Pest Report- April 23, 2021

Hello and welcome to the first Pest Report of 2021!
The Pest Report is a compilation of short discussions of pests, diseases and practical growing considerations relevant to the time of year. The Pest Report is not prescriptive – just a warning of current issues in the broader Northeast region that may or may not reach you, and/or issues that tend to be perennial. Many pest and disease issues come to southern New England before they find us here in Maine, allowing the Pest Report to be an early warning for you.
These discussions have been written by my predecessor Eric Sideman, myself, and sometimes other regional crop advisors. Discussions of pest and diseases typically include the biology of the organisms themselves, symptoms and effects on plants, and recommended management options. The report is sent by email to all of the MOFGA certified vegetable growers, and other growers who have requested it. I aim to keep these discussions updated with the most relevant information, and you can always help me do that by sending along pest and disease sightings, or new resources. 
New this year, we are working to transition elements of the Pest Report that are updated less frequently, such as specifics about pest and disease cycles that stay pretty much the same from season to season, into standalone factsheets on MOFGA’s website. This will make these emails a bit shorter and easier for you to get through, while also building an archive of easy-to-reference pest reports at mofga.org. The transition will take some time, and many editions of the Pest Report will likely blend the prior “everything goes in the email” style with our new style of directing you to MOFGA’s website for additional information. New growers are encouraged to click on and read through all the upcoming factsheets to learn about pests and diseases before they become a problem for you.
In this report
Spring has sprung in fits and starts, and it looks like it’s going to be blustery for a little while longer. This report focuses on common issues in seedling production, and the first couple pests we commonly see this time of the year. Flea beetles are likely only an issue in high tunnels for the moment, but I’m sure they will begin to emerge outdoors soon. Predictive models suggest that Seedcorn and cabbage maggot adults are likely flying in Massachusetts already, and may begin flying in Maine soon.
SEEDLING PROBLEMS
At this time of the year, most plants are still in greenhouses – a much more controlled setting than gardens and fields. As such, a lot of the problems that pop up are abiotic (i.e., not infectious). Sometimes abiotic issues are transient (e.g., cold temperatures) so it’s good to both, check new growth to see how it looks relative to symptoms elsewhere on the same plant, and to check on the health of roots and crown of the plant.
Seedling problems can occur in compost based potting mixes because of the possibility that the compost may be carrying plant pathogens, and it is much more difficult to predict the availability of nutrients from natural sources. Nutrient availability depends on biological activity – which is greatly affected by temperature, moisture and other factors. Many commercially available organic mixes are quite good – consistently free of pathogens, and good with nutrients – but occasionally even these fail.
Damping-offDamping-off is a disease most commonly seen in young seedlings but may (rarely) effect older seedlings. It is caused by species of fungi that commonly live in the upper layers of soil and jump at the opportunity to infect germinating seeds and seedlings, when conditions allow it to. The most common species that cause damping-off are in the genera Pythium and Rhizoctonia.
There are two types of damping-off. The first is pre-emergence damping-off, which rots the sprouting seed before it breaks through the soil. This can be confused for poor seed germination. The fungus attacks any part of the germinating seed especially the tiny growing tips. Post-emergence damping-off begins as a lesion on the root which extends up to the stem and/or above the soil line. The young stem is constricted by the attack and becomes soft, and the plant falls over and dies.
Damping-off cannot be cured, but it can be prevented by starting seeds in better conditions (or a “sterile” media). First make sure you are using seeds of the highest quality that will germinate and start growing quickly. Old, mistreated, and weak seeds are more susceptible to damping-off, as anything that slows germination increases the risk of infection. Excessive watering, poor drainage, and “less than optimum” temperatures should be avoided. Allowing the surface of the soil mix to dry a bit before each watering helps. If using seedling trays, adding perlite to yourpotting mix can help to facilitate soil drying and aeration.
