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 MOFGA's 2007 Pest Reports - Compiled by Eric Sideman, PhD Minimize

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Strawberry (adapted from Dave Handley's Umaine Small Fruit Newsletter)

Strawberry Rootworm - Strawberry rootworm adults and feeding injury has been found on strawberry
leaves in fields this week. The adult stage of this insect is a small (1/8”) dark brown beetle. The beetles feed on strawberry leaves during the spring and late summer, causing numerous small holes in the leaves. The adults in fields now will soon lay eggs. The larvae are small grubs that feed on the roots of strawberry plants, causing them to be stunted and weak. There is no organic spray that helps significantly, so this is another pest that demonstrates the importance of rotation to organic strawberry growers.

Root Weevil - From info supplied by Richard Cowles, CT Agricultural Experiment. Station; Peter Shearer, Rutgers Cooperative Extension; and others We have observed some plantings of strawberries infested with the grubs of black vine weevil and strawberry root weevil this spring. Infested plants appear week and stunted, usually in somewhat circular patches in a field. Digging under the plants will reveal small (1/4 -1/2”) crescent-shaped legless grubs. It is not too late to put on an application of nematodes to control the grubs (optimal timing is about mid-May). Two species of nematodes appear to offer the best control of root weevil grubs. Heterorhabditis bacteriophora (Hb) appears to be the best candidate for control of root weevils when the soil temperature is above 60 degrees (‘J-3
Max Hb’ from The Green Spot; ‘GrubStake HB’ from Integrated Biocontrol Systems; ‘Larvanem’ from Koppert Biologicals). Beneficial nematodes can also be applied in late summer (August 15 - September 1), and in that case,
Steinernema feltiae (‘Nemasys’ from Griffin Greenhouse Supply, ‘Gnat Not’ from Integrated Biological Control Systems, ‘Entonem’ from Koppert Biological) should be considered in northern locations since it tolerates cooler soil temperatures and completes its life cycle quickly. Once the grubs begin to pupate (usually early June) nematodes should not be applied, because they do not attack the adult (beetle) stage. Neem-based products containing azadiractin (such as Aza-Direct) may be acceptable for organic production, and while neem will not kill the adults it can disrupt egg-laying if applied at high rates at least twice.

Nematodes are living organisms and they can be killed if they are misapplied. Order nematodes ahead of time and be ready to apply them through a sprayer or irrigation soon after they arrive, refrigerating if delay is necessary. Do not apply nematodes using a sprayer with a piston pump. Use clean equipment, removing all screens finer than 50-mesh. Apply nematodes in early morning or evening in a high volume of water to already moist soil, pre-irrigating if needed. Apply another 1⁄4 inch of irrigation after application to wash them onto and into the soil. Although references suggest rates of several billion nematodes per acre, I found researchers and suppliers recommended 250 (if banded in the row) to 500 million per acre, at a cost of about $100 to 200 acre depending on volume and source. Ironically, nematodes probably work best in the worst weevil-infested fields. High populations of weevil larvae allow explosive growth in nematode populations, while low populations of larvae may not permit efficient nematode reproduction. Strawberry plants can recover their vigor remarkably well if crown feeding has not occurred and diseases haven’t taken over the roots.

Root weevils cannot fly, so they infest new plantings by wandering into fields from surrounding weedy and woodland vegetation, or in large numbers from recently plowed, infested strawberry plantings. Even plantings several hundred feet away can become generally infested as a result of mass migration from plowed fields. A good rotation program with substantial distance between strawberry fields can help to manage root weevils. Also, when turning under old, infested strawberry plantings, it is critical to leave a row or two at the perimeter of the field as a trap crop to protect
other plantings. Adult weevils will be intercepted in these rows before they leave the field and thus lay their eggs where the larvae will not do any damage. At the end of the season the trap rows should be turned under prior to planting winter rye. Do not spray the trap rows as this may repel weevils and result in more migration to other fields.

