Newsletter

Vegetables

Vegetable Insects - Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Cabbage.

Imported cabbageworm and diamondback moths have been observed laying eggs in cabbage, so larvae should be detected this week. Once DBM eggs hatch, young larvae will first mine between the upper and lower leaf surfaces before moving to the heart of the plants. Treatments should be applied when 5% of the plants are infested with larvae and before larvae move to the heart of the plants. Avaunt, Bt insecticides, Proclaim, or Spintor will provide effective control of both species. Be sure to rotate between these classes of insecticides to avoid the development of resistance.

Peas.

Be sure to watch for pea aphids in your earliest plantings of peas. The recent weather has been favorable for aphid development. As the weather fluctuates between the current hot temperatures and anticipated cool weather next week, aphid populations often explode and beneficial insect activity can lag behind. On small plants, you should sample for aphids by counting the number of aphids on 10 plants in 10 locations throughout a field. On larger plants, take 10 sweeps in 10 locations. A treatment is recommended if you find 5-10 aphids per plant or 50 or more aphids per sweep. Dimethoate or Lannate will provide aphid control. Be sure to check the labels for application restrictions during bloom.

Sweet Corn.

In addition to cutworms, be sure to watch for flea beetles on your earliest planted corn. A cutworm treatment should be applied if you find 10% leaf feeding or 3% cut plants in one-two leaf stage corn. A pyrethroid will provide effective control. Fields should be treated early in the morning or in the early evening when cutworms are close to the soil surface to achieve the best control. In order to get an accurate estimate of flea beetle populations; fields should be scouted mid-day when beetles are active. A treatment will be needed if 5% of the plants are infested with beetles. A pyrethroid or Sevin will provide control.

Potatoes.

Since we currently have a number of insecticides labeled on potatoes (Actara, Fulfill and Provado) that move into the leaf at application (translaminar), it is important to consider which fungicides are used in combination with these insecticides. The most effective control will be achieved when translaminar insecticides are able to penetrate the leaf surface. Therefore, these materials should not be combined with "sticky fungicides" such as Bravo Weather-Stik or spreader-stickers that might prevent movement into the leaf. If you plan to use a translaminar insecticide in combination with a fungicide, then select a fungicide that does not contain a spreader-sticker like Dithane, Polyram, Quadris, or Bravo Ultrex.

Vegetable Diseases - Kate Everts, Extension Vegetable Pathologist, University of Delaware and University of Maryland; everts@udel.edu

Watermelon Seedling Diseases in the Greenhouse

Many abiotic problems (scorch from high temperatures, excess nutrients and chemical burn) may cause spots on the leaves of plants in the greenhouse. There are also several seedborne watermelon diseases that may show up on seedlings in the greenhouse. Gummy stem blight (GSB) is the most common, but Alternaria leaf blight and anthracnose also affect watermelons. There are several greenhouse practices that minimize infection by the pathogens of these diseases, including GSB. The greenhouse should be disinfected before planting (benches, walls, walkways, etc.). The seed source should have tested negative for the pathogen with a minimum assay number of 1,000 seeds. Use clean transplant trays (disinfect trays if they will be reused) and new soil. Destroy any volunteer seedlings and keep the area in and around the greenhouse weed free. Avoid overhead watering if at all possible, or water in the middle of the day so that the plants dry thoroughly before evening. Keep relative humidity as low as possible through proper watering and good air circulation in the greenhouse.

As the seedlings develop, inspect them carefully. Infected seedlings will have small brown lesions on the leaves and water-soaked lesions on the stem. Initial infections will occur as ‘foci’ or clusters of diseased plants.

If the seedlings have lesions or appear diseased, destroy the flats where any seedlings show symptoms. Remove adjoining flats to a separate area for observation. Monitor these seedlings daily and destroy those that develop symptoms. Do not ship any trays containing plants with symptoms of GSB. Spray with a labeled fungicide when symptoms are observed and continue until plants are shipped.

Bacterial fruit blotch (BFB) of watermelon is caused by a bacterium that may also be seedborne. Initial symptoms of BFB are water-soaked areas on the lower surface of the cotyledons. Lesions turn necrotic often with yellow halos, are frequently deliminated by veins and subsequently the seedlings collapse and die.