It is possible to “sterilize” your media by baking the mix in the oven at 350 °F for about 45 minutes. The mix should reach 160 °F and should stay at that temperature for 30 minutes. Do not allow it to go higher or stay hot longer because overheating kills the beneficial microorganisms and may releases toxic materials. Many growers prefer not to “sterilize” their mixes because organic mixes are living systems and often the interactions of organisms in the mix reduces the impact of the fungi causing damping-off. In other words, if the damping-off pathogens get into a “sterile” mix, they are off and running free of antagonists and competitors (friendly or neutral microbes present in many biologically active composts). Adding a biological treatment to your potting mix, like Rootshield, may help.
Cold Soil/Wet SoilIn addition to encouraging damping-off, cold soil can cause other problems. For example, growing seedlings on window sills is often a problem because of cold night time temperatures. The roots of plants do not function when cold, and plants frequently display symptoms of malnutrition even though the media may have plenty of nutrients. Purple undersides of leaves, stunted growth, pale weak seedlings, etc., are often the results of cold or wet “feet”.
Wet potting mix, or potting mix that doesn’t get a chance to dry out on the surface can also foster algae growth. While algae is harmless to plants, it can form crusts on seedling cells that make even watering more difficult, or possibly slow air diffusion in and out of the potting media, which can hinder root growth and further exacerbate wet potting mix problems. Algae may also foster fungus gnats, which feed on algae and fungus in damp potting media, and then sometimes on your tender seedlings’ roots. The first step in responding to fungus gnats is cultural – reduce the amount of time your cells have surface moisture. In severe infestations, there is an OMRI approved Bt product, and predatory nematodes that can be effective in killing larval and pupal stages of the gnats.
Trying to grow a tiny seedling in a large container may worsen the problem of wet potting mix because the plant is just not big enough to use the water, and over watering may then be a problem the plant cannot grow out of. Perlite can help to improve drainage in a poorly-draining potting mix.
Excess mineral salts on greenhouse soil surface.SaltSome composts are high in soluble salts. Even if the salts are nutrient salts such as nitrates, high salts will cause water absorption problems and may prevent seeds from germinating. Salty composts are not a problem in field use because they become diluted with the soil, but in a potting mix it is a real problem. Compost used for potting mix should be the best you can get and should not have salts – measured as conductivity on a compost analysis – higher than 1 mmhos.
Carbon:Nitrogen ratioThe C:N ratio is critical for compost used in potting mixes. A high C:N ratio will result in nitrogen lock up, wherein all the nitrogen in the potting mix, and any you add with fertilizers, is being grabbed by the bacteria feeding on the carbon-rich material. It is a sign that the compost was made from an improper mix of feedstock, or perhaps is just not finished yet. Compost used to make potting mixes should have a C:N ratio of 15:1 to 18:1.
AmmoniaAs nitrogen is released from decomposing proteins it passes through a phase where it is an ammonium ion. Unfinished compost will have ammonium ions that may revert to ammonia gas which can kill roots and damage leaves. Ammonia in a compost used for a potting soil should be less than 0.1%. Problems from high ammonia levels can be seen as very weak, dying seedlings which have roots that are brown instead of a nice healthy white. It’s recommendable to get an analysis of any compost you plan to use to build your own seedling mix. The University of Maine Soil Testing Laboratory does compost testing, just be sure that when you send a sample you ask for the compost analysis. If you are buying a commercial compost-based mix you may want to speak to other growers and see how that brand has worked for them.
EDEMA (oedema)Edema is an abiotic symptom that can look concerningly like a disease, but typically isn’t a major concern. For more information check out the Edema Factsheet at MOFGA.ORGEdema on rooted sweet potato cutting.
SEEDCORN (Hylemya platura) and OTHER MAGGOTS
Now is the time to be aware of the problem. Seedcorn maggot larvae feed on seeds and young seedlings of many crops (corn, beans, beets, peas, spinach, onions, brassicas, etc.). The first symptoms are usually poor germination (or failure of seedlings to emerge), or wilting transplants that have lost their roots to feeding. Symptoms can be difficult to distinguish from other problems, such as damping-off due to Pythium or other soilborne fungi, or wireworm feeding. At least until you dig around a bit and find the maggots.
There are two other common maggot problems here in the Northeast, which are specific to certain crops. These are the onion maggot, and the cabbage maggot (which is a problem in cabbage, broccoli, cauliflower, radish, and other brassica family members). All of these maggots are problems early in the season, and even though they are still around they become less of a problem by late May/early June because there is much lower egg survival in warm soil.