Some Beneficial Nematode Suppliers:
The Green Spot: 603-942-8925 or www.shopgreenmethods.com
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


CABBAGE MAGGOT FLIES
(adapted from UMass Extension Newsletter)

A good indicator of the start of cabbage root maggot flight is blooming of the common weed yellow rocket. Cabbage maggot flies are likely to be found only near their host crop, brassicas. Flies spend the winter as small brown pupae in the soil. Adults emerge in spring and can then travel up to a mile in search of host plants. About 6 to 10 days after the flight of adult flies eggs are laid. Female flies seek out brassica plants to lay eggs at the base of the stem. Cool, moist soil conditions favor survival of the eggs. By late June, if the soil temperatures in the upper ∏ to 1 inch are above 100 degrees F then the heat of the soil itself provides control.

When eggs hatch, larvae feed on roots and can cause complete destruction of the root system. In brassica root crops such as turnips, radishes and daikon, feeding tunnels make the root unmarketable. 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 roots are gone. You may still find the little white maggots feeding, or the small brown, oblong pupae.

Monitoring for adults. Cabbage root maggot flies are rather delicate, hump-backed gray-brown flies, about 5-7 mm long. The flies are attracted to yellow sticky cards, which are inexpensive and easy to use. Attach them with small wire stakes and place near the soil. It‚s best to check traps twice weekly, as they often get coated with dust when left out for a whole week. This will tell you when the flight peaks, and when it declines. In cabbage, flight typically declines after mid-May so that some late May or June plantings do not get attacked (Maine is a week or two later). You can time your transplanting to try and avoid peak flights, and thus damage.

Monitoring for eggs. To check your field for eggs, look for the 1/8-inch long, torpedo-shaped white eggs that are laid along the stem, or on the soil next to the stem of young transplants. Often eggs are laid in neat rows, or inserted into the soil. They may be under a small clod of dirt near the stem. Eggs may be more abundant in wetter areas of the field. A pencil point helps stir the soil to look for them. Check 25 or more plants, in groups of 2 to 5 plants, scattered around the field. If you have several plantings, scout each planting (it takes about 15 minutes).

Organic growers do not have effective insecticide options for this pest so cultural practices are critical.

Floating row covers provide an effective barrier against this pest. 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. Crop rotation contributes to keeping populations low. Fall tillage to bury crop residues and to expose over-wintering pupae is also important. If the pest is present in a healthy 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 due to maggots. Natural enemies in the soil may also help to suppress the population of maggot eggs and larvae.


ONION MAGGOT

The onion maggot is in the same genus as the cabbage maggot and has very similar habits. I have received calls this year about it and I lost an entire crop a few years ago. It is one of the most serious pests of onions. Onion is the preferred host and related crops such as scallions, garlic, leeks and shallots are only occasionally infested. Wild onion is not an important host either.

The greatest damage is caused by the first-generation larvae that attacks young seedlings in the early spring. They move up rows to new plants as they devour the young seedlings quite quickly and need more food. The first symptom seen in very young seedlings is a slight wilting of the plant. Later they will simply disappear. Plants attacked at later seedling stages will turn a grayish yellow and wilt and will later detach at the ground level as the maggot consumes everything below ground. Onion plants attacked later are rarely killed and fewer plants are attacked as the maggot does not move to new bulbs. However, plants attacked at the later stage are likely to have misshapen bulbs and usually are later attacked by fungi and rot.

The onion maggot overwinters as a pupa in the top five or so inches of soil. When spring soil temperature rises above about 40 degrees the overwintering pupae begin to develop and, depending on the weather, in a number of weeks the adult onion maggot flies will emerge and fly around and mate. Later they will search for onions to lay eggs around. Eggs are laid around the base of onion plants and the resulting larvae crawl down to start feeding on
the roots. There can be three generations in a season.

