MOFGA Pest Report 2004 #3
4 June 2004

(View List Of All 2004 Pest Reports)

CROP CONDITION AND RUSHING THE SEASON

COOL WEATHER PRESENTS PROBLEMS FOR TRANSPLANTS

-Anne Carter, Dept. of Plant and Soil Sciences, University of Massachusetts

Warm weather, alternating sunny and rainy cloudy weather has presented a number of problems for growers who use transplants. Early seeding and transplanting so far this year have probably not given growers the jump on the season they have been hoping for. First, if you are growing your own vegetable trans-plants, a number of problems may have arisen in the green-house. Several days of clouds creates low light conditions in the greenhouse. Transplants tend to etiolate (grow taller) as they "seek" the light. Transplants grown under these conditions tend to have weaker stems and longer stem area between the nodes. Studies done several years ago have shown that if you brush across the tops of the plants with a piece of styrofoam or even your hand on a daily basis, the transplants will not grow as tall. This is especially true for tomatoes. Though this is something to start as soon as the tomatoes emerge from the soil media, there is still some beneficial effect from the brushing even now. I tried it with tomatoes plants in my research greenhouse this year and was pleased with the results. If possible, plant tomatoes and peppers deeper than usual. More roots will form along the stem and transplants are sturdier. Frank Mangan did studies that showed that peppers could be planted up to the first true leaf without any detrimental effect on the plant or later yields.

Another problem under cloudy conditions is that the stomates (the holes in the leaf where carbon dioxide and oxygen exchange) tend to stay closed. Open stomates are what moves water and nutrient uptake from the roots to the leaves. Thus, on cloudy days, plants may look yellow even though you are fertilizing them. Usually, this goes away once the sun comes out and translocation occurs at a more rapid rate. Remember that soils may not be dry-ing out as rapidly under cloudy conditions. Over watering can cause nutrient leaching, increase problems with fungus gnats, and cause damping off and botrytis.

Weather conditions in the field have the same effect on transplants. Air temperatures have averaged in the [low 50's to ]60's and there has been little solar radiation to warm the soil. As in the greenhouse, water and nutrient uptake is virtually non-existent when soil temperatures are below 60 °F. Transplants can actually dry out even if they are sitting in field-capacity soils, especially if there is wind. Cool, cloudy days exacerbate the problem as stomates are only partially open or completely closed. Growth is slow and the transplant basically sits in the ground in the same condition as it was in the greenhouse. This can be remedied by planting into plastic mulch to warm the soils.

Transplants that have been under low light in the greenhouse are more light sensitive in the field. Try to plant in the afternoon, with the sun waning, or on cloudy days (not a problem). A clear day can cause sun burn. Symptoms of sunburn include pale, tan or silvery blotches on the leaves which will die back quickly.

If there is enough room in the greenhouse and your transplants are not too big, keep the transplants in the greenhouse as long as you can. Warmer greenhouse temperatures (especially root temperatures) will at least allow growth to continue. Some growers have found that trans-plants of the same age that are held in the greenhouse until the soil temperatures warm-up will actually blossom and yield before transplants that were set out in the field when soil and air temperatures are low. This is especially true for tomatoes and peppers. However, if the plants are tending to become root bound it may be a benefit to get them into the field. This is also a good year to apply a dilute solution of fertilizer to the root ball just prior to planting or into the transplant hole. The plants could use the extra boost to recover from all its potential problems.

STRAWBERRIES

Situation:
The cool, cloudy weather has persisted in much of the state this week, and continues to slow strawberry plant development. Fields in Southern Maine are just passing the full bloom stage, while fields in central Maine remain in the early to full bloom stage. High winds and cold temperatures during last weekend have resulted in some wind damage appearing on strawberry foliage. The upper surface of wind-damaged leaves appears to have brown or purple discoloration, primarily between the veins. The injury is typically not serious, and the plants will soon recover, provided that there is some improvement in the weather

Strawberry bud weevil
The "clipper" has been fairly active in strawberry fields where flower buds are still present. The IPM threshold of 1.3 or more clipped buds on average per two feet of row. Small scale growers should be hand picking off the clipped buds before they drop and leave an egg to develop. Large scale growers need to rotate strawberry plantings to avoid ever increasing populations of the clipper. We have found significant clipper injury and/or live clippers in Monmouth, Litchfield, Wayne, Poland Spring, Dresden, New Gloucester and Cape Elizabeth this week, indicating that these insects are likely to be active in any fields where flower buds are present. Once fields are beyond the full bloom stage clipper is no longer a threat to strawberries.