There are only a few fungicides that are specifically labeled for greenhouse use on vegetable transplants. Dithane F-45 and some copper formulations are labeled. Do not use Quadris or Bravo; use in the greenhouse is prohibited for these fungicides.

Pickling Cucumber Rotations - Ed Kee, Extension Vegetable Crops Specialist; kee@udel.edu

Pickle growers are reminded to rotate as best as possible. This is especially helpful in avoiding phytophthora outbreaks. Phythphthora overwinters and is present at various levels throughout the year. A three year rotation away from cucurbit crops, peppers, tomatoes and potatoes is helpful.

Vegetable Diseases - Bob Mulrooney, Extension Vegetable Pathologist, University of Delaware bobmul@udel.edu

Potatoes.

Foliar fungicide programs for potatoes. Fortunately, we had another year free of late blight and the prospect of late blight occurring in Delaware is very low. The only source of late blight we have, as best I can tell, is infected seed potatoes. Seed growing areas have been free of late blight or have controlled it to prevent tuber infection. Without the threat of late blight what other foliar diseases or diseases that can be controlled with foliar applied fungicides should we be concerned about? Early blight is a disease that we see late in the season on our early maturing varieties that rarely causes problems except on a few susceptible varieties, such as BelRus, Norkotah Russet, and Snowden. Fungicide applications for early blight can be delayed until the later part of the season to prevent premature defoliation on susceptible varieties. The EDBC fungicides (Dithane, Polyram, Pencozeb, Manzate), chlorothalonil (Bravo, Equus, Echo) and Quadris all provide good control of early blight here. Pink rot is a soilborne disease that we have not seen much recently, but some growers have had problems with some fields or low areas in fields over the years. Two applications of Ridomil Gold MZ, Ridomil Gold/Copper, or Flouranil are recommended when tubers are nickel-sized and two weeks later if you have a history of pink rot. Now growers have the option of applying Ridomil Gold or UltraFlourish in the row at planting as well. Recent research has shown that strobilurin fungicides such as Quadris are also effective in controlling a disease called black dot. This disease is present in every potato field that I have ever surveyed, but it is not known to reduce yields in this area. The fungus is soilborne in old potato debris and seedborne as very small sclerotia. Symptoms appear late in the season on stressed potatoes (heat, poorly drained soil, low nitrogen), but could be confused with early dying because the fungus does some of the same things that Verticillium does to potatoes. Longer season varieties are also more prone to increased levels of black dot. The yield response that has been seen ocassionly when Quadris has been applied to potatoes in the absence of early blight (which it controls extremely well) has been attributed to black dot control. For control of early blight and black dot during the period of rapid growth consider 2-3 applications of Quadris beginning at bloom alternated with Dithane or Bravo every 7 days if black dot is suspected. Those are the primary potato diseases growers need to know and decide what type of fungicide program they need for control. Weather patterns, cropping history and diseases that you have seen on your farm can help you decide what you need. I would recommend several early season applications of either Dithane or Bravo. Once potato rows touch it is difficult to get fungicides to the lower leaves and older tissue is most likely to get infected first with late blight or early blight so at least one application should be made prior to row closure for insurance. None of these foliar applications will control Rhizoctonia. Our use of mostly early maturing varieties, lack of significant levels of early blight, and the absence of late blight is a good reason to re-evaluate fungicide use on potatoes and use only what is needed to produce an excellent crop profitably.

Field Crops

Field Crop Insects - Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Field Corn.

Black cutworm moth activity reached peak levels in pheromone traps in the Bridgeville, Dagsboro, Little Creek and Wyoming areas at the end of last week (April 12). If you look at accumulated degree-days since peak activity, we should reach 200 DD base 50 by the end of this week (April 19). Although we can see leaf feeding earlier, we generally do not see cut plants from black cutworms until we reach 300 DD from peak catches. Often, early cutting occurs from variegated cutworm that we can find while sampling for grubs. Since the weather forecast calls for cooler weather next week, we should not see cutting activity until the end of April or first of May. However if the weather remains warm, we could see cutting activity on early-planted corn by next week. A treatment should be considered in 1-2 leaf stage corn if you find 3 % cut plants or 10% leaf feeding. Pyrethroids provide the most cost-effective control. For the most recent pheromone trap catches, see trap catch table on page 5 of the newsletter or check our website at http://www.udel.edu/IPM/traps/currentbcwtrap.html.

