Pathogens such as Fusarium, Pythium, Phytophthora, Thielaviopsis and Rhizoctonia that cause pre- and post-emergent damping-off can cause serious problems in organic (and conventional) transplant production. The key to controlling and/or suppressing damping-off pathogens with biological controls is keeping the biological populations high and continually present on root surfaces of the host, and by following good cultural practices. [Read more…]
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Controlling basil downy mildew in the greenhouse
Basil downy mildew (BDM) can cause significant losses in the greenhouse. Once introduced into the greenhouse it can be very difficult to manage and eliminate. In the past few years, a vast amount of research has been done on understanding BDM biology and controlling it in the greenhouse using different cultural practices. Before we get to control strategies, let’s review what we know about the pathogen.
First, basil downy mildew is an obligate parasite – meaning it needs a living host to survive. As long as basil is in production in the greenhouse there will be a potential source of inoculum. Sources of inoculum can include fresh intact leaves, but also leaves discarded and fallen on the floor or in an open garbage container. This is important for greenhouse growers who produce basil year round or growers who are looking to extend basil production to later into the fall or earlier in the spring. The simplest method to break the disease cycle would be to stop growing basil for a short period of time and keeping your greenhouse as clean as possible. This would help break the disease cycle by removing the host. Sporangia produced by BDM are short-lived. Without a host their survival is only a few hours to a few days depending on the temperature and environmental conditions. The latent period (the time between infection and symptom development) can range from 5 to 10 days depending on the temperature and environmental conditions. This informs us that plants which appear uninfected may actually be infected without symptom development. Therefore, it is critically important to remove all plants from the operation before restarting production (especially if BDM is already present). A good time to stop greenhouse production (i.e., in the mid-Atlantic region or more northern regions) would be after the first hard freeze in the fall – after the freeze kills all potential sources of inoculum that could come from sources outside the greenhouse.
Control strategies using cultural practices in the greenhouse
Reducing relative humidity in the greenhouse
Basil downy mildew requires high relative humidity (>95%) for 7.5 hrs and at least 4 hrs of leaf wetness for sporulation. Sporulation has been shown to be significantly reduced, or not capable when relative humidity is less than 85%. Thus, maintaining relative humidity below 85% in the greenhouse can significantly help reduce spore production. If this is not possible interrupting the dew cycle (i.e., leaf wetness) with 10 minute periods of drying via fanning/venting every 2 to 4 hours can help reduce spore production.
Control using light
Research has shown that infected plants kept under 24 hours of continual light are unable to sporulate, this was also shown to be temperature-dependent. The type of lighting is also important. Incandescent light was fully inhibitory at 15 to 25oC, but not 10oC. Narrow band LED illumination with red light has been shown to be more inhibitory than blue light. Thus, lighting basil during the night every few hours at short periods of 10 minutes can help reduce sporulation.
Control using fanning and ventilation
Continuous fanning during the night has been shown to dramatically reduce BDM development by reducing leaf wetness (i.e., dew) and reducing relative humidity (keeping it below 95%). Recommendations from Israel are to initiate fanning when relative humidity reaches 70% in the greenhouse and to stop it when it is below 60%.
The key to controlling and mitigating BDM development in the greenhouse is controlling relative humidity and periods of leaf wetness to help reduce potential sporulation. Using a combination of cultural practices mentioned above can help reduce BDM development, but it will come at a cost to you in the form of additional hardware, temperature and relative humidity monitoring equipment and the cost of electricity. The first step in this process involves understanding where the initial source of inoculum may be coming from. Evidence for BDM being seed-borne is mixed. During the spring-summer-fall, greenhouse basil production will always be at-risk from infections coming from an outside source, including diseased seedlings you may be purchasing. Successfully breaking the BDM disease cycle (without the use of chemical inputs) in greenhouse operations has limited opportunities (e.g., in northern regions where freezing weather occurs). This can only occur in the fall, after freezing weather which can kill all outside sources of inoculum and by not carrying over infected plant material into the winter season, thus the need for a basil-free period during the production cycle. This is especially important in small greenhouse operations that produce basil organically or without the use of chemical input.
These management practices should significantly reduce your BDM problems though will require more of your attention and potentially additional equipment and supplies. Coupling best management practices with new downy mildew resistant basil varieties will further provide protection to you. Try the new basil downy mildew resistant varieties including Rutgers Obsession DMR, Rutgers Devotion DMR, Rutgers Passion DMR, and Rutgers Thunderstruck DMR or other DMR resistant sweet basils such as Prospera, and see which ones work best for you.
