by extension personnel and crop advisors as education and teaching tools in many production regions of the United States. Even with increased awareness and training, the proper use of these pesticides is ultimately placed upon the end-user (e.g., the farmer/applicator) to make sure that the pesticides are properly applied according to the label rate, its restrictions, and state and federal laws.

 Take home thoughts

There is still a lot to learn in the understanding of tank mixing and rotating HR and LR fungicides with each other, and the rotation of HR fungicides with different modes of action on a weekly basis. First, growers need to follow the label. The label is the law. Where appropriate, growers need to rotate HR fungicides with different modes of action (i.e., from different FRAC groups) as much as possible to limit the overuse of any one FRAC group during the production season. In general, tank mixing HR fungicides with LR fungicides will help reduce overall section pressure for resistance development to the HR fungicide. In crops, where there are but one or a few HR fungicides labeled for control of a specific disease, the use of the HR fungicide(s) needs to be done judiciously.

For more information on fungicide resistance management strategies using cucurbit powdery mildew and cucurbit downy mildew as examples, please click on the hyperlinks.

For more information on the specific fungicides recommended for disease control please see the 2020/2021 Mid-Atlantic Commercial Vegetable Production Recommendations.

Fungicide Resistant Management Guidelines for Vegetable Crops grown in the mid-Atlantic Region for 2020/2021 will be available soon.

Author citations in parenthesis are from peer-reviewed journal publications.

Filed Under: Commercial Ag Updates, Vegetable Crops Tagged With:
February 12, 2020

The A-B-C’s of cucurbit downy mildew control

In 2004, cucurbit downy mildew (CDM) re-emerged in the US with a vengeance causing significant losses in cucurbit production. In most years prior to this, concern for CDM control was minimal, since the pathogen arrived late in the growing season (in more northern regions), or the pathogen caused little damage, or never appeared. After 2004, with significant losses at stake, and with very few fungicides labeled for its proper control, CDM became a serious threat to cucurbit production. Importantly, at the time, cucumber varieties with very good levels of CDM resistance were no longer resistant, suggesting a major shift in the pathogen population. Research done over the past 15 years has led to a better understanding of the pathogen. Recent research has determined that the CDM falls into two separate clades: Clade I and Clade II. Some CDM (Pseudoperonospora cubensis) isolates fall into Clade I which predominately infect watermelon, pumpkin, and squash, where CDM isolates in Clade II predominately infect cucumber and cantaloupe. Research suggests that isolates in Clade II can quickly become resistant to specific fungicides (NCSU). Most cucumber varieties are resistant to Clade 1 isolates, but there is no resistance currently available for Clade 2 isolates. For pickling cucumber the varieties, Citadel and Peacemaker, are tolerant to clade 2 isolates. For slicing cucumbers, the varieties SV3462CS and SV4142CL are tolerant to Clade 2 isolates. All organic and greenhouse growers are encouraged to use tolerant varieties since chemical control options are very limited (NCSU). An extended list of cucumber varieties with CDM resistance from the University of Florida can be found here. For the past decade, researchers from around the US have been closely monitoring and forecasting the progress of CDM through a website hosted by NCSU. The CDMpipe website is currently in the process of an upgrade and will now be hosted by Penn State University. All cucurbit growers are encouraged to sign up to the CDMpipe website to help them know what cucurbit crops are being infected (and where) and to follow the forecasting to know where the pathogen may move to next. As a note, in recent years, CDM control with certain fungicides has varied significantly depending on the cucurbit host and geographic region. This is extremely important since two clades of the pathogen are potentially present (affecting host range) as well as having a potential impact on control strategies. How do you know which clade may be present on your farm? Follow the reports. If CDM is mostly present in cucumber crops as it works its way up the east coast, then you are most likely to see it infect cucumber and melon on your farm first. Scout your fields regularly, especially if CDM is in the immediate region. Pay very close attention to symptom development and on what cucurbit crop(s) you see it on, this is especially important if you grow more than one cucurbit crop. Like cucurbit powdery mildew, once CDM arrives in the region preventative fungicide applications will be necessary.

Fungicides for CDM control

As with cucurbit powdery mildew control, there is no reason to begin a preventative CDM fungicide program until it has been detected in the immediate region. Cucurbit growers need to pay special attention to the reporting system mentioned above to see what cucurbit crops are affected by CDM and follow the forecasting system to see if CDM is an immediate threat.

