Managing Onion Diseases
"Know your enemy" is the key to increasing success with onion disease management
Onions are plagued by a number of diseases in New York. One of these diseases, Botrytis leaf blight, has been dealt with aggressively. The "Blight Alert" system, developed several years ago under the leadership of Cornell plant pathologist James Lorbeer, gives growers warning when weather conditions favor the development of this disease. The idea is that they can withhold chemical treatment until such conditions exist. Monetary savings averaged $133 per acre for the 10 growers who used Blight Alert in 1996, even though blight disease levels were high that year. The savings were due to 66 percent reductions in pesticide applications.
While Blight Alert is an excellent program, it does not address other serious diseases such as black mold or bacterial soft rots. Black mold, for example, can render onion bulbs unmarketable for use in the production of onion seed crops. Growers are often reluctant to forego a fungicide application when the Blight Alert program suggests it, for fear they are leaving their crop vulnerable to these diseases.
In 1997 IPM Extension Educator John Mishanec and Orange County Extension Educators Maire Ullrich and Teresa Rusinek tackled black mold and bacterial soft rot. They looked at storage methods, onion varieties, and weather conditions as possible contributors to disease outbreaks. They also compared disease incidence and onion quality in Blight Alert fields to those in conventionally managed fields. Conclusions from this first year of what will be an ongoing investigation include
- "expertly maintained" storage facilities (careful attention paid to temperature and humidity) have lower infection rates than do less closely maintained ones
- certain onion varieties have higher incidences of both black mold and bacterial soft rot than do other varieties
- weather is the dominating factor for bacterial soft rot incidence in the field
- harvest quality in Blight Alert fields was equivalent to that of conventionally treated fields
For the past two years Professor Lorbeer has also been working on the disease of black mold. He used seed samples from the 11 onion fields with which Mishanec, Ullrich, and Rusinek were working, testing both home-grown and commercial seed for the presence of black mold. He has found that the black mold fungus may be perpetuated annually on certain farms in Orange County. In most cases, both home-grown and commercial seed tested from those farms have been infested with the disease. Onions harvested from the 11 Orange County fields will be tested later in 1998 for black mold so that more can be learned about the dynamics of both seedborne and airborne infection.
Further study of these diseases should result in grower guidelines that supplement and strengthen the existing Blight Alert program.