Scaffolds 00 index

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

Fire blight symptoms appeared last week in several orchards in the northern part of the Hudson Valley. Most of the farms reporting fire blight last week had previous problems with blight sometime within the past three or four years. In several cases, fire blight developed even in apple and pear blocks that had two streptomycin sprays during bloom. Where inoculum was present, two sprays of streptomycin proved insufficient for protecting trees throughout the entire period from May 6 to 11 when conditions where almost continuously ideal for blossom blight infection.

The fact that fire blight has been reported on relatively few farms in the Hudson Valley adds credence to the hypothesis that many orchards in eastern New York are free of fire blight inoculum. If inoculum had been present in most orchards, we presumably would be seeing a more widespread outbreak of fire blight, given the favorable weather conditions during bloom. Only a small proportion of the total Hudson Valley apple acreage was sprayed with streptomycin during bloom. Unfortunately, we still lack effective tools for predicting which orchards will have blight inoculum present during bloom and which ones will not. Therefore, streptomycin sprays are sometimes omitted where they are needed and, conversely, streptomycin may be applied unnecessarily in orchards that lack inoculum.

Cedar apple rust lesions are appearing on leaves that were not protected with effective fungicides during the two weeks of rainy weather that occurred after petal fall. Flint or Sovran have only marginal activity against apple rust diseases, so growers who used either Flint or Sovran after petal fall will probably see some rust infections on terminal leaves of susceptible cultivars such as Rome Beauty. Apple fruit are not susceptible to rust infection after petal fall, so there is no risk of fruit infection at this time, even though leaves may have numerous rust lesions. Furthermore, rust spores can infect only young leaves, so rust lesions appear only on those leaves that unfolded during the unprotected infection periods. Rust cannot spread from one apple leaf to another. Therefore, there is no need to fear secondary infections.

The bottom line is that rust infections on terminal leaves may be unsightly, but they are economically unimportant unless infections are severe enough to cause large numbers of terminal leaves to senesce and drop from the tree. This level of infection is likely only when apple trees are planted immediately adjacent to heavily infected cedar groves.

Apple leaf spot diseases have also become prominent within the past week. Apple leaf spots cause only cosmetic damage to apple trees in the northeastern United States. By the time the leaf spots become visible, the events that contributed to their development are long past and it is too late to apply control measures. Fortunately, the fungi causing leaf spotting in the northeast do not have secondary cycles on leaves. Therefore, most of the spotting will be limited to those leaves that become infected during the extended wetting periods between bud break and first cover.

The most common causes of leaf spotting are the black rot fungus (Botryosphaeria obtusa), the cedar apple rust fungus (Gymnosporangium juniperi-virginianae), other weakly pathogenic fungi (Alternaria species, Phomopsis species), or spray injury from oil sprays and/or from captan. The exact causes of leaf spotting are often difficult to determine because spots attributable to various causes all look very similar. Most leaf spots are uniformly circular and one to four millimeters in diameter. Individual leaves may have a single spot or as many as 30-50 spots. Severely affected leaves will turn yellow and drop from the tree within the next several weeks. The number of leaves lost is usually insignificant and does not adversely affect fruit growth.

The black rot fungus is the most common cause of leaf spotting. Spotting caused by the black rot fungus is called "frog-eye leaf spot" because the lesions are dark brown with an almost black center and a yellow halo around the edge. Other fungi can cause similar symptoms, so the spots themselves are not diagnostic. However, frog-eye leaf spot can often be identified based on its irregular distribution within the tree. Inoculum usually originates with small mummified fruitlets that remained in the tree after the fruitlet was killed by the previous year's thinning treatments. Inoculum can also originate from cankers, from branches killed by fire blight the previous year, or from pruned twigs left in the tree. Frog-eye leaf spot on the new foliage is usually concentrated in a cone pattern beneath one of these inoculum sources.

Severe frog-eye leaf spot infection on leaves surrounding a mummified fruitlet left from the previous year.

Captan, Benlate, Topsin M, Flint, and Sovran are all effective for preventing frog-eye leaf spot, but the SI fungicides and the low rate of mancozeb fungicides (3 lb/A) are relatively ineffective.

The same fungus that causes frog-eye leaf spot also causes black rot fruit decay. However, the fungus does not appear to spread from the infected leaves to fruit. Fruit infections that appear as a calyx-end rot when the apples begin to ripen are probably initiated at the same time and from the same inoculum sources as the leaf infections that are appearing now. Fruit infections can remain quiescent during summer because the green fruit have natural inhibitors that limit growth of the fungus. These natural inhibitors disappear as the fruit begins ripening. We do not know whether or not fungicides can eradicate quiescent black rot infections if the fungicides are applied to fruit several days or weeks after infections were initiated. However, it seems likely that Benlate, Topsin M, Flint, and Sovran might provide some eradicant activity against black rot infections on fruit.

In areas where cedar apple rust is prevalent, rust-induced leaf spotting can be confused with frog-eye leaf spot. Cultivars such as Empire, Cortland, and Liberty that are considered resistant to cedar apple rust can develop rust-induced leaf spotting. Rust spores can germinate on the foliage of rust-resistant cultivars. The rust fungus then invades a few cells and may even appear macroscopically as a pin-point yellow or orange spot on the upper leaf surface. However, the infected leaf cells soon die because of incompatible host-pathogen interactions within rust-resistant cultivars. The host cells that are killed or damaged by the rust infections are subsequently invaded by Botryosphaeria, Alternaria, or Phomopsis. These fungi use the dead or dying cells as a food base to get energy for invading adjacent healthy tissue.

Rust-induced leaf spot: Note that some rust infection sites have been invaded by other fungi that produced typical frog-eye leaf spot symptoms, whereas other rust infections are still visible as light yellow or tan lesions the size of pin-heads.

Rust-induced leaf spots are indistinguishable from frog-eye leaf spot except that rust-induced leaf spots are usually more uniformly distributed throughout the tree canopy. Sometimes the original orange-yellow rust lesion is visible in the center of the brown leaf spots initiated by rust infections. There is no reason to spray trees after these leaf spots appear because infected leaves do not produce inoculum for secondary infections.

> 6.5 Insects