Scaffolds 99 index

SCARF SKIN ON APPLES

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

The dry conditions that prevail in parts of New York State could make this a bad year for scarf skin and other fruit finish disorders. Fruit finish problems often are more severe in drought years because apple fruit growth becomes a stop-and-go process. Slow growth during dry spells is often followed by very rapid fruit expansion when rains finally arrive. The rapid fruit growth following drought can contribute to scarf skin and split lenticels. Some fungicides can make the problems worse.

Scarf skin is a fruit finish disorder that makes the waxy surface of 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 Gala, Stayman, and Law Rome, but it can appear on nearly all cultivars in severe years. Scarf skin does not affect the internal quality of the fruit, but this disorder is important in a marketplace that places great emphasis on the physical appearance of fruit.

A description of the disorder and the origin of the term "scarf skin" dates back to the 1905 publication of "The Apples of New York" by Beach et al. They described scarf skin as "a dull or clouded appearance to the red skin as in 'Sweet Winesap' or 'Black Gilliflower'". Researchers have since shown that the disorder occurs when the epidermis and cuticle separate from the underlying tissue. The resulting air space beneath the waxy fruit surface disrupts light transmission and produces the milky or cloudy appearance. Unfortunately, we still do not understand what causes scarf skin to develop. Various researchers have noted that scarf skin is consistently more severe in some orchard blocks than in others. We also know that some cultivars are more susceptible than others and that the problem is more severe in some seasons than in others.

Scarf skin was studied by Dr. David Ferree, Mike Ellis, 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. Severity of scarf skin was not affected by applications of Solubor, calcium chloride, or dimethoate, but it was reduced by applications of giberellic acid (GA 4+7).

Ferree et al. also showed that scarf skin was more severe on trees receiving a seasonal program of Benlate or Dikar fungicides than on trees sprayed with Polyram, dodine, captan, or mancozeb. However, these fungicides do not consistently cause a scarf skin problem. Other researchers have compared various fungicides for their impact on scarf skin and have found that, in some seasons and some orchards, Benlate and Dikar had no deleterious effects. Nevertheless, the work by Ferree and observations that I have made in New York both support the hypothesis that Benlate applied within 40 days of Petal Fall can contribute to development of scarf skin in some years. No other fungicide appears to stimulate scarf skin as frequently or as severely as does Benlate. The effect of Benlate is probably dependent on interactions with environmental, and possibly nutritional, conditions at critical periods in the development of the fruit.

The period of greatest mechanical stress at the surface of rapidly growing apple fruits occurs as fruit reach approximately one inch in diameter, and this period coincides with the period of high susceptibility to both scarf skin and russet. Any factors that contribute to stressing the fruit during this critical period may promote scarf skin development. Various researchers have shown that environmental conditions during the 40 days after bloom are important in determining the amount of scarf skin that will develop, but the exact weather conditions that contribute to scarf skin have not been defined. Ferree et al. suggest that 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 cause the separation in cell layers that results in scarf skin. An application of Benlate during this critical time might decrease elasticity of the cells on the fruit surface and thereby contribute further to the problem whereas Benlate applications under other conditions may have no adverse effects.

Given the current state of our knowledge (or lack thereof), we cannot provide recommendations that ensure scarf skin will not appear. We can only suggest that growers concerned about this problem avoid using Benlate during the 40 days after Petal Fall and, if possible, irrigate trees as needed to minimize water stress during this critical period of fruit development.

References:

Ferree, D. C., M. A. Ellis, and B. L. Bishop. 1984. Scarf skin on 'Rome Beauty': Time of origin and influence of fungicide and GA 4+7. J. Amer. Soc. Hort. Sci. 109(3): 422-427.

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