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 MOFGA's 2011 Pest Reports - Compiled by Eric Sideman, PhD Minimize

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MOFGA's Pest Report - April 11, 2011
Compiled by Eric Sideman, PhD - MOFGA's Organic Crop Specialist

In this Issue:

Seedcorn Maggot
Damping-off and other seedling problems
Onion Maggot
Cabbage Maggot


The first Pest Report of 2011 and the forecast today is for 70 degrees. The topic for this issue has to be "don’t be fooled and don’t rush." There are many insect and diseases of plants that take advantage of a crop that is struggling. Looking out my window I see signs of spring that could easily get growers out on their tractors or behind their tillers. My warning is to not grow by the calendar or a stray warm day. Today is warm, the calendar says mid-April. But, here at my farm we still have cold and very wet soil. In order to avoid setting your self up for problems, wait for the conditions to be favoring the plant and not the pest. Here are a few of the problems lurking in the shadows of a faux spring.

SEEDCORN MAGGOT (Delia platura)
The seedcorn maggot is a larva of a fly. The maggot mostly feeds on decaying vegetable matter in the soil, but if seeds are slow to germinate they fall prey too. Peas and beans are the most commonly injured seeds because people rush peas into the ground early in the spring and beans are slow to germinate in cool soil no matter what the calendar says. Corn, melons, cucumbers, potato sprouts, cabbage, beet, onion (here often confused with onion maggot), spinach, radish and more crops are also frequent victims.

The fly lays its eggs in moist soil. They are attracted to soil high in fresh organic matter. The fly eggs can hatch at very low temperatures. The larva feed on the seed, especially the embryo. Seeds may be killed before they sprout, or may sprout but be missing parts such as a cotyledon or growing tip. I often see beans germinate that have two cotyledons but no more growth because this pest ate the growing tip.

Cultural Control
Everything that can be done to encourage and hasten seed germination is important in early spring plantings. For example, waiting for warm soil, waiting for a good 5-day weather forecast, planting shallowly, etc. Slow to germinate seeds are sitting ducks for the seedcorn maggot. Avoid adding organic matter that is not fully decomposed to fields of early spring planted crops (ex. unfinished compost, livestock manure). Clean cultivation is recommended for early plantings.


DAMPING-OFF IN THE FIELD
Below I will discuss damping-off of seedlings in greenhouses and on window sills. Last year I received calls about peas dying just after or just before germinating and I thought I better mention that damping off can also occur in the field. This time of year the problem is with peas.

The disease is caused by several different species of Pythium, which is a common soil inhabitant that persists in soil (often 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 pea 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 peas.

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


CABBAGE AND ONION MAGGOT (Reprinted and modified from The Umass Veg Notes)
Onion maggot (Delia antiqua) and cabbage maggot (Delia radicum) flies look nearly identical but are likely to be found only on or near their host crop. Cabbage root maggot attacks all types of Brassica crops, while onion maggots are highly specific for the onion family including onions, garlic, leeks, chives, and shallots. A good indicator of the start of cabbage root maggot flight is blooming of the common roadside weed, yellow rocket.

Life cycle: Onion and cabbage maggot flies spend the winter as small brown pupae in the soil. Adults emerge in spring and adults can travel considerable distances in search of host plants (1/2 to 1 mile). Cabbage root maggot flies are rather delicate, hump-backed gray-brown flies, about 5-7 mm long. Onion maggot flies are very similar. Female flies seek out their host crop to lay eggs at the base of the stem. Cool, moist soil conditions favor survival of the eggs, and soil temperatures over 95 F kill them. When the soil temperatures in the upper half to 1 inch are high (>95 degrees F) that soil temperature itself then provides control. Last year we reached these conditions in April, but it didn’t help because eggs were not yet present. This is the reason these pests were still a problem last year, and are a usual problem in the spring. Summer generations of the pest are rarely noticed.

When eggs hatch, larvae feed on roots and can cause complete destruction of the root system. In crops such as broccoli or cauliflower the first sign of a problem is wilting of the plant on sunny days and yellowing of outer leaves. Later, plants collapse, wilt down, and die. If you pull one up you will see that the reason it is wilting is the roots are gone. You may find the legless white maggots feeding, or the small brown, oblong pupae. In Brassica root crops such as turnips, radishes and daikon, feeding tunnels make the root unmarketable.

In onions, newly hatched larvae crawl behind the leaf sheath and enter the tiny bulb, and feed on the roots, stem, and developing bulb. Feeding damage also encourages entry of soft rot pathogens.

