MOFGA Pest Report 2005
13 August

(View List Of All 2005 Pest Reports)

HORNWORMS

Hornworms are probably the most destructive insect attacking tomatoes and they are out in force now both in hoophouse and field tomatoes. They are giant caterpillars that can do a vast amount of eating in a very short time. Sometimes it seems that overnight healthy looking tomato plants are striped of their leaves leaving bare stems. The hornworms will also attack the fruit eating gouges out that look more like bites of a furry animal than an insect. Look now for the damage and the frass which is black pellets laying all around plants hosting hornworms.

The adults are large, fast flying hawk moths, which in flight may look like a hummingbird. At dusk they hover over flowers sucking nectar. Eggs are laid on tomato leaves and hatch in 5 days.

Hand picking is a bit frightening but does work and chickens enjoy fighting with the challenging pest. The problem is that they blend in very well and it is easy to overlook one or two caterpillars that can do significant damage in a day or two. Bt works very well on this caterpillar, especially when they are small.

WHAT ARE THOSE WHITE THINGS ON TOMATO HORNWORMS?

CABBAGE: INTERNAL TIPBURN

Symptoms

The inner leaves of heads of cabbage and Brussels sprouts are affected, but there are no external symptoms. Margins of inner leaves turn brown, beginning at the hydathodes, and later desiccate to become thin and papery at the margin or over large portions of the leaf. The affected tissue may turn dark brown to black, occasionally being invaded by secondary bacteria that cause a watery soft rot. In cauliflower, internal leaves turn brown and fold over the developing curds. When secondary microorganisms attack these leaves, they become mushy, smear over the curd and make it unmarketable.

Causal agent

Tipburn and internal browning are caused by inadequate transport of calcium to rapidly growing tissues. Low levels of calcium at the leaf margin result in tissue collapse. The disorder can be caused by a lack of Ca in the soil, but usually results from the plant's inability to move sufficient Ca to the young, actively growing inner head leaves at a critical point in their development. On a daily basis, Ca moves with the transpiration stream to the outside leafy parts of the plant that are actively transpiring on sunny days. At night, especially when dew forms, transpiration is reduced, and water movement generated by the roots is direct to the inner part of the head. However, on warm, dry nights the outer leaves continue to transpire and CA is diverted away from the head. Once Ca is fixed by the outer leaves, it cannot be translocated to the interior of the head. Environmental conditions that favor rapid plant growth favor tipburn. Abundant soil moisture promotes rapid growth, while excess moisture reduces soil oxygen levels, which in turn reduces calcium uptake and movement. Drought or root pruning also stress the root system and can impair the plant's ability to take up Ca and translocate it to the young leaves at the center of the head. A warm dry spell after a period of abundant moisture may aggravate the disorder. Excess nitrogen results in large outer leaves that accumulate calcium at the expense of young expanding leaves within heads, and also encourages rapid growth. Wide spacing also encourages large outer leaves and rapid growth. Cruciferous crops grown on sandy soil are usually more prone to tipburn compared to plants grown on heavier textured soils.

MANAGEMENT

-Cultural Practices
Factors that promote rapid plant growth should be avoided. Maintenance of optimum but not excessive fertility (including N) is important. Maintaining a phosphorous to potassium ratio of 1:1 should help to minimize the incidence of tipburn, because excess phosphorus can reduce calcium transport. Some of our soils have excessively high phosphorus relative to potassium. Irrigation may be necessary to maintain steady and optimum levels of soil moisture. Addition of high levels of calcium to the soil and foliar applications do not seem to alleviate the problem. Close plant spacing and prompt harvesting of crops when mature are beneficial practices. Internal symptoms grow worse as heads become larger and more mature.

-Resistant cultivars
Cultivars that grow less vigorously are less prone to this disorder. Resistant cultivars of Brussels sprouts and cabbage are available; check your seed suppliers for their recommendations. Growers are reporting that the culivars Green Cup and Bronco had worse symptoms than other cultivars when the problem occurred in 2004 or 2005. It always impresses me that the disorders that are caused by lack of calcium in critical tissues - such as blossom end rot in tomato and tipburn of lettuce and internal tipburn of brassicas-are not really about lack of calcium in the soil, but about other factors that influence the availability and movement of Ca into the tissues.

