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Black Spot on Apples and Pears

Black Spot on Apples and Pears

Black Spot, or Apple Scab as it is known in many countries, is the most serious widespread disease of apples and pears. The species which attacks pears is closely related to but distinct from the species which attacks apples. As such cross infection from apples to pears and vice versa does not occur. The life cycle of both species are quite similar but there are some differences. Apple Black Spot is the species we will discuss here.

The fungous Venturia inaequalis which causes Black Spot produces two types of spore, ascospores and conidia. Ascospores are produced over winter on the ground in the previous seasons' diseased leaves. During spring millions of ascospores mature and are released into the air during wet weather events principally during September, October and November. These ascospores are the major source of primary innoculum in spring. No matter how good your disease control was during the previous season you can assume ascospores will be produced the following spring.

In a typical year the first mature ascospores are capable of causing infection as soon as green tissue is present during bud break in spring. Peak numbers of ascospores are generally produced within a couple of weeks either side of full bloom. This period coincides with large amounts of susceptible tissue eg young leaves and fruitlets. Ascospore germination occurs as soon as a spore lands on new green leaves or fruit providing moisture is present. Whether or not infection results depends on the number of hours of continuous leaf wetness and the temperature during the wet period. Once the fungus is established the resulting lesions produce conidia (spores) which are the source of secondary infections during the remainder of the season.

Infection charts such as the abbreviated version below estimate the number of hours of continuous leaf wetness required for infection by ascospores at a given temperature.

Temperature oC Hours of Leaf Wetness
5 21
7 15
10 11
12 8
16 - 24 6

In the major apple growing regions specialized weather prediction services, and weather station monitoring data are available to assist growers and advisers in assessing the risk of an infection period occurring, or alternatively if one has occurred, after a period of wet weather. This type of data can be downloaded to a PC and interpreted with specialized software or received via subscription fax services.

Fungicides are essential to control Black Spot on all current commercial apple varieties. Successful control of the disease revolves around the prevention of primary infection in spring. Successfully preventing the establishment of primary infection significantly reduces the need for fungicides later in the season and the potential for secondary infections to establish.

There are two types of fungicide available to combat Black Spot infection. Protectant fungicides such as dodine, Delan, captan, the pyrimidines Chorus and Scala, the strobilurins Flint and Stroby, dithiocarbamates metiram, mancozeb, thiram and mizar. Protectant fungicides form the basis for disease prevention and must be applied before any spores land on the plant, that is in anticipation of an infection period. It is important to use weather forecasts to 'predict' likely infection events and then apply a protectant before the infection event begins.

The second type of fungicide have some 'curative activity against Black Spot. While some of the protectant fungicides listed above have limited curative activity they should be used primarily as protectants and generally will not prevent an infection establishing once spores have germinated and penetrated susceptible tissue. If this occurs the only way to prevent establishment is the use of a curative fungicide. The frontline curative fungicides all belong to the DMI group and include Nustar, Systhane and Score. These products are effective at preventing infection for up to 96 - 120 hours after the onset of rain. Check the label for the individual product capability.

Fungicide resistance management strategies have been devised to prolong the effective life of the products that we have available.

DMI fungicides - no more than 4 applications recommended per season and always mixed with a protectant fungicide.

Dodine - no more than 6 applications per season with no more than 4 in sequence.

Pyrimidines - no more than 4 applications per season.

Strobilurins - no more than 3 applications per season.

In Summary:
  • Successful Black Spot management depends on the prevention of primary infection in spring.
  • Use protectants or if necessary curative fungicides as close as possible to the start of infection periods. Adhere to resistance management guidelines.
  • Use weather forecasts to predict likely infection events and monitoring data to determine if a curative is required.
  • Use regionally based ascospore discharge data (if available) to determine when the primary infection risk is over. Sample for Black Spot after this time to determine Black Spot risk and fungicide use patterns for the remainder of the season.
Kevin Manning
Horticultural Consultant

September 2001

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