Avoiding damage by later planting. The first flight and egg-laying period is generally most intense in the first half of May, depending on accumulated growing degree days – thus, it will vary with the season and location. After the first flight is over, and as soils heat up, fewer eggs are laid and those that are laid are less likely to survive. I have observed that in most years that Brassica transplants set out after the flowers fall from forsythia did not suffer damaging infestations of cabbage maggots. In cooler areas of the state, however, scouting has sometimes found damaging levels into June. Each season will be different. It is impossible to name a consistent and reliable date after which it is safe to plant onions or cole crops, but late May into June will likely be safer than the first half of May. By the way, if you find that your early May plantings of onions were hit hard, there would still be time to do another planting that would be less at risk. And, there would be time for many more plantings of brassica crops.

Monitoring. Flies are attracted to bright yellow colors. Yellow sticky cards (3X5 inches) are inexpensive and easy to use; attach them with small wire stakes and place near the soil. Check and change traps twice weekly to record changes in fly activity. (sources: Great Lakes IPM, Gemplers)

Using Growing Degree Days. The beginning and peak of each fly generation can be forecasted using degree day accumulations. Most growing degree day information for
plants and insects is based on a base temperature of 50 F, but maggot flies are active at a lower base temperature of 40 F. For more information on using growing degree days, go to http://www.umassvegetable.org/ and look at the May 6, 2010 issue of the Umass Newsletter.

Control
Floating row covers provide an effective barrier against these pests. Place the cover on as soon as the transplants are set. Use in a rotated field, as flies overwinter in soil after late season crucifers and could emerge under the cover if the same field has spring brassicas. Replace cover after weeding operations. As soil temperatures rise, the first flight ends and crops grow large, covers can be safely removed.

Crop rotation contributes to keeping populations low; greater distances are more effective. Fall tillage to bury crop residues and to expose over-wintering pupae is also useful. For onions, bury or haul away onion cull piles. Rotting onion smell attracts the onion maggot fly. In an vigorous Brassica crop, cultivation that brings soil up around the stem may help encourage formation of adventitious roots from the stem, which can help compensate for root loss even if maggots are present.

Naturally-occurring fungal diseases occasionally will reduce onion maggot numbers significantly, particularly when flies are abundant and relative humidity is high. During a fungal epidemic dead, diseased flies, can be seen clinging to the highest parts of plants along field edges. Predaceous ground beetles, which eat onion maggot eggs, larvae and pupae, can also be important in reducing maggot numbers.

Nematodes for biological control. One alternative method that has shown promise but has not been widely field-tested is soil application of entomopathogenic nematodes, especially Steinernema spp. Steinernema feltiae has been found to be more effective compared to other Steinernema or Heterorhabditis species in attaching to and penetrating cabbage root maggot larvae at low temperatures (10C) which is an important trait for use in spring when soils are cold. Common application methods include suspension of nematodes (infective juveniles) in water and application of water to transplants prior to setting in the field (as a spray or soaking drench), in transplant water used in the water wheel transplanter, as a drench after transplanting, or a combination of pre-plant and post-plant applications. Rates of 100,000 to 125,000 infective juveniles per transplant have been shown to be needed to achieve reduction in damage.

Some Beneficial Nematode Suppliers:
The Green Spot: 603-942-8925 or www.shopgreenmethods.com
IPM Labs: http://www.ipmlabs.com/315 497 2093
Arbico Organics: http://www.arbico-organics.com/
+-800 827 2847
Griffin Greenhouse Supplies: 978-851-4346 or www.griffins.com
Integrated Biological Control Systems: 888-793-4227 or www.goodbug-shop.com
Koppert Biologicals: 800-928-8827 or www.koppert.com

--R Hazzard. References: Network for Environment and Weather Applications (NEWA) of NYS IPM Program; Univ of Wisconsin-
Minnesota Degree Day Calcuator (http://www.soils.wisc.edu/asigServlets/asos/SelectDailyGridDD.jsp); Ontario
Ministry of Agriculture, Food and Rural Affairs online fact sheet ; Schroeder et al 1996, Journal of Economic Entomology
89:1109-1115; Chen et al, 2003, BioControl 48: 713–724


SEEDLING PROBLEMS
Raising seedlings in a "sterile" mix such as Pro Mix is easy. These mixes are called sterile, however they are not really sterile. On the other hand they are called sterile by growers 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, vermiculite with 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. We either purchase mixes that are generally based on compost, 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 biological activity 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.

Damping-off
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 tip. 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 the "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. Allow the surface of the container to dry a bit before watering.

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

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 thought the media may have plenty in it. Purple undersides of leaves, stunted growth, pale weak seedlings are often the results of “cold feet” 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.

Salt
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
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.

Ammonia
As nitrogen is released from decomposing proteins in a compost pile it passes through a phase where it is an ammonium ion. Unfinished compost will have ammonium ions that may revert to ammonia 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 if you send a sample to follow the directions and 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.

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