Surprising impacts on pests and diseases

Doug Walsh (left) and associate scout for mealybug infestation.
Leafminer lays an egg on a leaf.
Mealybug infestation on grapevines.
Onion thrips are extremely small and easily dispersed in rainstorms.

Weather is always on the minds of growers and is usually cited as the number one challenge. Whether from changing weather patterns, weather cycles or routine weather events, changes in weather create new conditions. These conditions affect pest populations and diseases have impacts on fruits and vegetables across the country.

California leads the nation in production of many fruit and vegetable crops. While a number of weather variations occur, climate change that could produce a growing environment with higher carbon dioxide has received a great deal of attention and is addressed in California Agriculture 2009. According to Dr. John Trumble, University of California, Riverside, entomologist, studies in the 1980s suggested that larger, more drought-tolerant plants would produce better yields under elevated levels of carbon dioxide. However, Trumble noted that insects eat more when plants are grown in elevated levels of carbon dioxide, primarily because leaves contain less nitrogen, and more loss to insects can eliminate better yields.

According to entomologists, warmer temperatures benefit some insect species while reducing or eliminating other species.

Southeast tomato yellow leaf curl

The whitefly was identified in the United States in the 1980s as a primary vector for a devastating tomato disease, tomato yellow leaf curl virus (TYLCV). The U.S. tomato crop is a major income producer; any crop loss is significant and can represent millions of dollars. The disease has been well-established in Florida and recently identified in Georgia. Unseasonably cold winters bringing freezing temperatures to south Florida the past two winters have been helpful in reducing the populations and offering some relief to growers.

When the disease hits tomato fields, tomato leaves turn yellow and flowers drop off, preventing fruit production. While Florida typically has hot, wet summers, crops are not growing in the summer. Rains tend to stop in the spring, and whitefly populations increase at a time when crops are growing. While first identified as the sweet potato whitefly, biotype B, the culprit whitefly was later identified as the silverleaf whitefly.

Dr. David Schuster, University of Florida entomologist, said, “In southern Florida, the silverleaf whitefly is the key insect and tomato leaf curl is the key disease.”

While it is thought that the largest source is infected plants, studies are in progress that may indicate that weeds also serve as hosts.

Schuster noted that currently lower than normal populations of whiteflies are present. “We think that is due at least in part to the freezing temperatures of the last two years,” he said. “Apparently one effect of the cold temperatures is that these temperatures are also killing some of the wild hosts.”

Responses to the disease have included chemical control of whiteflies, disease-free transplants and disease-resistant varieties of tomatoes, as teams of specialists continue to work toward solutions.

Whitefly-transmitted cucurbit melon virus in the Southwest

Across the southern United States, into the desert Southwest, the same silverleaf whitefly is wreaking havoc on another important crop. Cucurbit yellow stunting disorder virus (CYSDV) is transmitted by the whitefly in much the same way the tomato virus is transmitted. Dr. Steven Castle, USDA-ARS research entomologist at Maricopa, Ariz., is among a large number of scientists looking for answers on the movement of this virus. “The silverleaf whitefly is very adapted to high temperatures such as those we have here in the desert,” Castle said. The silverleaf whitefly biotype B was identified in southern Arizona and California’s Imperial Valley in 2006, and it quickly displaced the original whitefly.

Dr. Bill Wintermantel, research plant pathologist with USDA-ARS in Salinas, Calif., said, “We know that weather plays a significant role in the prevalence of the virus. We need to know just how variable weather affects the whitefly population and the broader hosts.” While it was originally thought that melon plants were the only hosts for the virus, studies have indicated that other plants such as alfalfa may show no symptoms, but serve as hosts.

The melon virus is a major issue in Yuma County, Ariz. Dr. John Palumbo, University of Arizona entomologist, and Dr. Kurt Nolte, Yuma County Extension director, are working to help growers cope with this threat and to find answers for long-term solutions.

Nolte noted that newly emerged melon plants are particularly vulnerable. Palumbo is working with growers on chemical control, but there is a very short window in which to treat melon plants. “If melon vines can grow to a larger size, they can carry the crop to harvest,” Nolte said. If the newly emerged melon plants are infected by whiteflies, they are damaged and are not likely to survive and produce a crop.

Nolte is conducting studies to help identify cantaloupe varieties that are less susceptible to the virus. “We can’t say there are resistant varieties out there yet,” he said. “But we have two years of data to identify some varieties that are less susceptible.”

