- Tomato and pepper transplanting has begun in southern New Jersey. With the on and off again heavy rains we have been getting all growers should consider applying at-transplanting fungicides for root rot control. Please click here to see this article from last week.
- Root rot in pea has been reported. Infected plants will wilt and collapse, especially in wet, low areas of the field. Look for black stem lesions at the base of the plant at the soil line. In general, Pythium infections won’t extend beyond one inch above the soil line in infected plants. Under ideal conditions a white, cottony growth may be present and is a diagnostic feature. In Pythium infected plants, the outer cortex of the root can easily be pulled off. For, more information on damping-off please click here.
- Scouting and controlling Septoria leaf spot in parsley.
- Bacterial leaf spot has been reported on savoy cabbage in southern New Jersey.
- Timber rot has been reported in greenhouse pepper in southern New Jersey. For more information on white mold on tomato and pepper click here.
- Cold weather injury has been reported in numerous vegetable crops throughout the state.
- Bacterial leaf blight in Cilantro was reported in southern New Jersey.
- Common leaf spot was reported in strawberry. For more information click here. Weather conditions have also been ideal for fruit rot infections. For more information on controlling fruit rots in strawberry please click here.
- White rust and anthracnose have been reported in spinach over the past few weeks. For more information click here
- For the updated list of fungicides for use in the control of important diseases in the greenhouse please click here.
- Transplant producers need to remember to vent their structures to reduce relative humidity buildup going into the evenings to help reduce leaf wetness during the overnight.
- The 2020/2021 Commercial Vegetable Production Recommendations Guide is available for free online! With many county offices running reduced hours or being closed this is the easiest way to obtain the newest recommendations.
- For a quick review on managing fungicide resistance development using tank mixes and fungicide rotations, and information on FRAC group 4, FRAC group 7, and FRAC group 3 and FRAC group 11 fungicides please click on hyperlinks.
April 30, 2020 Andy Wyenandt
Vegetable Disease Update – 4/30/20
April 23, 2020 Andy Wyenandt
Understanding phenylamide (FRAC group 4) fungicides
The phenylamide fungicides (FRAC group 4) are a highly active class of fungicides that target oomycete pathogens such Phytophthora and Pythium spp. FRAC group 4 fungicides are also highly effective against downy mildew pathogens such as Pseudoperonospora cubensis (cucurbit downy mildew), Phytophthora infestans (Late blight), and basil downy mildew (Peronospora belbahrii) and many other important pathogens in vegetable production. Like other fungicide classes, FRAC group 4 fungicides have a high-risk for fungicide resistance development. The phenylamides (PA) inhibit ribosomal RNA (rRNA) biosynthesis in oomycete pathogens which inhibits several life stages in oomycetes including hyphal growth, haustoria and sporangia formation. The exact mechanism for resistance is not completely understood although research has shown that PA resistance is control by a single incompletely dominant gene although multiple mutations or mechanisms may be involved in PA resistance development. Pathogens such as cucurbit downy mildew, late blight, and basil downy mildew can be disseminated over vast geographic distances in any given year, thus this migration can be responsible for the introduction of new pathogen genotypes (including PA resistant) to new locations along with local selection pressure due to PA fungicide use resulting in changes in the pathogen population. Additionally, pathogens such as P. infestans and P. capsici (Phytophthora blight) are highly sexually active at the local level, because of the potential presence of two mating types (A1 and A2), you have a “recipe” that is always evolving. Resistance development to PA fungicides is often described as sensitive, intermediate, or resistant based on EC50 (Effective Concentration) values of the different fungicides needed to kill 50% of the pathogen in laboratory assays. This type of collected information is useful in determining what proportion of a given local, or wide geographic pathogen population, may be PA resistant. Recommended resistance management guidelines developed FRAC have not changed since 1997and are intended as general recommendations that must be adapted to respective pathosystems, fungicide use and patterns, and resistance levels.
The following are general recommendations for PA fungicide use as stated by FRAC.