For information on Rutgers DMR sweet basils, where to purchase seed, as well as control strategies, and ongoing research efforts please follow the Rutgers basil downy mildew breeding program on Instagram at #Rutgersbasil.
Resources:
Tracking basil downy mildew in the US
Fungicides for the control of BDM
Controlling basil downy mildew in the greenhouse
Authors: Andy Wyenandt and Jim Simon, Department of Plant Biology, Rutgers University
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.
Got humidity in the greenhouse? Get rid of it!
Unfortunately, closed greenhouses retain heat and unwanted humidity which can spell problems if left unchecked, especially during prolonged periods of overcast days and cooler weather.
Remember, high relative humidity equals wet leaves which can favor the development of diseases such as Botrytis or bacterial diseases such as bacterial leaf spot. To avoid potential problems, do your best to vent out as much humidity as possible on a daily basis in the early morning or late afternoon. Turn on circular fans to promote air movement and finish watering earlier enough in the day so leaves have enough time to dry out before nightfall. Any flats with transplants with suspect leaf spots should be pulled out of the production facility and isolated until a proper diagnosis can be made.
As a side note, stressed transplants where water and/or fertility have been withheld are also more prone to disease development. A light fertilizer application may go a long way in improving transplant health before setting in the field.
Please see Table E-14 in the upcoming 2020/2021 Mid-Atlantic Commercial Vegetable Production Recommendations guide for a list of control options for important diseases in greenhouses.
Understanding The Differences Between FRAC Group 11 and FRAC Group 3 Fungicides
FRAC Group 11 Fungicides
The strobilurin, or QoI fungicides (FRAC group 11) are extremely useful in controlling a broad spectrum of common vegetable pathogens.
You may know some of strobilurins as azoxystrobin (Quadris), trifloxystrobin (Flint), pyraclostrobin (Cabrio), or Pristine (pyraclostrobin + boscalid, 11 + 7). For example, FRAC group 11 active ingredients such as azoxystrobin are also now available generics or in combination products as Quadris Top (azoxystrobin + difenoconazole, 11 + 3), Quilt (azoxystrobin + propiconazole, 11 + 3), or Quadris Opti (azoxystrobin + chlorothalonil, 11 + M5).
All strobilurin fungicides inhibit fungal respiration by binding to the cytochrome b complex III at the Q0 site in mitochondrial respiration. Simply said, the fungicide works by inhibiting the fungi’s ability undergo normal respiration. The strobilurin chemistries have a very specific target site, or mode-of-action (MOA).
Although highly effective, fungicide chemistries like those in FRAC group 11, with a very specific MOA, are susceptible to fungicide resistance development by some fungi. Why is that? In the strobilurin’s, a single nucleotide polymorphism of the cytochrome b gene leads to an amino acid substitution of glycine with alanine at position 143 of the cytochrome b protein.
For us, knowing the specifics on the technical jargon isn’t so important, it’s understanding what is at stake. So, if we hear someone speak about G143A resistance development to the QoI fungicides (where resistance is already known in cucurbit Powdery mildew and Downy mildew), we know what they are talking about and how important it is! So much so, if cucurbit powdery mildew develops resistance to one strobilurin compound it may develop what is known as cross resistance and become resistant to all chemistries in FRAC group 11, even if only one chemistry has been used!
Damping-off: Identifying and Controlling Pathogens in Transplant Production
It is extremely important to know which pathogen is causing damping-off problems and which fungicide to properly apply. The key to controlling damping-off is being proactive instead of reactive. Always refer to the fungicide label for crop use, pathogens controlled, and application rates.
Damping-off is caused by a number of important vegetable pathogens and is very common during transplant production. Damping-off can kill seedlings before they break the soil line (pre-emergent damping-off) or kill seedlings soon after they emerge (post-emergent damping-off). Common pathogens that cause damping-off include Pythium, Phytophthora, Rhizoctonia and Fusarium spp.
Control of damping-off depends on a number of factors. First, is recognizing the conditions which may be leading to the problem (i.e., watering schedule/greenhouse growing conditions) and second, identifying the pathogen causing the problem. Reducing the chances for damping-off always begins with good sanitation practices prior to transplant production.
Conditions Favoring Damping-off
Although all four pathogens are associated with damping-off, the conditions which favor their development are very different. In general, Phytophthora and Pythium are more likely to cause damping-off in cool, wet or overwatered soils that aren’t allowed to dry out due to cloudy weather or cooler temperatures. Conversely, Rhizoctonia and Fusarium are more likely to cause damping-off under warmer, drier conditions especially if plug trays are kept on the dry side to help reduce transplant growth. [Read more…]