Like insects, diseases develop in predictable ways based on growing-degree accumulations, but diseases are driven more by moisture than are insects, which makes management more complicated. For both insects and diseases, predictive models are used, but insect models are more precise than disease models.
The models used by New York growers are built into the http://newa.cornell.edu Web site to help growers with disease management. The NEWA network, developed by Cornell University, is being proposed as a national model that could be used by growers everywhere.
Cornell University pathologist Dr. Kerik Cox says it requires time and effort to use models intelligently, and it can be frustrating otherwise.
For growers now, he said, “It is easier, cheaper, and safer to be imprecise for numerous diseases.”
By that he means growers still rely on preventive and protective methods that impede the development of disease organisms. With diseases, growers don’t wait until they have infections and then try to cure them.
Eastern growers have fewer diseases than insects to worry about. Basically, Cox said, they fight apple scab, fireblight, powdery mildew, and the summer diseases flyspeck and sooty blotch.
While fewer, diseases can be more devastating. Rarely do insects completely strip a tree of foliage, as apple scab might, or kill a tree to the root, as fireblight can.
On the other hand, trees can be bred or genetically engineered for resistance to scab or fireblight, something not ordinarily thought of as a control method for insects like codling moth or plum curculio.
Cox’s first choice for disease control is variety selection: Avoid varieties like McIntosh that are highly susceptible to apple scab, choose fireblight resistant rootstocks, and avoid mildew-susceptible varieties. But that’s easier said than done.
The Liberty apple, for example, has broad resistance to scab, fireblight, cedar apple rust, and powdery mildew. It can be kept disease-free with at most one spray per year of a fungicide like captan, Cox said. But consumers won’t choose Liberty apples merely because they don’t need to be sprayed with fungicides.
On the other hand, existing varieties could be improved by genetic engineering, adding disease-resistance genes. “It would alleviate considerable disease management costs,” Cox said, “but (such varieties) are expensive to develop, deregulate, and convince consumers of their merit.”
So the standard approach to control of a disease like apple scab control is to reduce inoculum levels in the orchard by applying urea to and/or shredding fallen leaves, applying copper when trees are still dormant, then using captan, mancozeb, and sulfur to keep new tissue covered during the primary scab infection period.
After that, sprays may be needed and materials should be chosen to which the scab organism has not developed resistance, and used in ways that don’t encourage resistance.
For fireblight, growers apply streptomycin during bloom if conditions are conducive to infection. Streptomycin is also the tool of choice for “trauma blight,” where trees are injured by high winds or hail. Sprays of Apogee (prohexadione calcium) are used to shorten shoot growth and prevent shoot infection.
Models on the NEWA network are used to predict scab infection periods and to help growers precisely time fungicide applications.