Small Grains.

We can now find cereal leaf beetle larvae in barley and wheat in Kent and Sussex Counties. If you are growing high management wheat, a treatment should be considered if you find 25 eggs and/or larvae per 100 tillers and at least 50% of the eggs have hatched. If fields are scouted on a routine basis, a threshold of 0.5 larvae per stem (flag leaf and next 2 stem leaves) and 10% defoliation can also be used. Lannate, Mustang or Warrior will provide effective control and will also provide control of any worms that have hatched. Lannate is labeled on both crops. Mustang and Warrior are only labeled on wheat. Sevin will also provide control; however, it can only be applied to wheat and we have seen aphid outbreaks after Sevin use. Furadan will also provide control, but it can not be applied after heads emerge from the boot. True armyworm moth catches have increased significantly in the Harrington, Greenwood and Rising Sun areas. Trap catches totaling 200 or more moths for the month of April generally indicate a potential for outbreaks. You should begin scouting for armyworms by the end of next week or as soon as the heads have emerged. Examine 5 linear foot of row in 10 locations for armyworm larvae. Armyworms are generally nocturnal, so carefully check for larvae at the base of plants. Since they often cause more damage in barley, the treatment threshold is one per foot of row. In wheat, the treatment threshold is 2 per foot of row. You should also start looking for sawfly larvae during the next week. Since adults often lay their eggs on field edges and in lodged areas, you should check these areas first for sawfly larvae. A sweep net can be used to detect the first larvae. Once larvae are detected, sample 5 foot of row innerspace in 5-10 locations in a field to make a treatment decision. You will need to shake the plants to dislodge larvae that feed on the plants during the day. No treatment will be needed until you find 2 larvae per 5 foot of row innerspace or 0.4 larvae per foot of row.

In the last newsletter, I included an article written by Galen Dively regarding winter grain mites in orchardgrass. I have received a few calls about this mite in wheat and if you look closely you will find them in most wheat fields. At this point, it appears that the warm weather should make the mites inactive; therefore, we do not anticipate the need to treat wheat. However, we will let you know if problems develop. Damage symptoms were observed in wheat in Virginia in January and February.

The following information, provided by Ames Herbert at VPI, provides additional information on winter grain mites and discusses what has been found so far in Virginia:

" An infestation of winter grain mite, Penthaleus major (Dugès), was first detected in wheat fields in Chesapeake and Virginia Beach during the months of January and February 2002. Several fields were infested and infestations appeared to be at least somewhat more common in fields that had been previously treated with sludge. Populations declined to unobservable levels in late February to early March, but re-emerged in early April – indicating a second generation. The early spring infestations were associated with dead patches where plants began dying in January. However, we are still not certain that plant death was the result of mite injury, alone, but was associated with additional plant stress caused by a high soil pH condition and manganese deficiency that is common in that area of the state. Discussions with mite experts and perusal of web information have not provided a clear understanding of whether mites are capable of killing large areas of wheat or orchardgrass, what the economic thresholds might be, or what control strategies or acaricides would be most effective.

Temperature and moisture are the most important factors influencing mite development and abundance. Cool, rather than warm temperatures, favor their development. Egg laying is heaviest between 50° and 60°F; the optimum conditions for hatching are between 44° and 55°F. When temperatures drop below or rise above these extremes, the mites stop feeding, descend to the ground, or burrow into the soil. Mite activity in the spring drops rapidly and the eggs fail to hatch when the daily temperature exceeds 75°F. Aestivating (oversummering resting stage) eggs do not hatch in the fall until rains provide adequate moisture. On hot, dry days it may be necessary to dig into the soil to a depth of four or five inches to find mites.

We are uncertain at this point about which products to recommend for chemical control of winter grain mite. The available recommendations from other states appear somewhat outdated. For example, although Di-Syston 8E is listed, it would not be effective. Dimethoate is labeled, but may not provide good control under cooler temperatures. Both ethyl and methyl parathion are listed, but these are restricted use products that would require careful use. We will investigate this further if mite problems persist. More information will be passed along as we learn more about this new and potentially damaging pest. "

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