The 1993 growing season started out like summer, but the last week has seen a return to early spring conditions, even some snow in parts of the state. Some early-season disease management observations and suggestions:
Fire blight. Orchards in the southern part of Iowa came into bloom 7 to 10 days ago, when the weather was unseasonably warm. The MARYBLYT program, which helps growers interpret the weather in order to apply streptomycin sprays only when they are needed, noted an infection period in some of these orchards last week, following light rains. The event most growers remember best about last week was high winds. Many orchards experienced winds in excess of 40 miles per hour, with gusts even higher. Under these "trauma" conditions, leaves and blossoms become tattered and bruised. Some orchards in full bloom were stripped of all their flower petals overnight. The wounds created by wind are ideal sites for entry of fire blight bacteria. Paul Steiner, the originator of the MARYBLYT model, calls infections resulting from this process "trauma blight." If temperature conditions are favorable for fire blight activity when winds exceed 40 mph, a streptomycin spray should be considered on fire blight-susceptible varieties, especially if the orchard has a history of fire blight.
Apple scab. Although several episodes of rainy weather have occurred statewide over the last week, cool temperatures have lessened the likelihood that scab infection periods occurred. If your scab problem was minimal (less than 1% of apples with symptoms) last year, a good spray program to follow is the Integrated, Reduced Spray program designed at Cornell University and tested throughout the eastern U.S., including Iowa. This program involves spraying an SBI fungicide (Nova or Rubigan) four times during primary scab season: tight cluster, pink, petal fall, and first cover. Many growers tank mix the SBI fungicide with a contact fungicide, such as Captan or Mancozeb, for some or all of these sprays. Caution: this program will NOT be effective if you had a significant scab problem last year. In such a case, you are better off with a traditional protectant spray program until the scab problem gets cleaned up.
Cedar apple rust. The high-risk period for infection of apple by this disease runs from pink through second cover. Once the orange, gelatinous galls have swelled up on the cedar trees, wetting periods required for infection by this fungus are somewhat shorter than for apple scab. Keep in mind that Benlate, Topsin-M, and Captan will not control rust; Nova, Bayleton, Funginex, Rubigan, Mancozeb, or Polyram will control it.
Frost injury. Freezing temperatures caused some damage to king blossoms at scattered locations in southern Iowa during the last week.
Tarnished plant bug. The nymphs of this insect, whose feeding activity causes buttoning of berries and loss of marketability as a result, begins feeding when the blooms appear. Our IPM on-farm program starts weekly counts of TPB nymphs when the first blooms appear. If the number of TPB nymphs per fruit cluster exceeds 0.5, an insecticide spray is recommended.
If you would like to participate in 1995 in the IPM on-farm trials, on apples, strawberries, tomatoes, or cucurbits, contact Donald Lewis (515-294-1102), Mark Vitosh (515-294-1613), or me (515-294-0579).
This article originally appeared in the May 4, 1994 issue, p. 64.
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