by George Looby, DVM
Growers work hard to stay current in the area of plant pest control. It is an area of management which requires a lot of study to stay ahead of all of the pests out there. If the average consumer knew how much work goes into just this segment of vegetable production they would be amazed the produce they consume is so carefully monitored in the field.
It seems every vegetable has at least one major pest which puts its survival as a marketable crop at risk. Asparagus is one of the first crops to hit the market in the spring and the common and spotted asparagus beetles are waiting for it to push its crown through the ground. There are a group of cultural practices which can be incorporated into almost any control program and so it is with the control of this pest. Removing spent vegetation in the fall applies to almost any good control program. Insecticides such a Sevin, Assail and Lannate can be used on spears and ferns. Other conditions affecting asparagus include the yellow striped armyworm, Fusarium wilt/crown rot, and asparagus rust.
The cabbage maggot is a problem wherever cabbages are grown and here we can expand the list of cultural practices which will aid in their control. Eliminate weeds of the Crucifer, or mustard family, avoid spring applications of manure and/or compost, rotate crops, delay planting until late May, plastic mulch, bait over row, and incorporate crop residue after harvest. Several caterpillars enjoy feasting on cabbage and one of these is the Crossed-striped cabbageworm. It is the hope of investigators a predatory wasp will be the answer to this particular pest. The cabbage whitefly is a pest for which there is no effective control at this time and has been said many times the folks in the lab are working on it. One further management practice which has a place in the control of many pests is eliminating the use of overhead watering, especially in the control of fungal diseases such as Alternaria leaf spot.
One of the major problems with members of the cucurbit family is powdery mildew. Included in this family are pumpkins, cucumbers, gourds and squash, among others. Site selection is an important consideration in attempting to minimize this disease. Hilltops are preferred where good air circulation is apt to be the best, avoiding hollows at the bottom of slopes. Select resistant varieties and do not crowd plants thus allowing for good air circulation. Trickle irrigation is best for those varieties which are susceptible. Other diseases and pests affecting these varieties are Black rot, cucumber beetles, bacterial wilt, squash vine borer and squash bugs.
Tomatoes constitute a major component of most commercial vegetable operations so when the crop is affected in any way it represents a major disruption in the entire operation. This crop can be affected by a wide variety of different problems, one of which is early blight. Conditions which favor its development are long periods of leaf wetness accompanied by temperatures between 65-85 F. Spores of the fungus can survive the winter in crop debris and seeds and is spread by wind, water and workers. There are many steps, in addition to some already mentioned, which can be taken to reduce the impact of this disease: Crop rotation every three years, plant as far away as possible from potatoes and eggplant, select tolerant varieties, use trellises where possible, prune to allow for good air circulation and control weeds. When attempting to diagnose many tomato diseases it may be necessary to enlist the services of an expert to make the treatment plan as effective as possible. Among the other diseases and pests in tomatoes are Septoria leaf spot, Anthracnose ripe rot, bacterial canker, bacterial speck/spot, verticillium wilt and the Colorado potato beetle.
Late blight is arguably the most destructive disease to infect a tomato plant. The fungus can survive the winter in a living host such as potatoes and the spores can carry in the wind for up to 40 miles. Its development is favored by cool, wet weather with optimal temperatures of 54-75 F. Control measures include separating plantings from potato fields and crop rotation. Trickle irrigation is advised and the use of resistant varieties is recommended. If an infestation is suspected it is critical to alert other growers of its presence. Specimens should be submitted to the Diagnostic Laboratory immediately for positive confirmation of the disease. The Extension Service has a system in place which makes everyone concerned aware of its presence and the measures to take to minimize damage.
A highlight of the summer season is the arrival of the first native sweet corn. This crop has a list of pests which can serve to make the growers life miserable if not managed aggressively. The three major pests which view sweetcorn are the European corn borer, the Fall armyworm and the Corn earworm. Scouting is essential to determine the level of infestation in any given cornfield and once determined, the information should be recorded. Another fungal disease, northern corn leaf blight, kills the entire plant if left untreated. Management measures include chopping or burying crop residues, crop rotation (especially where reduced till is practiced), good air drainage and proper timing of fungicides.
To the untrained eye it would seem onions would have few natural pests. Onion thrips prefer to feed at the base of the plant using both piercing and sucking mouth parts giving rise to a condition called “white blast” or “silver top”. Farmers should avoid planting onions, cauliflower or cabbage downwind of small grains or alfalfa. Red onions seem to be most susceptible, yellow intermediate and white most resistant. The onion maggot is an insect whose larvae feed on internal plant tissue. A parasitic wasp is useful in developing a control program. Other diseases which affect onions include stem and bulb nematode and Allium leafminer.
Basil is an increasingly important crop and one of its important pests is the Japanese beetle. A point emphasized was to no longer use traps to capture the beetles as they tend to attract beetles from far greater distances than the home plantings, thereby compounding the problem. Downy mildew is a major problem with Basil and some of the suggestions already made will help in control such as plant spacing, good air circulation and trickle irrigation.
Beets, swiss chard and spinach all belong to the same family of plants, the Chenopodiaceae, and share some of the same problems. Cercospora leaf spot can be identified by light centers and dark borders. Its development is favored by high temperatures and high moisture so controlling leaf moisture is very important. Flea beetles chew tiny holes in the foliage rendering them unattractive to the consumers’ eye. Wood ash applied to the soil is helpful in control as are row covers. It was emphasized that when using row covers, the edges of the covers should be tucked in to make entry by insects on the outside difficult. The adult Leafminer is a fly which lays its eggs on the underside of leaves. When the eggs hatch, the larvae emerge and feeds on the leaf structure between the two layers of epidermis creating a mine. There is a wasp parasitoid which is useful in control.
Beans and peas are in the legume family and the insect which comes immediately to mind is the Mexican bean beetle, the control of which can be helped by crop rotation, control and disposal of crop residues and the introduction of a specific parasitoid wasp. Other problem pests affecting legumes include the potato leafhopper, cutworms, seed corn maggot, and at least three caterpillars which also infest other crops already mentioned. One bug which requires special recognition is the Brown Marmorated Stink Bug which has a voracious appetite and is not selective in the plants it chooses to attack. Other conditions commonly observed include anthracnose, White Mold and bean rust. Damping off of emerging seedling can be caused by a number of different fungi and water molds but a constant seems to be high soil moisture.
Lettuce is not without its problems. One of which is aphids (usually one of four varieties) — lettuce, potato, green peach and lettuce root. Biocontrol, insecticidal soaps and conventional insecticides may help in control. Slugs and White Mold are other possible pests of lettuce. Carrots are affected by Alternaria leaf spot, Alternaria blight, carrot weevil, tarnished plant bug and the parsley worm.
As an aid in digesting all of the material the reader should go online to to reference a most valuable resource which has excellent photos of many plant pests and more information regarding control measures.