Loss of efficacy in the control of CDM has also been documented in FRAC code 4 (mefenoxam), FRAC code 11 fungicides (azoxystrobin), FRAC code 28 (propamocarb HCL), and FRAC code 43 (fluopicolide) in the mid-Atlantic region and elsewhere. Insensitivity to fluopicolide (43) and propamocarb HCL (28) have been reported in multiple states (Thomas et al., 2018). In some cases, individual isolates of CDM were insensitive to both chemistries. Recent research in Michigan in a three year field study using pickling cucumber determined that cyazofamid (21), (ametoctradin, 45 + dimethomorph, 40), (zoxamide, 22 + mancozeb, M03), mancozeb (M03); chlorothalonil (M05), and oxathiapiprolin (49) alone or in a premix provided the best level of control (Goldenhar & Hausbeck, 2019). In a recent study evaluating different fungicide chemistries in field trials done in different states (OH, NY, & SC) determined that propamocarb HCL (28), cymoxanil + famoxadone (27 + 11), and fluopicolide (43) were ineffective in 1 or 2 states during both years of the trial (Keinath, Miller, & Smart, 2019). In one year of the study, famoxadone (11), dimethomorph (40), cymoxanil (21), and mancozeb (M03) were ineffective for CDM control (Keinath, Miller, & Smart, 2019). In bioassay studies done during this trial, cyazofamid (21), oxathiapiprolin (49) suppressed CDM >80%.

Most fungicides labeled for the control of CDM are at-risk for resistance development because of the specific modes of action. These include Ranman (cyazofamid, FRAC code 21), Gavel (zoxamide, 22 + mancozeb, M03), Zing! (zoxamide, 22 + chlorothalonil, M05); Curzate (cymoxanil, 27), Previcur Flex (propamocarb HCL, 28), Forum/Revus (dimethomorph, 40), Zampro (ametoctradin, 45 + dimethomorph, 40), Orondis Opti (oxathiapiprolin, 49 + chlorothalonil, M05), and Orondis Ultra (oxathiapiprolin, 49 + mandipropamid, 40). Importantly, just like with cucurbit powdery mildew control, there are a number of CDM fungicides with different modes of action from different FRAC codes to chose from. As noted in the paragraph above, the efficacy of individual fungicide chemistries may vary significantly by state or region. Thus, growers need to scout their cucurbit fields on a weekly basis, note the efficacy, or lack thereof, they are seeing in the field, and incorporate the use of as many different FRAC groups as possible to help mitigate fungicide resistance development.

Fungicide programs for CDM control

An example of a fungicide program for CDM control in the mid-Atlantic region might look like this, where a CDM specific fungicide from a different FRAC group is used on weekly basis:

A – B – C – D – E

where A= Gavel (zoxamide, 22 + mancozeb, M03); B= Orondis Opti (oxathiapiprolin, 49 + chlorothalonil, M05); C= Ranman (cyazofamid, FRAC code 21); D= Orondis Ultra (oxathiapiprolin, 49 + mandipropamid, 40); E= Curzate (cymoxanil, 27)

Not all of the fungicides listed above are labeled for all cucurbit crops. Some fungicides, such as the Orondis products have  limited number of applications. Growers will need to refer to local recommendations and the label for crop specifics. Remember, the label is the law.

A protectant fungicide such as chlorothalonil or mancozeb should be added (if not already included) to the tank mix with each high-risk fungicide to reduce selection pressure and to help control other important diseases such as anthracnose and plectosporium blight. All growers should follow use recommendations on labels and avoid overusing one mode of action, even if it works well. If loss of efficacy is present, the grower should avoid using that particular fungicide (FRAC group) for CDM control the rest of the growing season.

Growers should remember that fungicides specifically labeled for CDM control won’t control CPM, and fungicides labeled for CPM control won’t control CDM. Therefore, carefully following the disease monitoring and forecasting website, choosing varieties with CDM resistance, paying close attention to host crops, scouting fields on a regular basis, noting fungicide efficacy, and following proper fungicide resistant management guidelines remain critically important for successful CDM control.

For more information on the specific fungicides recommended for CDM control on cucurbit crops please see the 2020/2021 Mid-Atlantic Commercial Vegetable Production Recommendations.