Raspberry Growers Take Note:
Strawberry clipper will move onto raspberry buds when strawberry buds aren't available. Check raspberry flower clusters for clipped buds and live clippers. Insecticide sprays to control raspberry fruit worm adults, which are also active at this time, should provide some control of clipper as well. Products registered for clipper on raspberries include PyGanic

Tarnished plant bug
TPB activity remains fairly low, although we seen see a few more adults and nymphs this week in the Monmouth and Cape Elizabeth fields we scouted. Once the weather improves, we expect to see more nymph activity, so it is important continue scouting for tarnished plant bug through the bloom period.

(modified from Umaine Strawberry IPM Newsletter by David Handley)

SPINACH LEAF MINER

This critter is the larvae of a fly the burrows between the layers of a leaf eating everything but the epidermis. Early damage is a slender, winding mine but later these expand and become blotches on the leaves. The fly overwinters as pupae in the soil and hatches in late April and May. The adult fly then lays eggs on the leaves and the resulting larvae begin their damage. The maggots may migrate from leaf to leaf down a row. They become fully grown in just a few weeks and drop into the soil to pupate. There are three to four generations per season. In most seasons the damage is minimal and the plants will out grow it leaving only early leaves with cosmetic damage. In other years, or other fields in the same year, the damage may be great and if the plants are hit early and the growing is slow because of weather conditions, the loss may be great. If you catch it early enough and you think the crop is worth it, Entrust is labeled for spinach leaf miner. The spinach leaf miner also feeds on beets, chard and many weeds including chickweed, lamb's quarters and nightshade. Weed control and crop rotation are the first line of defense.

EUROPEAN CORN BORER

The ECB is a reliable pest in sweet corn every year, unlike corn ear worm and armyworm that must migrate in on the winds, the ECB overwinter here in the Northeast in the bottom of corn stalks and to a lesser extent in some other crops such as potatoes and weeds. Adult corn borer moths emerge in mid to late June and spend daylight hours hiding in grassy areas. On warm nights, they fly into sweet corn fields to lay their eggs. They may also lay their eggs on other hosts, such as beans, broccoli, peppers, potatoes, dahlias and many weeds. The eggs, in masses of up to 50, resemble overlapping fish scales and are usually found on the underside of the corn leaves. The eggs hatch in four to nine days. Moth populations can be monitored by using pheromone traps. These traps are typically plastic nets or containers placed in or near the corn field and baited with a chemical attractant called a pheromone. Insect pheromones are excreted by the females to attract mates of their species. Synthetic pheromones, impregnated into small plastic or rubber "lures," are put in the traps to attract the male moths. The traps by themselves do not offer a means of control, but are very useful to know when sprays are needed. Traps and lures are available from: Great Lakes IPM, (800)235-0285, www.greatlakesipm.com and Gemplers, (800)382-8473, www.gemplers.com

Now is the time to get ready to monitor for corn borer. You will need to place two Scentry Heliothis© traps or traps of similar design in grassy weeds bordering the field, spaced at least 100 feet apart. One trap should be baited with Scentry© ECB I pheromone or equivalent (for the NY strain). The other trap should be baited with Scentry© ECB II (for the Iowa strain). Empty the traps and count moths on at least a weekly basis. Early in the season the presence or absence of moths gives you a heads up as to the importance of scouting the field. Always scout at least 100 plants per field or block by examining 20 plant samples in five locations. Later in the season there are thresholds for moth counts to determine whether a spray is needed.

In the whorl stage of sweet corn (8 leaves), look for shot-hole injury in the leaves. Treatment is recommended if 30% or more of plants scouted show fresh feeding injury. In the pre-tassel stage scout plants as above, look for feeding damage on the leaves and in the developing tassels. A treatment is recommended if 15% or more of the plants scouted show injury. In the tassel-silk stage, look for fresh feeding injury on leaves, in tassels or on the sides of ears, especially where the ears meet the stalk. Give the stalk a gentle twist near the ear to detect larvae in the stalk. A treatment is recommended if 15% or more of the plants scouted show fresh injury.

The first line of defense is crop sanitation, i.e., destroy corn stubble in the fall. Long distance crop rotation will help. Dipel DF and Entrust are registered for ECB.

(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.)