Aug 26, 2020

Late August challenges for vegetable producers in Pennsylvania

During the summer, Penn State Extension’s network of educators and specialists are on the road visiting and consulting with farmers throughout the state.

Currently, we are entering the peak harvest season and our rainfall amounts have been varying from farm to farm. As precipitation levels vary, so does the severity and appearance of key pests and diseases on Pennsylvania produce farms.

Several educators are seeing an upswing in the number of corn earworm moths that are being caught in the traps that they have placed on cooperator farms. While these trap counts can provide growers with the information they need to determine what the appropriate spray interval should be, Penn State Extension educators typically defer the selection of a specific insecticide for sweet corn pest management to the grower. Unfortunately, most growers’ first choice in protecting their sweet corn crops are synthetic pyrethroid insecticides. Synthetic pyrethroid insecticides are economical and will target worm pests and sap beetles effectively, but pesticide resistance is being observed in some corn earworm populations across the United States. A greater and less obvious concern for growers may be the impact that synthetic pyrethroid insecticides pose on pollinators like bees.

Pollinator protection in sweet corn

When I visit Amish and Mennonite farms early in the morning, freshly tasseling sweet cornfields are just buzzing with pollinator activity. On a recent visit to a field in Sinking Valley, you could observe as many as 3–5 honeybees working each tassel. This same field also had rising trap catches of corn earworm and fall armyworm and the grower needed to initiate a more aggressive spray program to prevent worm pests in his sweet corn.

When selecting an insecticide, the grower is faced with a dilemma regarding insecticide choice. If the grower elects to use a synthetic pyrethroid insecticide, he or she could kill-off their bees which may compromise the pollination needs of their other crops. If the grower elects to use an insecticide with a lower bee toxicity rating like Coragen, Radiant, Blackhawk, or Entrust, they could see their profit margins shrink a bit due to the higher cost of these insecticides versus synthetic pyrethroid insecticides. They may also see a greater risk of sap beetle and brown marmorated stinkbug injury on their sweetcorn ears. While there are no easy solutions for growers, a switch to a boom sprayer from an air blast sprayer may reduce the amount of insecticide contacting the pollen shedding tassels. Ultimately, the best solution to protect bees and other pollinators in tasseling sweetcorn fields may lie in the selection of an insecticide that has lower toxicity to bees.

Bacterial canker in greenhouse and high tunnel tomatoes

Bacterial canker caused by the pathogen Clavibacter michiganensis subsp michiganensis is one of the most devastating diseases that can impact tomato plants in the field, high tunnel, or greenhouse. Inoculum for bacterial canker can emanate from infected plant debris, infected weeds, infected tomato transplants, contaminated tomato stakes, and infected seeds. Once the pathogen is introduced into a production area it can easily be spread by workers.

Infected seed is frequently the initial source of infection in tomato hydroponic operations. According to researchers, a 1% seed transmission rate can quickly lead to a 100% infection rate in a matter of weeks. In tomato greenhouse production operations, workers routinely prune, sucker, thin fruit clusters, and lower plants. In some cases, infected plants may not appear symptomatic at first which could lead to the rapid spread of this pathogen through the entire production area.

Since infected seed is the leading cause for infection, growers should consider disinfecting the seed using sodium hypochlorite (chlorine bleach) or by using a hot water seed treatment . Hot water seed treatments to limit pathogen spread and infection in tomatoes require a grower to bring a water bath temperature up to 100°F for ten minutes (pre-warming). Next, the tomato seed is placed in a cloth bag or wrapped in cheesecloth. The bagged seed will then be immersed in the hot water bath at 122°F for exactly 25 minutes. Upon the seed’s removal from the hot water bath, place the still-bagged seed into a container of cold water to stop the heating process. Once the seed has cooled it should be spread in a single layer on a screen to dry. Once dried the seed should be dusted with Thiram 75 WP (1 tsp per 1 pound of seed). The seed should be sown shortly after undergoing the hot water seed treatment. *Hot water seed treatments can impact seed viability.

Fixed copper fungicides, which are very effective in preventing many fungal diseases, can be used to provide some level of protection against bacterial canker, but fixed copper fungicides will not “cure” infected plants. If bacterial canker is confirmed in your tomato greenhouse or high tunnel remove symptomatic plants as soon as possible. I would consider placing all symptomatic plants in large 55-gallon poly bags prior to transporting them from the production area to your trash dumpster to limit the spread of this pathogen in the greenhouse or high tunnel. After disposing of the infected plants, the grower should wash their hands thoroughly before handling any other plants or before entering adjacent fields.

At the conclusion of the production year, it is imperative that all plant debris and potting media be removed from the greenhouse floor to prevent infecting next year’s tomato crop. After the tomato debris and media have been removed from the floor all trellises, clips, ground cloth, etc., should be sanitized with an appropriately labeled greenhouse disinfectant.

Recently, several growers have tried to reduce their input costs by re-using poly bags, coir, and perlite to grow tomatoes. While there are some growers that can provide anecdotal testimony that this practice worked fine, there are countless other growers that have lost their entire crop to bacterial canker as their first fruits ripened. Quality seed, hot water seed treatments, and good sanitation practiced in the greenhouse or high tunnel are the best means to prevent bacterial canker from destroying your crop.

Tomato pith necrosis

A grower was seeing blotches on his tomato leaves and called Extension because he was concerned it may have been late blight. While the leaf lesions looked like late blight, they did not have the late blight pale hallow or the typical sporulation on the underside of the leaf. A sample of the infected plants was delivered to the Penn State Plant Disease Clinic. The clinic determined the problem was the bacterial disease tomato pith necrosis. In the photo below, the stem behind the leaf has collapsed. When the stem interior was examined, it had a ladder appearance to the discolored pith tissue. Tomato pith necrosis is favored by high nitrogen levels in the plant. The bacteria survive in the soil and may also be seed-borne.

The stem behind the leaf has collapsed. Photo: John Esslinger/Penn State

Bacterial pith necrosis (Pseudomonas corrugata) close up of a cross-section through a diseased tomato stem. Note the “ladder-like” appearance of the discolored pith. Photo: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

Two-spotted spider mites

Two-spotted spider mites have been a problem in the dryer areas of the state. Spider mites will feed on a wide range of fruit and vegetable plants. Watermelon seems to be one of its favorite hosts and many infestations start in the melons then move out from there. You will probably need a hand lens (magnifying glass) to see them. Look on the undersides of the older leaves. They will appear as a small speck that is tan in color. If you run your finger across the speck and it leaves a smear, you know you have mites. With a lens, you can see them slowly move. The foliage will get a pale color as the mite population builds. See the pictures below of the top and bottom of a mite-infested leaf. Be aware that mites are not in the insect family. Mites are in the spider family. Many of the common insecticides do not control mites. Pyrethroids can make an infestation worse by taking out beneficial insects that are feeding on the mites. You will need to apply an acaricide to control mites. Some farms have a mite problem, and some do not. You can often avoid mite problems by not overusing pyrethroids early in the season.

Top and bottom of a mite-infested leaf. Photo: John Esslinger, Penn State

– John Esslinger and Thomas Ford, Penn State University

Photo at top: Honeybee on corn tassels. Note the bright yellow pollen “saddlebags” on the bee. Photo: Tom Ford/Penn State