Biological pressures and chemical resistance

Since the first refrigerated railcars rolled north to Chicago from southern Illinois, the region has been a significant fruit producer. Newer distribution channels allowing shipment from more distant locations have relegated the region to a more local market, with Chicago markets now primarily served by larger producing areas. Imported fruits have taken an additional market share.

Sooty blotch and flyspeck are summer diseases that can be controlled by timely chemical spraying.
Photos courtesy of Elizabeth Wahle, University of Illinois, unless otherwise noted.

“It’s hard to compete with the big growing areas such as Washington state and Michigan,” said Jeff Flamm of Flamm Orchards, Cobden, Ill. “Our fruit hangs on the tree longer,” Flamm said, noting the primarily local market areas. Longer time on the tree increases the sugar content and improves the taste of the fruit. Established in 1888, the orchard produces mostly Red Delicious, Golden Delicious and Jonathan apples, along with peaches. The orchard is now in the hands of the fifth generation of Flamms, following the pattern of many family-owned orchards. A number of challenges exist to maintaining healthy orchards in this region, many of which are felt by growers across the country.

Brad Taylor, associate professor in soil and water, Southern Illinois University at Carbondale, noted that the industry is changing: “With more fruit and vegetable servings being promoted, the region is poised to serve the interest in local produce as it is close to its consumer base.” Taylor noted that a resurgence of biological pressures exists.

While family ownership of many orchards has remained stable, changes have occurred over the years in threats to the orchards, and changes in the mode of action of chemicals have changed the way that orchard managers approach controlling those threats. Carefully timed chemical applications can be managed by accessing available weather data or by using leaf wetness monitoring devices. Gaining an understanding of chemical resistance is essential to effective use of chemicals.

Threats to orchards

A number of threats to orchards have emerged, and university and private company specialists throughout the industry have worked to find ways to prevent or mitigate the effects of these threats. Mohammad Babadoost, University of Illinois associate professor of plant sciences, cited several threats to healthy orchards, including fire blight, apple scab and summer diseases of sooty blotch, flyspeck and fruit rot. “Warm, moist conditions encourage disease,” Babadoost said.

“We have a wide range of temperatures with rainfall and high humidity,” Flamm noted. Similar conditions of heat and humidity are found through much of the Midwest, contributing to disease pressures, which are more prevalent in apples. While many varieties of apples are susceptible to fire blight, resistant varieties have been developed. Starting over with fire blight-resistant varieties isn’t easy for established orchards, and Flamm noted that the Jonathan apples are susceptible to the disease. “We try to prevent the disease by using only certified varieties and by using chemicals,” Flamm said. The apple scab fungus can overwinter in leaves, and ascospores produced within the leaves cause the infection. Flamm noted, “We use dormant oil as a barrier before any green is showing. The dormant oil suffocates it.”

Fire blight is a serious threat to some apple varieties.

Babadoost pointed to summer diseases, fruit rot, sooty blotch and flyspeck, as major concerns for growers and noting that chemical application timing is extremely important to control them. Members of the juniper family are hosts for cedar-apple rust, a fungal disease. Eastern red cedar and southern red cedar trees are common hosts. The codling moth is among the most troublesome pests as it bores into the apples, and Japanese beetles have increased in recent years.

Flamm noted that timely applications of fungicides are essential in controlling diseases and the codling moth. “We look at rain, temperature and the degree days,” Flamm said.

Managing water in orchards is an important element in preventing diseases, according to Babadoost. “Drainage is very important to move the water away quickly,” Babadoost said. He noted that new orchards should be placed on slopes in light soil, avoiding the heavy soils often found in the upper Midwest. “Drain tiles can be installed to help with drainage in existing orchards,” he said.

With timely chemical applications essential, collection of weather data plays a major role in identifying the appropriate time to apply. In addition to timely chemical applications, pruning plays a major role. Fire blight in particular is spread by diseased trees, and removal of diseased limbs is necessary. Flamm noted that while pruning is essential to keep orchards healthy, pruning too heavily could make trees more susceptible to disease.

“Pruning works like fertilizer promoting new growth,” he said. He noted that new wounds created by pruning could increase susceptibility of trees to disease.

Elizabeth Wahle gathers data during a leaf wetness monitoring device study.

Many of the tasks associated with maintaining a healthy orchard are learned over time. “I learned from my father,” Flamm said. “I’ve modified some things, and I talk to the university researchers. We’ve lost some tools when we lost some chemicals, although new ones are coming out. We’re seeing some resistant strains in the codling moth.”

Understanding resistance

Elizabeth Wahle, University of Illinois Extension, is a fruit and vegetable specialist who works with growers in the southern half of the state. Wahle said, “Some growers are using leaf wetness monitoring equipment, and others access weather data from various sources. Some rely on other information, such as our extension newsletters.” A 2008 study conducted by Babadoost and Wahle on eight southern Illinois orchards found that using a weather-based disease warning system was effective in controlling summer disease. A Watchdog Wetness/Temperature sensor manufactured by Spectrum Technologies, Inc., Plainfield, Ill., was used in the study.

Wahle said, “For many years, growers used chemicals that worked very well. EPA restricted the use of many of these chemicals when they were found to be harmful to humans.” Some of the new chemicals have different modes of action. For example, some older chemicals had multiple modes of action, while some newer chemicals have a single mode of action. Resistance is more likely to develop to chemicals with a single mode of action.

Codling moth is one of the most troublesome pests that attacks apples.

Understanding the mode of action helps growers select the chemical that will be most effective based on the number of sprayings and the likelihood of resistance developing to the chemical.

Fungicides, insecticides and herbicides are evaluated by action committees made up of technical representatives to determine the likelihood of resistance developing to that chemical. The committees identify resistance, both existing and potential, collect and distribute information, provide guidelines on use and recommend studies. Designated committees include the Fungicide Resistance Action Committee (FRAC), Insecticide Resistance Action Committee (IRAC) and Herbicide Resistance Action Committee (HRAC). The committees designate group numbers to indicate the likelihood that resistance will develop.

This understanding is important because resistance develops based on the mode of action rather than a particular product. “If we lose one product with a particular mode of action, we lose all with that mode of action,” Wahle said. She noted that the cost of bringing new chemicals to market is extremely high, and that companies do not want to lose any to resistance development. Growers are encouraged to rotate use of chemicals, especially among the groups to which resistance is more likely to develop. It is important to understand that rotating chemicals, with different modes of action is important to prevent the development of resistance.

Flamm Orchards uses carefully timed chemical and dormant oil applications to help prevent diseases.
Photos courtesy of Flamm Orchard.
Jeff Flamm inspects apple trees.

“Growers have gained a good knowledge and understanding of potential resistance to these chemicals,” Wahle said. “Many growers have added acronyms such as FRAC, IRAC and HRAC to their vocabularies to discuss potential resistance development.”

Nancy Riggs is a freelance writer who resides in Mount Zion, Ill.