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Low Risk of Low Temperature Storage Disorders for 2005/06

Low-temperature disorders develop in apples during storage following cool and wet conditions during the months of July and August. As CA storage temperatures drop below the critical level of 3^oC, the incidence and severity of these disorders increase. 

Low-temperature breakdown (internal flesh browning) begins as diffuse browning of the outer cortex. This is often a well-defined area, and is separated from the skin by some normal tissue. It is different from senescent breakdown, in that the brown tissue is usually moist rather than dry and mealy, and in the early stages is separated from the skin by an area of healthy tissue. Low-temperature breakdown is favored by light crops, large apples, advanced fruit maturity, cool weather in the latter part of the growing season, and high levels of humidity and carbon dioxide in storage. It has also been associated with low levels of phosphorus, and to a lesser extent low levels of magnesium and potassium. Increased calcium levels in the fruit help to reduce the incidence of low-temperature breakdown. 

Vascular breakdowndevelops as browning of the main vascular bundles and some of the adjacent cortical tissue. The incidence and severity varies with the season, orchard and tree. Vascular breakdown is associated with cool growing seasons and becomes significant after 6-7 months of storage.

Core browning is characterized by diffuse browning of the flesh next to the carpels, with no clear distinction between normal and affected tissue. It rapidly becomes more extensive when the fruit is removed to warmer locations. Core browning is likely to be more prevalent in apples that are large, picked early, have high nitrogen content, from shaded parts of the tree, and harvested after an extended period of cloudy, wet and/or cool weather. Either excess or a deficiency of boron can induce core browning. Core flush is reduced by phosphorus sprays, calcium sprays, dips in diphenylamine (DPA), and waxing. In CA storage, a high concentration of CO₂ is likely to increase core browning, particularly if O₂ is also high. Low O₂ (<2%) is one of the most effective treatments for controlling core flush. In McIntosh apples, core browning is often accompanied by browning of the skin and underlying flesh in the stem cavity.

CIPRA is a computer-based program used to predict disease and pests of several horticultural crops, based on weather data. The program was developed by the research team of Dr. Gaétan Bourgeois, Agriculture and Agri-Food Canada, St-Jean-sur-Richelieu, Québec. More recently, CIPRA has been expanded to predict the risk susceptibility of apples to certain storage disorders (Bourgeois, DeEll, and Plouffe). CIPRA is presently being used commercially in Quebec, while it is currently being evaluated and adapted for Ontario (Bourgeois, DeEll, and Nichols).

A risk model has been developed for low-temperature disorders using CIPRA, based on weather data observed during July and August. The figure below shows the results from 1985 to 2005 using weather data from the Simcoe-Delhi area in Ontario. In 2005, the months of July and August were both higher than average for growing degree-days and lower than average for the number of days with precipitation. Therefore, it should be a low risk year for low temperature disorders, which is predicted by CIPRA as 0% percent risk. However, it should also be noted that 2005 should be a high risk season for senescent breakdown, external CO₂ injury, and superficial scald for the same reason.

Figure 1: Risk of low-temperature disorders in apples during the last 20 years. (Source : Gaétan Bourgeois, AAFC-HRDC, 2005)

Text equivalent of chart