MOFGA Pest Report 2004
21 August

(View List Of All 2004 Pest Reports)

It is still raining. It seems as if it is raining almost every other day. This has been a hard summer to make hay and a hard summer to avoid leaf wetness that enables fungus spores to germinate. There are lots of diseases out there in the fields this year that we only see when people don't manage irrigation or ventilation wellŠ..or when it just rains almost every day and when it is not raining the humidity is nearly 100%. White mold is going wild in beans and causing widespread lettuce drop. Early blight is really kicking in now. Etc.

ONIONS

BOTRYTIS LEAF BLIGHT (Botrytis squamosa)
The first symptoms are lesions that have a white center and a light green halo. Under prolonged leaf wetness and relatively cool temperatures the fungus develops rapidly and causes blighting of the leaves and whole fields start going down. This causes losses in yields because of smaller bulb size. It does not lead to rotting of storage onions, which a related disease called Botrytis Neck Rot does (see below). Remember, onions normally have some dying leaves and so a few dead leaves does not mean problem. Onion plants produce about 15 leaves in a growing season but have only about 10 living at any time.

The fungus that causes Botrytis Leaf Blight overwinters as sclerotia (hard, persistant, dormant pieces of vegetative tissue) in cull onion piles or in leaf debris or in the soil. In the spring these sclerotia germinate and produce spores that blow around in the wind and are the primary source of infection. If there is sufficient leaf wetness for 6 hours or more the spores germinate. The longer the leaf wetness duration the greater amount of blight.

Sanitation and crop rotation are key organic controls for those who have had problems with leaf blight. Destroy cull piles and do not put waste onions into compost destined to be used on next year's onion fields. Rotation to non onion crops for a few years will help a great deal. If you have to stay in a field then plow deeply in the fall rather than disking to bury onion debris and sclerotia.

The fungus overwinters as sclerotia on rotting bulbs. It can be carried by seeds but most onion seed producing farms are in arid climates where the disease does not occur.

If necks are still succulent at harvest they are susceptible to invasion by the spores. The fungus is unable to penetrate well-dried neck tissue. Practices that hasten curing is the best defense. Undercutting bulbs at maturity helps. If onions are still standing at the end of the season, do not break necks as this creates a point of entry for the disease. Pull onions and allow to dry in well ventilated area. I hang onions up side down with the leaves hanging through slats in a table, such as a pallet. Do not cut leaves off until the neck area is dry.

PURPLE BLOTCH (Alternaria porri)
Purple botch symptoms first appear as lesions similar to Botrytis leaf blight but they become larger, zonate and change to brown or purple. After a few large lesions develop they may coalesce and girdle the leaf. This disease may invade the bulb at the neck with a rot similar to neck rot except that the pigment is soluble and the color diffuses through the scales starting as deep yellow and turns to purplish red.

Purple blotch is spread by spores produced in the lesions. It overwinters in onion debris. It may be seed borne but similar to Botrytis leaf blight it is not a common source because onion seed is produce mostly in arid climates. Champion WP is labeled for purple blotch, but the spraying would have to begin at the first signs of the lesions.

TOMATO HORN WORMS

I am seeing the first hornworms of the season in hoop houses of tomatoes. Be on the look out now. These critters blend in very well and the first sign is usually the frass. Hand picking works if you can find them. Bt (Dipel) works well.

PARSNIP CANKER

(Itersonilia perplexans) This disease is seen as canker on the shoulder and as leaf spots. The fungus that causes it is a peculiar one related to rusts and smuts and it produces a weird spore that is called a ballistospore, which somehow can be shot into space due to the surface tension of a drop of water stuck to the spore wall.

Spores that are produced on the leaves fall to the ground where they come into contact with the roots and can infect through wounds caused by insects such as the carrot rust fly.

The fungus requires a wet, cool season to cause much injury. When the soil and air are relatively dry it does not reproduce. Good drainage and long rotations are general control recommendations. Controlling weeds in the family is important, e.g., wild carrot and parsnip. Hilling the rows to keep the root shoulders covered helps. Copper fungicides have produced good results.

SWEET CORN

SITUATION
Corn harvest continues in full swing throughout the state. The remnants of the tropical storm that touched most of the state last week appears to have brought up a batch of corn earworm moths in addition to all the rain. This is fairly typical of this time of year. Any weather systems coming up from the south can be expected to bring an increase in the moth numbers we find in our pheromone traps.

European Corn Borer: There was a significant increase in the number of moths in our pheromone traps in some locations this week. This may be an indication that a second generation of corn borer is getting started in Maine. Larval feeding injury continues to be low, but may be expected to increase soon if moth counts remain high.

European Corn Borer moth counts were over the threshold for silking corn in Biddeford, Cape Elizabeth, Dayton, Farmington, Lewiston, Monmouth, New Gloucester, North Berwick, Oxford Warren and Wayne. However, all of these fields were already under a spray interval for corn earworm, so no additional sprays should be needed.

Corn Earworm: Moth populations were generally higher this week, continuing to break season records in some locations. The high numbers resulted in a three day spray interval recommended on silking corn in Cape Elizabeth, Dresden, Lewiston, No. Berwick and Poland Spring. A four day spray interval was recommended in Biddeford, China, Dayton, East Corinth, Farmington, Litchfield, Monmouth, New Gloucester, Nobleboro, Oxford, Warren, Wayne and Wells. A five day spray interval was recommended for Farmington. No spray was recommended in Levant.

Fall Armyworm: Fall Armyworm moth captures were higher in most, but not all, locations this week. Sprays for moths are recommended on silking corn when three or more moths are caught in a week. This week the threshold was exceeded in Biddeford, Cape Elizabeth, Dayton, Farmington, Lewiston, Litchfield, Monmouth, New Gloucester, North Berwick, Poland Spring and Wells. However, these locations were already on a spray interval for corn earworm, so no additional spray was required. Fall armyworm larval feeding injury has increased and was found in most locations in southern Maine. This injury is combined with European corn borer injury to determine if protection is needed. Control thresholds are 30% infestation in whorl stage corn or 15% for corn at pre-tassel and beyond. Injury exceeded control thresholds in Biddeford, Cape Elizabeth, Dayton, Lewiston, Monmouth, Oxford, Poland Spring and Wells. Twilight Meeting: Thursday, August 26th, 6:00 p.m. Johnny's Selected Seeds, Foss Hill Road, Albion, ME.

Potato leafhopper injury has been found on several crops. Symptoms include yellowing, stunting and curling leaves. When disturbed, the small, whitish insects can be seen flying off plants. Look for small, light green leafhopper nymphs on the underside of the leaves. They are about 1/16th inch long and bullet-shaped. Check the New England Vegetable Management Guide for appropriate pesticides, rates and precautions

(About the author: Eric is MOFGA’s Technical Services Director, essentially an organic "extension agent". He can be reached at the MOFGA office to answer your questions about farming and gardening. Link to MOFGA Contact Page, or email Eric directly.)