- PA fungicide should be used on a preventative basis; and not used as a curative or on a eradicative basis
- As foliar applications, PA fungicide should always be tank mix with a unrelated fungicide from a different FRAC group
- The total number of PA applications per season should be limited to 2 to 4 depending on label requirements
- PA sprays are recommended for use earlier in the production season during active vegetative growth
- PA should not be used as soil treatments against foliar disease development
Resistance development in P. capsici to mefenoxam has been known for many years in southern New Jersey. This is most likely to the widescale and overuse of mefenoxam in its early days because of the lack of an alternative chemistry, as well as a result of crop rotations where susceptible crops were planted in the same field for many years. In the past decade, there have been several new fungicides from different FRAC groups labeled for oomycete control. These include: Orondis Gold (oxathiapiprolin + mefenoxam, FRAC groups 49 + 4), Previcur Flex (propamocarb, 28), Ranman (cyazofamid, 21), Presidio (fluopicolide, 43), and Phosphites (33) for field use. Ranman, Previcur Flex, and phosphites have greenhouse use labels for Pythium control. Ranman and Previcur Flex can be applied in the transplant water. Orondis Gold, mefenoxam, metalaxyl, and the phosphites are the most systemic of the group and should readily be taken up the by plant via application through the drip. Presidio has locally systemic and translaminar activity and offers protection of the root system via drip. Ranman has protectant-like activity and thus will offer root system protection. Growers with a known history of mefenoxam-insensitivity on their farm should use Presidio, Previcur Flex, or Ranman plus a Phosphite fungicide in rotation in their drip application programs. Importantly, if mefenoxam has not been used in particular fields on any crop for a number of years (more than 5+) the fungus may revert back to being mefenoxam-sensitive and control with these products may return.
For more information please see specific fungicide labels, crops sections, and greenhouse uses in Table E-11 in the 2020/2021 Mid-Atlantic Commercial Vegetable Production Recommendations Guide.
Reference:
Hermann, D.C., McKenzie, D., Cohen, Y., and Gisi, U. 2019. Phenylamides: Market trends for resistance evolution for important oomycete pathogens more than 35 years after product introduction (FRAC code 4). Chapter 6 in: Fungicide Resistance in North America, 2nd Ed. Katherine L. Stevenson, Margaret T. McGrath, and Christian A. Wyenandt (eds). The American Phytopathological Society, St. Paul, MN.
Additional resources:
Damping-off: Identifying and Controlling Early-Season Damping-off Pathogens
Understanding Damping-off Pathogens During Seeding and Transplanting
April 22, 2020 Andy Wyenandt
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
April 21, 2020 Andy Wyenandt
Vegetable Disease Update – 4/21/20
- Bacterial leaf spot has been reported on savoy cabbage in southern New Jersey.
- Timber rot has been reported in greenhouse pepper in southern New Jersey. For more information on white mold on tomato and pepper click here.
- Cold weather injury has been reported in numerous vegetable crops throughout the state.
- Bacterial leaf blight in Cilantro was reported this week in southern New Jersey.
- Common leaf spot was reported in strawberry this past week. For more information click here.
- White rust and anthracnose have been reported in spinach over the past few weeks. For more information click here.
- Damping-off has been reported in pepper transplants this past week. For more information click here
Timber rot in greenhouse pepper
Bacterial leaf spot in savoy cabbage
Frost damage in strawberry (photo: T. Besancon)
April 21, 2020 Andy Wyenandt
Recognizing cold injury in spring crops
If the erratic, wet weather wasn’t enough, temperatures have fluctuated wildly this spring with night time temperatures dropping to near freezing in some parts of the state and region in recent days. With this comes the potential for cold injury on spring planted crops. Cold injury can take may different shapes on affected plants and developing fruit.
In some cases, sympoms may show up on the newest growth as a result of non-lethal injury to meristematic tissue, in pepper and tomato, new growth may be distorted with misshapen leaves. In some cases, new leaves may have a mottled, or mosaic look much like a plant infected with a mosaic virus. In these instances, plants will grow out of the problem.
In cucumber, symptoms on maturing fruit appear as brownish-tan areas on the epidermis of fruit. The fruit will also show cracking as if it has a dry rot. The effects are physiological where areas of young developing fruit got chilled by the cold night time temperatures.
We have collected a few images below of cold injury from crops from this spring.
Cold injury on cucumber fruit. The initial damage was done a few weeks ago while the fruit was very young.
Cold injury on young cucumber plant (Photo: M. Casella)
Cold injury on young cauliflower plant (photo: K. Holmstrom)
Frost damage in strawberry (photo: T. Besancon)
Cold injury on sweet corn under low tunnel. (photo: M. Casella)
Cold injury on cucumber seedling (photo: M. Casella)
Cold injury in emerging asparagus spear (photo: Rick VanVranken)
Freezing of young potato plant (photo: Rick VanVranken)
Cold injury on snap bean. (photo: Jack Rabin)
Freeze damage caused by ice crystal formation in veins of snap bean (photo: Jack Rabin)
April 19, 2020 Andy Wyenandt
Understanding Protectant Fungicides
(FRAC groups M01 – M11)
Protectant (contact) fungicides, such as the inorganics (copper, FRAC group M01) and sulfur (FRAC code M02); the dithiocarbamates (mancozeb, M03), phthalimides (Captan, M04), and chloronitriles (chlorothalonil, M05) are fungicides which have a low chance for fungicide resistance to develop. Protectant fungicides typically offer broad spectrum control for many different pathogens.
Why wouldn’t fungi develop resistance to protectant fungicides? Protectant fungicides are used all the time, often in a weekly manner throughout much of the growing season.