Regularly occurring conditions affect pest outbreaks

While Santa Ana conditions, usually associated with hot, dry winds, occur with regularity in California, the winds definitely affect the leafminers that attack celery fields in the state, Trumble noted. “If we have winds for four or five days and temperatures rise, populations explode,” he said. The leafminers can quickly devastate celery fields, as well as any other leafy vegetables.

Cucurbit yellow stunting disorder virus shows up in older foliage, first with yellowing at the crowns, then toward the branch terminals.

“Catalina eddy conditions in which counterclockwise winds flow out to the Pacific Ocean can reach into Sinaloa, Mexico, and sweep insects up into the San Diego area,” Trumble said. Corn earworm outbreaks have become major problems for Southern California strawberry growers. Specific tolerance standards exist for insects in strawberries to be processed. Both temperature and rainfall are indicated as possibly influencing the survival of the pest in its pupal stage. Severe outbreaks of the rare pest have occurred only following mild winters, and the timing of emergence suggests that wind-borne immigration of the insect has contributed to outbreaks.

The impact of cool temperatures

While Midwestern springs and summers are usually quite warm, two successive cool, moist springs have produced environmental conditions affecting pests that have both helped and hurt growers.

“Two springs in a row in the upper Midwest have produced below-average temperatures,” said Russell Groves, an entomologist at the University of Wisconsin, Madison. “These cool, moist springs have produced a few things. Some are positive and some are negative.”

On the positive side, growers have been able to see a savings in the cost of insecticides and in the labor required for applications. Onion thrips can completely destroy onion crops. “Thrips populations have been set back the last two seasons, so that’s one of the pluses,” Groves said. “These two springs have allowed our growers who are good about scouting to delay the beginning insecticide applications to midsummer. Some only just began in early July.”

Weather continues to affect thrips throughout the growing season. Groves noted that the thrips are incredibly small, and that big thunderstorms with heavy rains are a major factor in removing thrips. “Rain and wind in thunderstorms just knock them off,” Groves said.

At the opposite end of the spectrum, three types of maggots that overwinter usually emerge in early spring, around April when few vegetables are planted. The two successive cool, moist springs have led to their emergence at a more damaging time, around the first of June. Seed corn maggots, cabbage maggots and onion maggots are becoming a significant problem, emerging at a time when transplants are extremely vulnerable. They do not differ greatly in appearance from houseflies. Recently seeded plants or transplants are vulnerable.

A cantaloupe plant during testing of whitefly infecting plants.

“We tell growers to watch the heat units for fly-free times for planting. The Wisconsin Department of Agriculture keeps track of heat units and provides the information for growers,” he said. “We tell people to avoid incorporation of cover crops or organic matter, or that they must incorporate at least a week to 10 days prior to planting, and they need to plant into a warm seedbed.”

Whatever the cause of changes in weather conditions, these conditions can promote changes in pests. Groves, who came to Wisconsin from central California three years ago, cited the increasing prevalence of squash bugs as an example. “I’m told that squash bugs were once very intermittent in Wisconsin,” he said. “They have now become very prevalent.”

Cool summers require greater observation for symptoms

The state of Washington has significant wine grape production, about 33,000 acres, and unlike California’s wine grape country, Washington has cool summers. Grapevine leaf roll disease is considered to be the most economically destructive disease, and it has made its way into Washington. The primary vector for transmitting the complex virus is the mealybug. While no direct relationship between weather and the disease is established, the cool summers that promote wine-growing success make it more difficult to diagnose the disease and manage the mealybug.

“Symptoms require greater observation up here,” said Dr. Doug Walsh. With mealybug movement into the Northwest, Walsh and others are offering growers extensive information on prevention, diagnosis and management to help prevent spread of grapevine leaf roll disease.

Virus-tested cuttings are encouraged to help prevent the disease, and an emphasis on avoiding the disease is paramount for preventing economic loss.

Weather and growing

While growers continue to focus on how they can best work with weather to enhance yields, understanding just how weather encourages or discourages specific pests and diseases is essential. Entomologists emphasize that avoiding transporting untested plant material is crucial to limit introduction of new pests and new diseases.

Keeping abreast of what is happening in new pest infestations is critical for growers. Understanding the best ways to spot and control any new pests is essential, and diligent scouting remains a major focus.

Nancy Riggs is a freelance writer and frequent contributor to Growing. She resides in Mount Zion, Ill.