Warmer-than-normal conditions this spring are causing a few concerns for apple growers. The biggest threats right now are cedar rusts and fire blight. Cedar rust galls are developing. As galls swell, they produce spores that threaten apple (and sometimes crabapple and hawthorn). Swelling galls were observed on cedar in Lexington in mid-March. Warmer temperatures in Western Kentucky provided conditions for even earlier gall development.
These galls indicate that rust pathogens are releasing or preparing to release infective spores. Growers should protect trees with fungicides. Once diseases symptoms develop on apple, it is often too late for control. Three types of cedar rusts affect Kentucky apples:
• Cedar-apple rust produces large brown galls on cedar and other species of Juniperus. Soon after a rain, galls produce slimy yellow or orange “horns” that are made up of infective spores (basidiospores). These spores immediately infect apples, causing yellowish leaf spots with red rings (called halos). Leaf yellowing and leaf drop follow. Infected fruit develop large spots near the calyx end. These fruit are often stunted and may prematurely drop.
• Cedar-quince rust produces orange swellings on twigs. Spores produced in these slimy lesions affect apple fruit, but not leaves. Infection of blooms and young fruit occurs early in the season, but symptoms do not develop until fruit mature. Diseased fruit are puckered and have spongy lesions at the calyx end.
• Cedar-hawthorn rust forms galls similar to cedar-apple rust, only smaller.Spores produced from short “horns” infect apple, crabapple and hawthorn. Leaf spots on apple appear similar to those caused by apple cedar rust, and can cause defoliation. Fruit infection is not common.
Growers should use fungicides to prevent rust pathogens from infecting apples. After symptoms develop on apples, they are often too late for control. As to fire blight, primary infections occur through blossoms, so it is critical to manage the disease before and during bloom.It is too late to spray after bloom. The disease is most prolific when conditions are warm and rainy at bloom. Our current conditions are an indication that fire blight may be severe this spring.
All growers should be utilizing UK’s Cougarblight predictive system. It is extremely easy to use, first visit http://wwwagwx.ca.uky.edu/plant_disease.html. Next, click “Fire Blight” on the left side of the screen. Choose the weather station that is closest to the orchard by clicking the arrow under “Station” in the center of the screen. There are a few options below, such as the history of fire blight in the orchard. Finally, click “Submit Choices.” The next screen will describe the risk of infection in the orchard.
Fire blight control measures include antibacterial pesticides applied during bloom. Applications made after bloom are ineffective. When fire blight risk is high (warm temperatures combined with rain) during bloom or if fire blight was a problem last year, the following spray schedule should be followed:
• Apply fixed copper at silver tip. Do not use copper fungicides after bud break.
• Apply streptomycin beginning at pink stage, repeating every four-five days, through petal fall. At least two applications are required, but you may apply up to four sprays, depending on rain and temperature conditions. Pay extra attention to susceptible varieties (i.e. Gala, Jonathan and Rome). Utilize Cougarblight or Maryblight predictive systems for assistance. Mycoshield (oxytetracycline) is also available for management of fire blight but is not as effective as streptomycin.
Refer to the Midwest Tree Fruit Spray Guide ID-92 for spray recommendations or our UK Disease blog at http://nicolewarduk.blogspot.com/ for more information on fire blight.
For more information or for fungicide recommendations, contact the Harrison County Cooperative Extension Service at 234-5510.