Scaffolds 00 index

(Dave Rosenberger dar22@cornell.edu, Plant Pathology, Highland)

Apple Scab

The period from bloom to the first cover spray is usually the period of peak risk for scab infections on fruit. Ascospores are usually still available for discharge during bloom-time wetting periods, and secondary inoculum from earlier scab infections may become available if prebloom scab control was less than perfect. Apple fruit are highly susceptible to scab infection for the first several weeks after petal fall. Most fruit infections in commercial orchards are secondary infections that occur when conidia are washed from earlier infections on leaves or leaf petioles. Primary infections on leaves can produce and release conidia for secondary infections beginning a day or two before the lesions are visible.

The ascospore maturity model developed by Gadoury and coworkers indicates that the supply of ascospores in overwintering leaves is depleted after the first daytime rain event following accumulation of 760 degree days (base 32°F) counting from bud break. Orchards in the Hudson Valley had accumulated 845 degree days as of May 8. However, our unusually dry spring may have slowed ascospore development, and growers should assume that primary inoculum will not be exhausted until we have two good wetting periods during daylight hours. Even if all ascospores have been discharged, good fungicide coverage should be maintained for at least 14 days after petal fall to allow time for an assessment of primary scab control and to ensure protection of the small fruitlets during their period of peak susceptibility to scab.

Petal fall is a critical time to scout orchards for evidence of primary scab. A standard approach is to examine leaves and fruit on 50 clusters on each of five trees per block. If scab is found in two or more clusters, apply cover sprays of a strobilurin fungicide (Sovran or Flint) or apply captan. In orchards with visible scab lesions, cover sprays should be applied at 10—14-day intervals either until terminal leaves stop growing or until summer temperatures exceed 85°F for three or four days. High temperatures reduce the viability of conidia. Remember that strobilurin fungicides can be applied only four times during a season with no more than three sprays in a single sequence.

Black Rot Fruit Decay

Petal fall and first cover are critical sprays for controlling black rot. Most black rot fruit decay in N.Y. probably originates from spores produced in dead fruitlets that were retained in the tree after chemical thinning the previous year. Spores from these retained fruitlets can infect the calyx end of new fruitlets beginning at petal fall. Infections that occur at or soon after petal fall remain quiescent until fruit begin to ripen. Then they appear as calyx-end rots.

Fig. 1. Shown here at the pink bud stage, retained fruitlets from last year can carry black rot inoculum for infecting new fruitlets at petal fall.

Applying effective fungicides at petal fall and first cover can prevent infection of this year's crop while at the same time reducing inoculum levels for next year's crop. Fruitlets killed by thinning sprays provide ideal infection sites for the black rot fungus during late May and early June. Fungicides applied at first cover can reduce the proportion of retained fruitlets that become infected this year, thereby reducing both the amount of inoculum available for causing lenticel infections during summer and the amount of inoculum that will be available at petal fall next year.

Fig. 2. Black rot fruit decay at the calyx-end of a mature fruit resulted from an infection that occurred early in the season.

The SI fungicides (Rubigan, Nova, Procure) are not effective against black rot. Mancozeb and metiram (Polyram) at the 3-lb/A rate will not control black rot under high-pressure situations. Sovran, Flint, Topsin M, and captan have good activity against black rot and white rot. Where black rot is a problem, one of these fungicides should be used at petal fall and first cover.

Scarf Skin

Scarf skin is a disorder that makes the waxy surface of mature fruit appear milky or cloudy. Researchers in the Cumberland-Shenandoah region have also referred to this disorder as "opalescence". The disorder is particularly severe on Stayman, Law Rome, and Gala, but it can appear on nearly all cultivars in severe years. Scarf skin does not reduce the eating quality or storage life of fruit, but it may make the fruit less acceptable to consumers because the fruit do develop a clear shine after waxing on the packing line.

Scarf skin was studied by Dr. David Ferree and coworkers in Ohio in the early 1980's. By bagging Rome Beauty fruit clusters in polyethylene bags at various times beginning at petal fall, they were able to demonstrate that scarf skin is initiated between petal fall and 60 days after petal fall. Fruit bagged for 60 days had no scarf skin. The greatest amount of scarf skin was initiated close to petal fall, and the severity of scarf skin from later exposures decreased gradually. Fruit protected for 40 days showed very little scarf skin.

Fig. 3. The white mottling of scarf skin on Law Rome destroys the natural luster and makes the apple appear dull at harvest.

Various researchers have shown that environmental conditions during the 40 days after bloom are important in determining the amount of scarf skin that develops, but the exact weather conditions that contribute to scarf skin have not been defined. Climatic changes that stress fruit during the critical period after petal fall may contribute to scarf skin. Thus, a period of cool rainy weather followed by a hot, sunny, windy day might constitute a stress that could contribute to scarf skin.

Applications of Benlate made within 40 to 50 days of petal fall can exacerbate scarf skin. The effect of Benlate is probably dependent on interactions with environmental conditions and nutritional status of the orchard. Scarf skin can still develop in the absence of Benlate sprays. However, avoiding Benlate during the 40—50 days after petal fall will reduce the severity of the problem in years and locations where it does occur.

Powdery Mildew

Sprays for mildew should have been initiated before petal fall (see the April 24 issue of Scaffolds). However, the petal fall spray is especially important because of the rapid shoot growth that begins shortly before petal fall. Apply Sovran, Flint, or an SI fungicide to control mildew at petal fall and first cover.