Insect Resistance, Late Blight Have Potato Growers Concerned

7/19/2014 7:00 AM
By Ann Wilmer Delmarva Correspondent

EAST NEW MARKET, Md. — Potato farmers from Delaware, Maryland and Virginia braved rain to gather in a potato field near East New Market for a July 10 workshop presented by University of Maryland Extension.

Test plantings of new potato genotypes of table stock and processing tubers that do well in other areas are being tested locally. One section of the test planting has been set aside to demonstrate the benefits of supplemental calcium applications. These plots, planted in May, are not yet ready for harvest, but organizers pulled plants for workshop participants to examine.

There were also discussions on insect management, targeted calcium nutrition, diseases that affect potatoes, nutrient management and weed control.

Virginia Tech professor Tom Kuhar kicked off the session with an update on Colorado potato beetles that overwintered in the soil and became active again in May. He said that severe winters usually knock back the population, but despite a particularly cold winter, there was no appreciable population decline of the insect this season.

“Without control, you can lose 30 percent of your production,” he told the group of farmers. “Any control measure is only good for the life cycle of the beetles.”

Insecticide resistance is a growing problem that can be exacerbated by repeated use of a single class of insecticide. Kuhar urged farmers to use different classes of chemistry at planting and at other times, too. He also said that 1 or 2 inches of straw mulch is a good beetle deterrent and feasible in a small patch.

Kuhar supervises field trials of various insect control mechanisms on farms using traditional cultivation as well as organic farming. For results of the trials, contact Kuhar at tkuhar<\@>

Another insect that bores holes in tubers is wire worms. Kuhar said that the best solution is crop rotation, and he recommended wild mustard as a good crop to plant and then plow under before planting potatoes again. The mustard contains isothiomate, which has biofumigate properties.

Assuming insects can be deterred, research has shown that supplemental calcium can be applied to improve tuber quality. Professor Jiwan Palta of the University of Wisconsin is a pioneer in the research into the effect of calcium in improving potato production.

He explained that calcium phosphate is the glue that holds human bodies together, while other forms of calcium are the glue that hold other biological forms together. Calcium is all around us, meaning that all life forms developed and evolved in the presence of calcium.

Palta has found that potatoes deficient in calcium do not store well.

“Jack up the calcium and bacteria won’t touch it,” he said.

Calcium is abundant, he said.

“Even sandy soil has roughly 1,000 pounds of calcium per acre,” he said. But not all of this calcium is water-soluble and calcium moves with water into the plant.

Gypsum and lime are cheap sources of calcium, but dry calcium doesn’t satisfy the requirements of potatoes. In addition to the roots of the plant, there are feeder roots that come out of the eye of the tuber, and this is how tubers get both their water and calcium. This is how liquid fertilizer came to be developed.

Because the leaves capture traditional applications of calcium and the tubers cannot compete with the plant, supplemental applications are needed during the time that the tubers are bulking up. This is from the time the plants are hilled until harvest. So some method of directing the calcium to the tubers has to be used.

Palta recommended applying 50-150 pounds of calcium per acre at hilling and at three-week intervals until harvest. He said the results are dramatic even with regard to internal quality deficiencies that are largely eliminated by the addition of calcium.

Professor Kate Everts, a plant pathologist at the University of Maryland, discussed blight, scab and other diseases affecting potatoes.

“Bacterial issues have increased this year,” she said, “and the best way to prevent bacterial problems is to create an environment that bacteria don’t like.”

She said that calcium also helps to reduce bacterial disease.

Potatoes are susceptible to pectobacterium and dickeya, both formerly classified as erwinia. Bacteria, as a pathogen, cannot invade the plant on their own; it relies on a crack in the skin of the tuber to invade. Potato diseases include blackleg, aerial stem rot and tuber rot.

Blackleg always develops from a seed piece, so planting bacteria-free seed potatoes is the only way to prevent this problem. Aerial stem rot develops from a wound in the stem. Blackleg and aerial stem rot are caused by different bacteria; tuber rot by several strains of bacteria.

Fungi, on the other hand, produce a strong enough structure to invade plants. Late blight, a serious problem for potato and tomato farmers, lands on the upper leaves of the plant. If the plant is wet, it will stick; otherwise, it might be moved by the wind. Blight needs 100-percent humidity to thrive. If it reaches the lower part of the plant, the canopy will offer a very hospitable environment.

This affects how pesticide may be applied. It must be mixed with water and can be sprayed from the air or applied through ground irrigation. Aerial application is the most economical in terms of the amount of pesticide used, and rain following an aerial application would be helpful. Everts said that it’s important to get a contact product out at least two hours before it rains.

She also warned that late blight has already appeared in Cambria, Chester and Lancaster counties in Pennsylvania, and is almost certain to show up in parts of Delaware and Maryland. The genotype of these isolates is US23, which is sensitive to menefoxam. Everts said producers should be aware of late blight and should have suspected plants analyzed by contacting the local Extension office. Some types of blight are consistently resistant to fungicides, she said, and these can spread throughout fields and to neighboring farms.

Professor Mark Reiter, a nutrient and soil management specialist with Virginia Tech, said that nitrogen is the hardest nutrient to manage.

“Timing is everything. The goal is to apply nitrogen when the plant needs it. The closer to plant uptake you can get, the higher the efficiency with which the fertilizer is used, meaning higher yields and higher profits, as well as less nitrogen lost to the environment,” Reiter said.

Slow-release fertilizers can help farmers match plant nitrogen uptake. These include low-solubility compounds, sulphur-coated and polymer-coated nitrogen granules. Each requires moisture to activate the release of nitrogen. Water from rain or irrigation leeches nitrogen out of the soil or releases the nitrogen in slow-release fertilizer

Additives can also include nitrification inhibitors that reduce the rate of conversion of ammonium to nitrate.

Reiter also demonstrated how to measure petiole sap concentrations using a hand-held meter that he said costs about $100. Petiole nitrate readings, best sampled between 10 a.m.-2 p.m. when the plant is actively growing and not drought stressed, can help to determine when additional nitrogen is needed.

When it comes to weeds, conventional farmers have to do a preapplication and post-application of herbicide, according to Sudeep Mathew, University of Maryland Extension agent. Four weeds that have the potential to seriously reduce potato yields include pigweed and lambs quarter, which are herbicide resistant, and morning glory and ragweed.

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