Filed Under: Commercial Ag Updates, Vegetable Crops Tagged With: ,
February 11, 2020

The A-B-C’s of cucurbit powdery mildew control

Cucurbit powdery mildew (CPM), caused by Podosphaera xanthii, is one the most important diseases of cucurbit crops throughout the world. The pathogen is an obligate parasite, just like cucurbit downy mildew, meaning it needs a living host in order to survive. In northern regions that have a killing frost in the fall the pathogen will die out when the crop freezes. Not being able to overwinter, the pathogen must be re-introduced each spring or summer in the mid-Atlantic region. The pathogen accomplishes this by re-infecting cucurbit crops in the spring as they are planted up the east coast starting in Florida, then the Carolina’s, Virginia, and so forth. By late May, as soon as cucurbit crops begin to germinate in the mid-Atlantic region, the potential threat for potential powdery mildew infections begin.

The first step in mitigating CPM begins with planting powdery mildew tolerant (PMT) or resistant (PMR) cultivars if they meet your needs. It is important to remember that these cultivars are not “immune” to CPM; they will become infected at some point in the growing season depending on disease pressure. Hopefully, this will occur later in the season when compared to CPM susceptible cultivars. Organic growers hoping to mitigate losses to powdery mildew should always chose CPM tolerant or resistant cucurbit cultivars first. There are a number of OMRI-approved fungicides labeled to help suppress CPM development, these should always be used in concert with CPM tolerant or resistant cultivars and a preventative fungicide program. Cultural practices such as increasing in-row plant spacing to improve air flow and cultivation to keep weeds to a minimum will also be advantageous. Avoiding the use of overhead irrigation will help reduce disease pressure from another important pathogen, cucurbit downy mildew. Thus, growing cucurbits on a mulch with drip irrigation has its advantages, but also increases costs.

In the past, a typical conventional fungicide program consisted of rotating two different FRAC group fungicides every other week, such that the pattern looked like:

A – B – A – B – A – B

Often a protectant fungicide such as chlorothalonil or mancozeb is added to the tank mix on a weekly basis to 1) help control other important fungal diseases, such as anthracnose or gummy stem blight and 2) to help reduce selection pressure on the high-risk fungicide that was being applied. This type of preventative program was used for many years, because, in most cases there were just a few effective fungicides available for CPM control depending on the crop. An example of this would be:

A = (azoxystrobin [FRAC group 11] + chlorothalonil (MO5) rotated weekly with B = (myclobutanil [FRAC group 3] + chlorothalonil (MO5)

This type of control strategy worked extremely well as long as the pathogen didn’t develop resistance to either the FRAC group 11 (azoxystrobin) or FRAC group 3 (myclobutanil) fungicide. To better understand modes of action and how fungicide resistance develops in FRAC group 11 and FRAC group 3 fungicides please click here. Unfortunately, because of fungicide resistance development this type of program is no longer effective and is no longer recommended for CPM control.

Over the past 10 years, there have been a number of new fungicides released with new modes of action (i.e., new FRAC groups) for CPM control in cucurbit crops. Unfortunately, all have a moderate to high-risk for resistance development because of their specific modes of action. The good news are these new fungicide chemistries have less effects on humans, non-target organisms, and the environment.

These fungicides include:

  • FRAC group 13 (quinoxyfen)
  • FRAC group 39 (fenazaquin)
  • FRAC group 50 (metrafenone)
  • FRAC group U06 (cyflufenamid)
  • FRAC group U013 (flutianil)

Not all of the fungicides listed above are labeled for all cucurbit crops. Growers will need to refer to local recommendations and the label for crop specifics. Remember, the label is the law.

These fungicides offer new strategies when it comes to controlling and mitigating losses to CPM. Instead of rotating two fungicides with a moderate to high-risk for resistance development every other week ( A – B – A – B), growers now have option to reduce the total number of times any single fungicide might be applied during the production season; further reducing the risk for resistance development to any one mode of action. For example, in pumpkin, a new CPM preventative fungicide program may look like this:

A – B – C – D – E – A – B – C – D – E

Where A=(FRAC group 3);B=(FRAC group 13); C=(FRAC group 50); D=(FRAC group U013); E=(FRAC group 11)

A protectant fungicide such as chlorothalonil or mancozeb should be added to the tank mix with each high-risk fungicide to reduce selection pressure and to help control other important diseases such as anthracnose and plectosporium blight.