(-reprinted from The UMASS Vegetable Newsletter-Compiled by R. Hazzard. Sources: Howard, R.J., J. A Garland, and W. L. Seaman. Diseases and Pests of Vegetable Crops in Canada, Canadian Phytopathological Society and the Entomological Society of Canada; Nonpathogenic Disorders of Cabbage, Robert Becker, Cornell Cooperative Extension Fact Sheet; A Sherf and A. Macnab, Vegetable Diseases and their Control.)

EGGPLANT AND POTATO: COLORADO POTATO BEETLE RISES AGAIN

The summer generation of Colorado Potato Beetle adults have been emerging (look for round holes in the ground where they came out from the soil, where the pupal stage hides out). Those that emerge before August 1 lay eggs and produce a second flush of larvae. Those that emerge after August 1 don't lay eggs. All of the adults feed rather heavily, in eggplant and potato. After about August 15, CPB adults begin to leave the fields and move to field borders where they will overwinter. Beetle eggs hatch quickly in the heat, and larvae feed and grow quickly. Try to control them before they are 2/3 grown; it is the final instar that does 85% of the feeding damage.

If potatoes are in the tuber bulking stage, they can tolerate quite a bit of feeding damage (>20%). Don't try to kill every beetle in the field. This is especially important for us organic growers because trying to kill every beetle increases development of resistance to pesticide formulations, and we organic growers are very limited in what materials we have. Remember, there are no formulations of rotenone or Bt for CPB that are approved for organic use. Since food reserves in the foliage two weeks prior to senescence add little to final tuber bulking it is ok to leave some feeding beetles who are likely to carry susceptible genes to next year's generation. If you try to kill everyone the only ones you miss are the resistant ones.

Beetle adults can be particularly damaging in eggplant, where they clip flower buds as well as feeding on leaves. Scout for adults, eggs and larvae to determine which stages are in your crop and at what density. For resistance management, given the famed and truly remarkable capability of CPB to develop resistance, it is critical to rotate among classes of insecticides. The most effective way to extend the useful life of an effective product is to use it on a single pest generation only, and then on the next generation, use a second pesticide with a different method of killing the pest (mode of action). Call Valent, who manufactures Novador, and as them to reformulate this Bt in a formulation approved for organic use.

(adapted from an article by Ruth Hazzard in the UMASS Vegetable Newsletter)

CURCURBIT DISEASE UPDATE:

Here is the section on Powdery Mildew taken from the new Resource Guide to Organic Insect and Disease Management that folks from MOFGA, OMRI, NEON and Cornell put together.... the Guide should hit the street soon:

Powdery Mildew

Powdery mildew, caused by the fungus Podosphaera xanthii, appears later in the growing season than bacterial wilt, and can reduce yields by decreasing the size, or number of fruit. Fruit quality can also be reduced because of sunscald, lower sugar content, or incomplete ripening. The disease is quite easily recognized by a white powdery growth on both upper and lower leaf surfaces. As the disease advances the leaves yellow, turn brown and die. All cucurbit species are susceptible although resistant varieties of cucumber, melon, cucumber, summer squash, winter squash, and pumpkin are available.

The fungus is thought to blow into the Northeast from southern states each year, and probably does not overwinter in this region outside of greenhouses.

Cultural Control
1) Growing the crop in smaller parcels may slow disease spread.
2) Field-grown plants are resistant until fruit start to enlarge, unless they are stressed such as by heavy weed competition.
3) Vigorous indeterminate varieties may maintain sufficient numbers of healthy leaves to tolerate PM longer in the season.
4) Grow resistant or tolerant varieties.

Materials Recommended for Organic Control
1) Sulfur
2) Copper (one good, one fair, and 5 poor results)
3) Mineral oil (two fair and one poor result)
4) Several plant oils are reported to reduce powdery mildew.
5) Potassium bicarbonate (two of 13 studies showed fair control; 11 poor)
6) Bacillus subtilis (Serenade®) (One of seven studies showed fair control; 6 poor)
7) Combinations of oil and potassium bicarbonate have been more effective than either alone

(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.)