May 24, 1999 Volume 8 No. 10
HUDSON VALLEY DISEASE UPDATE
(Dave Rosenberger email@example.com, Plant Pathology, Highland)
As a result of several extended wetting periods during the last week, the supply of apple scab ascospores in the Hudson Valley has been depleted and the primary scab season is over. However, secondary scab could still cause serious problems in orchards where primary scab was not completely controlled. Fruit and foliage are still very susceptible to infection. In our unsprayed check plots, scab symptoms from infections that occurred May 8-9 became apparent toward the end of last week.
Cedar apple rust
The rains of last week and this week are providing ideal conditions for cedar apple rust infections on terminal leaves of susceptible apple varieties. Teliohorns on cedar apple galls (the fruiting structures on cedar trees) become gelatinous during rains and appear as colorful orange masses on infected cedar trees. Basidiospores that are produced on these fruiting structures are blown from cedar trees to apple leaves where they infect the leaves and ultimately cause yellow-orange rust lesions.
Where cedar trees are present to provide inoculum, apple trees will remain at risk for cedar rustinfection until about June 10. Apple fruit become resistant to infection at or soon after Petal Fall, but newly unfolding apple leaves remain susceptible. By mid-June, the cedar apple galls will have exhausted their spore-producing capabilities for the season and the rust season will be over. Cedar apple rust does not have any secondary infection cycle. Infections that may appear on terminal leaves of apple do not pose any threat for further spread of the disease in apples.
The importance of spraying to control rust after the first cover spray is debatable because fruit will not become infected. Failure to protect susceptible varieties, however, will result in severe infection of two or three terminal leaves during each infection period in locations where inoculum is abundant. The bright yellow-orange lesions can make a tree look rather sick during early July when symptoms are most apparent, and severely affected leaves will probably drop from the tree prematurely. Susceptible cultivars such as Rome Beauty and Golden Delicious that are left unprotected after first cover may lose so many terminal leaves that fruit size could be affected. Where inoculum is less abundant, trees left unprotected after first cover will only develop an occasional rust lesion on terminal leaves. Light rust infection on terminal leaves may be unsightly (except to plant pathologists!), but occasional lesions are unlikely to affect fruit size or quality.
Captan will suppress rust (about 50% control), but only Polyram, mancozeb, or SI fungicides (including Bayleton) will provide complete control. If heavy rains remove protectant fungicides and trees are left unprotected during a subsequent infection period, then the SI fungicides or Bayleton will provide excellent eradicant activity when applied within 96 hours of the start of the infection period.