The species of maggots are very hard to tell apart. It would be good to know which you have because the onion and cabbage maggots will become more of a problem year after year if crop rotation is not followed. And, especially with the cabbage maggot, using the wrong cover crop in a field could be a problem (you would not want to use tillage radish as a fall cover crop in a field you plan to plant to cabbage the following spring).
Seedcorn maggots in soil
In contrast to the cabbage and onion maggots, the seed corn maggot is not specifically attracted to a particular crop but rather is attracted to the smell of decaying organic matter. Seed corn maggots have even been shown to be attracted to decaying soybean meal that was applied as a fertilizer at seeding or transplanting. The maggots can usually be found in the soil around and inside seedlings and seeds.
The adults look like small houseflies. The prior season’s last generation of adults lay eggs that hatch into maggots, which feed for a while and then pupate late in the fall. They overwinter in the soil as pupae. In early spring, the adults emerge, fly around, and lay eggs either at the base of their favorite plants, or in the case of the seed corn maggot, where they smell organic matter, such as compost or manure added to a field, decaying seeds (or as mentioned above, maybe even soybean meal added as a fertilizer), etc. The eggs hatch within 2-4 days at soil temperatures of 50F.
Research has shown that peak emergence of the first three generations occurs when a enough base 40 F degree days have accumulated:Seed corn maggot – 350, 1080 and 1800 degree days (base 40 F)Cabbage maggot – 450, 1250 and 2175 degree days (base 40 F)Onion maggot – 725, 1750 and 2975 degree days (base 40 F)
For predicting the peak emergence of these flies, (and thus, the greatest risk of them laying eggs by crops) degree day accumulation is calculated on a daily basis. To see the degree day (base 40 F) accumulation for your area, you can use the Climate Smart Farming Growing Degree Day Calculator. Make sure that the current location is correct for you, and then adjust the “GDD base” to 40 F. As you hover your cursor over the graph, you will see the current accumulation of growing degree days, as well as a forecast for the next six days and averages of prior years. The Oregon IPM Center also has a degree day calculator that is more detailed, but less user-friendly.
Another good indicator of cabbage maggot flight is blooming of the common roadside weed, yellow rocket (Barbarea vulgaris).
The first generation of seedcorn maggot usually causes the most damage. The adults prefer to lay eggs in wet soil. Crop seeds that germinate slowly are more vulnerable to attack. Crops that are planted in wet, and or cool soil, may be especially susceptible to damage because of slow growth.
Management strategies:By the time you see damage, it is too late to control the problem using either cultural or chemical methods. Prevention is the key.Avoid seeding fields (especially wet fields) too early. Seeds germinate more quickly and are less vulnerable in warmer and drier soils.Disk and incorporate organic matter (such as a cover crop) at least 4 weeks before seeding to give it time to break down and make it less attractive to the flies.Avoid applying manure or unfinished compost in late fall or early spring to heavy soils that you might want to plant early. Lighter, well-drained, sandy soils are less likely to have problems (because they warm up and dry out faster than others).Rowcovers can help – but only if the maggot flies are coming from elsewhere. Remember, the flies overwinter in the soil as pupae. If you grew a susceptible cover crop, or applied manure in the fall, pupae may be there overwintering, and then flies could end up underneath your rowcover.If you need to replant, wait at least 5 days if maggots that you find are a quarter inch long; if they are smaller than that, wait at least 10 days to make sure they have pupated and will not damage the new seeds.From UMass Extension: Soil application of the entomopathogenic nematode Steinernema feltiae has shown efficacy against cabbage maggot in trials. Apply by treating transplants prior to setting in the field, in transplant water used in a water wheel transplanter, as a drench after transplanting, or a combination. Post-plant treatments are likely to be needed if maggot flight begins >1 week after transplanting. Rates of 100,000 to 125,000 infective juveniles per transplant have been shown to be needed to achieve reduction in damage. Nematodes need a moist soil environment to survive.
FLEA BEETLESWhy do my brassicas look like they were shot by a shotgun? Learn more about flea beetles in the Flea beetle factsheet at MOFGA.org
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