Adults of later generations disperse very little from onion fields so crop rotation is a very important tool for avoiding infestations. Good sanitation is very important because cull onions will attract flies that will lay eggs that result in overwintering pupa. Damaged onions are the preferred site for egg laying so avoid mechanical damage to onions and dispose of any damaged onions rather than leaving them laying around the field. Discing onion fields in the early fall when the flies are still active actually makes things worse because it makes many pieces of onions and so increases the number of sites for egg laying. Ash and diatomaceous earth around the base of onion seedlings may deter egg laying and maggot survival.


COLD SOIL TEMERATURE AND SEED GERMINATION

I have received a number of calls about very poor seed germination both in the greenhouse and in the field. In my mind this is due to low soil temperature. This spring has been challenging with days on end of very cloudy, damp, cold weather and this effects greenhouses too since they do not warm during the day and we are all guilty of trying to save heating fuel at night.

Seeds of many crops are adapted to germinate in warm soil (corn, melons, etc) and will take dramatically longer to germinate in soil even only slightly cooler than their optimum temperature for germination. Here is a table of just a few examples of how temperature effects seed germination:

CROP OPTIMUM RANGE DAYS REQUIRED @ DEGREE F

590 F 770 F

Corn 60-95
12 4

Melon 75-95 does not germinate 4

Cucumber 60-95
13 4

Pepper 65-95 25
8

When a seed sits damp and cool for a long time it becomes very susceptible to some diseases and insects. And those of us using compost based soil mixes may have the organisms that cause problems waiting right there in the mix. As you can see above, greenhouse temperatures that we had a few weeks ago in that rainy stretch resulted in seeds just sitting….like sitting ducks for disease attack. Here are two problems that I think had a field day in crops this year and may still be a big problem if folks get fooled by this hot day today and rush to plant some things that do much better in warm soil:


PYTHIUM ROOT ROT

Pythium is one of the most common fungi found in roots of greenhouse crops and is usually present in field soil. It is a natural inhabitant of the soil and can survive there indefinitely as well as in debris in the greenhouse. Stunted growth and wilted plants are common above-ground symptoms caused by Pythium root rot. To examine plants, remove plants from pots and examine roots. Healthy roots are white and firm; decayed roots may be dark colored
and the rotted outer covering of the root slips from the central core.

Growers having re-occurring problems with Pythium, should review their overall production practices including fertilizing, watering and media handling in greenhouses, and may want to scale back on their rush to plant seeds outdoors. Over-watering and excessive fertilizer levels promote Pythium. Allowing the soil surface in containers to dry before watering again helps. Good sanitation is crucial for prevention in greenhouses. Keep hose ends off the floor, wash hands before handling plants and avoid contaminating growing medium.

Since symptoms can be confused with other causes such as high soluble salts or other diseases, suspicious plants should be diagnosed through your University diagnostic lab. -(Adapted from article by Tina Smith, UMass
Extension)


SEEDCORN MAGGOT

Peas, beans, corn, potato sprouts and even some seeds in the greenhouse are attacked by the larvae of this fly. They are yellow-white maggots about a quarter of an inch long and sharply pointed at the head end. The symptom is
usually that you see no germination, and when you dig around you may find nothing left or may find the maggots burrowing into the seed. Sometimes the seed germinates but only a weak or partially eaten plant is seen. The
injury is most likely to occur in cold wet seasons where the germination is slow, and also in soil high in organic matter.

The attack is early in the spring because the critter spends the winter as pupae in the soil or maybe free maggots in manure or unfinished compost. The adult is a grayish brown fly only about a third of an inch long. It emerges in early spring and deposits eggs in rich soil, compost piles or near seeds and seedlings. Exposed peat or potting soil mix of transplants can also serve as attractive sites for females looking for a place to lay eggs. There are a few generations each season.

The best method of dealing with this critter is to do everything you can to encourage quick germination and rapid growth. In the cold, wet soils we have in spring the seeds are just sitting ducks. Shallow planting helps when conditions are poor. Best yet, wait for things to warm up and dry out.

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