April 26th, 1999 Volume 8 No. 6Update on Pest Management and Crop Development
PREBLOOM SPRAYS FOR SCAB AND MILDEW
What a difference a year makes! One year ago primary scab lesions were visible in the Hudson Valley by April 21. This year, our first infection period did not occur until April 22-23. The delayed start of the scab season this year has benefits inversely analagous to waiting until age 55 to start saving for retirement. Waiting to age 55 to initiate a retirement plan is a bad idea because one loses the benefits of compounded interest over time. In the case of apple scab, a shorter season for "compound interest" (i.e., secondary spread of scab) means that the risk of a major scab epidemic is significantly reduced. With a delayed start to the scab season, the scab fungus has less opportunity to generate multiple secondary cycles during the period when leaves and fruit are most susceptible to infection.
The dry spring may have reduced the risk of a serious scab epidemic in 1999, but it is still too early to celebrate. Scab problems could still develop if we have an extended period of rainfall such as the 11 consecutive days that we had last year in mid-May. Good fungicide protection is required for the next several weeks. Primary infections that become established during pink and bloom could still provide inoculum for subsequent fruit infections during a cool wet summer. We are nearing the end of one of the driest Aprils on record, but I suspect that we will "catch up" on seasonal rainfall totals sometime during the next six weeks. Having a clean orchard when the rains come is the best protection against potential postbloom scab problems.
Years that are unfavorable for apple scab are often more favorable for powdery mildew. Powdery mildew is actually suppressed during very rainy seasons. The mildew fungus benefits from dry weather. Wetting periods are not required for mildew infection. Mildew infections on leaves can occur when temperatures are above 50°F and relative humidity is above 70%.
Mildew overwinters in infected buds. Conidia from these primary infections become available sometime between Tight Cluster and Pink. Mildew can be controlled with protectant fungicides (e.g., sulfur) if protection is initiated at Tight Cluster. The SI fungicides have post-infection activity against mildew, and the first SI spray for mildew can therefore be delayed until Pink. Highly susceptible cultivars such as Ginger Gold should definitely be protected with an SI fungicide beginning at Pink. An SI fungicide is also recommended at Pink for orchards that had mildew problems last year. Where mildew was well controlled last year, the first SI spray can be delayed until Bloom or Petal Fall provided that SI fungicides are applied at least three times during the period from Bloom to Second Cover.
Mildew infections on fruit can result in net-like russetting similar to that caused by phytotoxicity from prebloom copper sprays. Most fruit infections occur at Pink or early Bloom stages. Thus, pink sprays can be important for protecting fruit from mildew. Under New York conditions, however, fruit infections are relatively uncommon and seem to occur only in high-inoculum orchards of susceptible cultivars, and then only in years that are unusually favorable for mildew development during the prebloom period.
When choosing fungicides to control scab and mildew, remember that mildew is NOT controlled by dodine, captan, Vangard, Polyram, or the mancozeb fungicides. Benlate and Topsin M may still control mildew in some orchards, but resistant strains of mildew are present in many orchards. (Benlate and Topsin M are totally unreliable for apple scab because resistant strains of scab are present in most orchards.) Sulfur applied at 3 to 5 lb/A works reasonably well as a mildew suppressant, but SI fungicides are the only option for high-inoculum orchards and for highly susceptible cultivars. Bayleton controls mildew and rust diseases but does not control scab. Bayleton must be applied at a minimum rate of 3 oz/A for effective control of mildew. Lower rates worked well when the product was originally introduced, but rates below 3 oz/A have provided disappointing results in recent years. Rubigan, Nova, and Procure will provide good control of mildew when applied at rates recommended for scab control.