Market Diseases of Apples, Pears, and Quinces: Skin and Flesh Cracking
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Market Diseases of Apples, Pears, and Quinces
Skin and Flesh Cracking
Occurrence and Importance
Fruit cracking occurs most frequently on Stayman, Wealthy, and York Imperial apples. The condition occurs so frequently and so seriously on the Stayman that it is often termed "Stayman cracking." Rain cracking develops after periods of cloudy, rainy weather, when the rates of evaporation and transpiration are very low. Dumping and floating apples in water before packing them may produce some cracking in thin-skinned varieties or in fruits with numerous open lenticels.

Symptoms
On Wealthy and Stayman apples, cracking occurs chiefly on the cheek of the fruits in the form of irregular breaks in the skin and underlying flesh. The breaks vary from almost invisible short slits to cracks 1/2 inch or more deep that may extend almost completely around the apple. Late in the growing season, cracks may originate near the stem and extend out over the cheek in more or less straight lines towards the calyx. Cracks around the calyx basin are rare. The exposed flesh gradually becomes discolored and decay follows, often while the fruits are on the trees.

In York skin crack, the cracks are small and tend to run in a sidewise direction on the cheek of the fruits. The cracks may make a wavy line. They vary from being barely discernible to being open as much as 1/16 inch. In the late stages of York skin crack, the cracks are very numerous and are likely to be accompanied by wilting of the fruits.

Cracking may also occur in Golden Delicious apples from the absorption of rain water caught in the stem cavity of fully matured fruits. The cracks generally are small, but individual ones may exceed 1 inch in length and be up to 1/4 inch deep. They are oriented concentrically around the stem and are usually associated with russeting of the stem cavity. Cracking of Golden Delicious apples may also occur when immersion dumpers are used in the packing house.

Causal factors
Fruit cracking is more prevalent in the humid growing regions than in irrigated districts. Cracking occurs most frequently during periods of high humidity following rains. Absorption of rain water through the skin, coupled with the uptake of water from the roots, results in rapid enlargement of the fleshy cells. The internal pressure from the enlarged cells of the fruits creates a strain that cracks the skin. Differences in the thickness and composition of the cuticular layer of the fruits account for varietal differences in susceptibility to cracking. Russet is commonly associated with cracking in susceptible varieties.

York skin crack seems to occur on fruits grown on trees of comparatively low vitality or affected by drought during the growing season. It appears to be worse on light-crop trees than on those bearing heavy crops and on yellow and green parts of the fruits rather than on the red part.


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Monday, September 19, 2005