Scaffolds 99 index

CONTROLLING SUMMER DISEASES IN A DRY YEAR

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

Controlling apple diseases in wet years can be frustrating, but disease control in dry years is perhaps more complex. In wet years like 1998, everyone knows that continual fungicide protection is essential. In a dry year like 1999, anxieties arise because of the unknowns involved in NOT spraying.

In dry years, midsummer (late June – early July) is a good time for apple growers to save on fungicides. Apple scab is now inactive, even in orchards where scab appeared on leaves in late May. Lesions that appeared in May have probably received two fungicide cover sprays by now. The fungicides, hot weather, and the aging process of existing lesions have reduced production of viable conidia in existing lesions. Trees have (or soon will) set terminal buds, thereby terminating the production of susceptible new tissue for scab infection. Fruit have now reached a size where they are more resistant to scab than they are during the first 30 days after petal fall. Some of the scab on leaves could re-activate if we encounter a multi-day period of cool and wet weather, especially during late August or September. However, scab is of little concern given our current dry weather patterns. Even a few showers during midsummer (which would be welcome) will not reactive scab enough to cause problems.

Sooty blotch and flyspeck are the two diseases that generally require regular sprays during summer, but these diseases are also inactive during late June and July in dry years. Most of the inoculum for sooty blotch and flyspeck comes from wild hosts in orchard perimeters. Ascospores of the flyspeck fungus mature shortly after bloom. Visible symptoms appear on fruit only after fruit have been wet for a cumulative total of approximately 270 hours following infection.

Release of ascospores by the flyspeck fungus peaks about 10 days after petal fall. However, only a few ascospores land on apple fruit and most of these are killed by fungicides used to control apple scab. Although ascospores do not play much of a role in commercial orchards, they are important because they initiate secondary infections in the border areas. The secondary infections produce conidia in wild hosts (presumably after about 270 hours of accumulated wetting). These conidia are blown into apple orchards and cause the infections on apple fruit that appear during late summer, after another 270 hours of accumulated wetting from the time of infection.

Summer fungicides for controlling flyspeck are not needed from the end of the scab season until the time when 270 hours of wetting have accumulated counting from 10 days after petal fall. At that point, flyspeck conidia will become available in the orchard perimeter and will begin blowing into the orchard.

In dry years, a single fungicide application in late July sometimes provides adequate control of flyspeck. Depending on a single application is risky, however, because effectiveness of a single spray depends on achieving perfect spray coverage. A safer approach, even in dry years, is to use a minimum of two summer fungicide applications with one timed for mid- to late July, and the second about three weeks later in early to mid-August. These sprays will also help to prevent lenticel infection of fruit by the black rot fungus. A slightly earlier timing may be advisable in orchards where poor pruning and/or an exceptionally heavy crop load will make it impossible to get good spray coverage by mid-August. If August is exceptionally wet, an additional late-August application may be needed in orchards where a lot of primary scab lesions (May infections) are present in the orchard. More conservative application schedules are also advised for those "hot spots" where flyspeck is a perennial problem.

As explained below, the July-August applications should include Benlate or Topsin M (in combination with captan or ziram) to maximize both eradicant and residual activity against flyspeck. In dry years, however, Benlate should be avoided before mid-July. If a sudden heavy rain breaks the drought, fruit will size very rapidly. Rapid growth following drought stress often causes "lenticel splitting". Lenticels that "split" appear as enlarged and roughened lenticels on fruit at harvest. Benlate sometimes increases the severity of this phenomenon. The interaction between Benlate and lenticel splitting is probably related to Benlate applications made during June, but later applications may also be involved.

Captan and ziram do not have any eradicant activity against flyspeck and therefore must be applied before the conidia cause infections, i.e., before 270 hours of wetting have accumulated. The benzimidazole fungicides (Benlate and Topsin M) provide about 100 wetting hours of eradicant activity against flyspeck. As a result, development of flyspeck on fruit can be arrested if Benlate or Topsin M is applied sometime between 270 and 370 hours of accumulated wetting counting from 10 days after petal fall.

Captan and ziram provide good control of flyspeck under New York conditions if they are applied on a 14-day interval. Shorter intervals may be needed to compensate for wash-off by rains. Benlate, Topsin M, and the combination of ziram plus sulfur (1 lb. of each per 100 gal) provide excellent residual protection that will control flyspeck for about 30 days or through three to four inches of rain during summer. Benlate, Topsin M, and the ziram-sulfur combination generally provide adequate control of flyspeck during the preharvest interval if applied within 45 days of harvest. The ziram-sulfur combination provides excellent residual activity against flyspeck, but it does not provide any eradicant activity and therefore must be in place before the first flyspeck conidia arrive in the orchard. Combinations involving Benlate or Topsin M will provide better control of black rot than ziram-sulfur.

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