Pest Report – April 28, 2016



I will concentrate in this first pest report on problems that are common when a grower simply tries to push a crop too early. Basically the recommendation will be to calm down, sit tight, and wait for the soil and air to be more hospitable to tender crops. Of course, I don’t follow my own advice and I push everything. My peas are sitting under a few inches of snow!

The four most common early problems that I see in vegetables, which arise simply because the grower jumped the gun are seed corn maggot, seedlings rotting in cold soil, warm weather crops planted in cold soil, and bolting onions.

In this issue:
Onion Seedling Planting Time
Seedcorn Maggot
Seedling Problems in the Greenhouse
Damping-off in the Field

Bolting (flowering) onions in a greenhouse. Becky Sideman photo.
Seedcorn maggots on a spinach seedling.


Onions are biennials that flower their second year of growth. What causes an onion to sense it is in the second year of growth is winter temperatures. If onion seedlings are transplanted too early in the spring, and the temperature alternates from warm to winter-like cold that causes the plant to go dormant, and then warm again, the plant may be triggered to prematurely bolt (flower). The onion is tricked into sensing two seasons of growth. The solution is to plant small (no larger than a pencil) seedlings that are the right variety at the right time. But, another complicating factor is that onions need to be sufficiently large when they get the day length signal to form a bulb. That means onions need to be planted fairly early. Generally, planting long day onions is mid May works fine. April is frequently too early and causes boltling, June is too late and onions may never form bulbs.

SEEDCORN MAGGOTS – Hylemya platura

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, cole crops, 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 (see below), or wireworm feeding. Also, symptoms are similar to damage caused by the cabbage or onion maggot, which I will discuss a bit later in the season when it is time to plant those crops.

If seedcorn maggots are the culprit, maggots can usually be found in the soil around and inside seedlings and seeds. The seedcorn maggot is yellow-white, 1/4 inch long, legless, with a wedge-shaped head. The adults look like small houseflies. Seedcorn maggots overwinter in the soil as pupae. In early spring, the adults emerge and lay eggs where they smell organic matter, such as compost or manure you added to a field, decaying seeds (or maybe even soybean meal that you 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 occur when 200, 600 and 1000 degree days have accumulated. Degree days are calculated on a daily basis by using the formula: (Max temp – Min temp)/2 – 39F.

The first generation usually causes the most damage. The adults prefer to lay eggs in wet soil that is rich in organic matter. 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 germinsation. Two years ago my earliest planted spinach seedlings wilted within a week of transplanting. When I pulled one up for examination I saw that it was swarming with maggots. This year I covered the early planting with row cover to keep the adult flies away.

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. Damage can occur if the flies have overwintered in the soil and thus end up underneath the rowcovers.

• 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.


Raising seedlings in a “sterile” mix such as ProMix is easy. These mixes are called sterile, however they are not really sterile. They are called sterile because they don’t have plant pathogens in them, and that beats my home-made mix. These “sterile” mixes are mixes of pathogen free materials such as peat, perlite, and vermiculite. They often also include synthetic and very available sources of plant nutrients added in just the right amount to get newly germinated plants off and growing. Organic growers do not have the option to use these mixes because of the synthetic fertilizers. We either purchase mixes that are generally based on peat and compost, and other natural sources of nutrients, or mix up our own. Seedling problems are much more likely in organic mixes because of the possibility that the compost may be carrying plant pathogens. In addition, it is much more difficult to predict the availability of nutrients from natural sources because of how that depends on biological activity, which is greatly affected by temperature, moisture and other factors. Many commercially available organic mixes are quite good and consistently free of pathogens, and good with nutrients, but occasionally even these fail. Home made mixes, such as the one I make, fail more regularly. Here are some of the problems I have seen frequently in my home made mix, and occasionally in commercial mixes.

Seedling affected by damping-off disease.


Damping-off is a disease most commonly seen in young seedling but may (rarely) effect older seedlings. It is caused by species of fungi that commonly live in the upper layers of soil and when things are not right these pathogens jump at the opportunity to infect germinating seeds and seedlings. The two most common species that cause damping off are in the genera Pythium and Rhizoctonia.

There are two types of damping off. The first is one growers often misinterpret because it is pre-emergence damping-off, which rots the sprouting seed before it breaks through the soil. Growers often blame the seed companies for poor seed. 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 the stem to 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. Old, mistreated, and weak seeds are more susceptible to damping-off. Anything that slows germination increases the risk of infection. Excessive watering, poor drainage, and less then optimum temperatures should be avoided. Allowing the surface of the soil mix to dry a bit before each watering helps.

It is possible to “sterilize” your media by baking the mix in the oven at 3500 F for about 45 minutes. The mix should reach 1600 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.

Cold Soil/Wet Soil

In addition to damping off, cold soil can cause other problems. 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 in it. Purple undersides of leaves, stunted growth, pale weak seedlings, etc., are often the results of cold or wet “feet”. Trying to grow a tiny seedling in a large container may have the same effect because the plant is just not big enough to use the water, and over watering may be a problem the plant cannot get out of.


Some 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 seedling media it is a real problem. Compost used for media should be the best you can get and should not have salts -measured as conductivity on a compost analysis- higher than 1mmhos.

Carbon:Nitrogen ratio

The C:N ratio is critical for compost used in potting mixes. A high C:N ratio will result in nitrogen lock up, which is a case where all the nitrogen in the media, and any you add with fertilizers, is being grabbed by the bacteria feeding on the carbonaceous 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 media should have a C:N ratio of 15:1 to 18:1.


As 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 and kill roots. Ammonia nitrogen in a compost used for a potting soil should be less than 0.1%. Problems from high ammonia can be seen as very weak, dying seedlings, and when you pull them up you see that the roots are brown instead of a nice healthy white.

My recommendation is 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 that 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.


Above I discussed damping-off of seedlings in containers. I often receive calls about peas dying just after or just before germinating and I thought I better say that damping off can occur in the field too. This time of year the problem is with peas and spinach.

The disease is caused by several different species of Pythium, which is a common soil inhabitant that persists in soil and in root debris as spores. The species have a wide host range of crops and weeds and so crop rotation will do little to avoid the problem.

During or just after germination the seedling begins to show symptoms. The symptoms may be as simple as yellowing and stunting because sometimes only the root tips are infected and this root pruning interrupts growth. Sometimes a soft rot of the stem kills the plant. Sometimes you just don’t see any germinate.

High soil moisture and warm soil temperatures (warmer than optimum for pea growth, i.e., 65-75) favors Pythium. You can’t control the weather but you can choose when you plant your peas. If the soil is very wet and warm weather is forecast it may make sense to delay planting a few days for the soil to dry a bit.

There are resistant varieties. In peas, the resistant gene is tied to some visual characteristics of the seed. Wrinkled seeds are more severely affected than smooth, round seeds. Also, some biological seed treatments such as Rootshield may help.


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