In this type of CPM preventative program any one high-risk fungicide would only be applied twice per growing season and 5 weeks apart greatly reducing the risk for fungicide resistance development. Importantly, for cucurbit growers, the easiest method to mitigate the potential for fungicide resistance development are to reduce the total number of applications of any one high-risk fungicide during the production season.

When to start spraying for CPM

Initiating a preventative spray programs begins with paying attention to Extension reports, scouting, and when the crop was seeded. If the crop is seeded the early-spring (i.e., early to late May) there is a very good chance CPM is not present in the mid-Atlantic region. If CPM is not present, there is no need to initiate a spray program using high-risk fungicides. In this instance, general protectant fungicides such as chlorothalonil will help mitigate other foliar diseases. As cucurbit crops are seeded into early to mid-June (and afterward) the risk for CPM development will rise in the mid-Atlantic region. This is when scouting and paying close attention to Extension reports becomes important. The first application should be done when CPM has been detected in the immediate region or when it is detected by scouting (e.g., with one lesion found on the underside of 45 mature leaves per acre). This will help reduce the use of unwarranted high-risk fungicide applications early in the production season. Importantly, the use of PMR or PMT cucurbit varieties will also help delay the onset of CPM development as well. Once CPM preventative fungicide programs are initiated, applications need to occur at every 7 to 10 days (at the latest) for as long as you expect to harvest (e.g., summer squash) or hold the crop (e.g., pumpkin and winter squash). During harvest, growers need to pay careful attention to pre-harvest intervals because they may vary significantly between different FRAC groups or fungicides within the same FRAC group (a good example are fungicides in FRAC group 3). Once harvest is complete, those blocks or fields need to be destroyed immediately to help reduce the spread of CPM to other blocks or fields that are scheduled to be harvested later in the production season. This is especially important for other diseases such as cucurbit downy mildew.

In some instances, rotating between many different FRAC group fungicides are not an option because the chemistries aren’t available for use. An example would be leaf spot control in spinach, where FRAC groups (7, 11, 7 + 11, 7 + 12, and 9 + 12) are available. In this example, options for control might look like this:

A – B – C – D

Where A=(FRAC group 7); B=(FRAC group 9 + 12); C=(FRAC group 11); D=(FRAC group 7 + 12)

Here, we have maximized the use of as many different FRAC groups as possible and spread their use as far apart as we can during the production season. Its important to remember that fungicides with more than one active ingredient (e.g., 7 + 11) should also be rotated as far apart as possible with fungicides that contain the single active ingredient (e.g., FRAC group 7 or FRAC group 11).

Monitoring fungicide efficacy

With the use of high-risk fungicides, all growers need to monitor fungicide efficacy accordingly. Once the lack of efficacy is detected there is a chance that fungicide resistance might be present. Importantly, the lack of efficacy should not be misconstrued with poor applications or waiting too long between fungicide applications. Reports of poor efficacy from Extension personnel from one region may not reflect fungicide efficacy in another region. Therefore, fungicide efficacy needs to be done at the farm level and the only way to accomplish this is to scout your fields and know what is and isn’t working for you.

The principles mentioned above also extend to other important diseases in vegetable production where there are multiple FRAC groups with high-risk fungicides available to control specific diseases. As a general rule, growers need to rotate as many different modes-of-action (i.e., fungicides from different FRAC groups) as possible during the production season to help mitigate fungicide resistance development in conjunction with best management practices.

For more information on fungicide use, FRAC groups, and specific control recommendations please see the 2020/2021 Mid-Atlantic Commercial Vegetable Production Recommendation Guide.

 

Filed Under: Commercial Ag Updates, Vegetable Crops Tagged With: ,
April 5, 2016

2016 Fungicide Resistance Management Guidelines Available

The 2016 Fungicide Resistance Management Guidelines for Vegetable Crops is now available. This free publication, targeted to New Jersey and the surrounding mid-Atlantic region, helps vegetable growers understand and manage potential fungicide resistance development on their farm.

In order to understand and use fungicide resistance management strategies effectively, first learn how and why fungicide resistance may develop; a series of articles is available on this website.

Filed Under: Vegetable Crops Tagged With: