June 7, 1999 Volume 8 No. 12
HUDSON VALLEY DISEASE UPDATE
(Dave Rosenberger firstname.lastname@example.org, Plant Pathology, Highland)
Apple scab is appearing on a few terminal leaves in some orchards despite the relatively dry spring weather. In some cases, infections occurred primarily on vigorous terminal shoots that may have outgrown the fungicide coverage during the spring growth flush. We had several lengthy wetting periods with very little rainfall. The rainfall may have been insufficient to redistribute protectant fungicides to newly developed foliage. Other factors that may have contributed to appearance of scab at this time are low fungicide rates, alternate row spraying, poorly calibrated sprayers, or poor spray coverage caused by windy conditions when spray were applied.
What should a grower do if scab is appearing on terminal leaves at this time of year? There is no single correct answer. First, recognize that primary scab appearing in June is a totally different and much less serious situation than the one we faced last year when primary scab lesions appeared as early as April 27. Scab that appears several weeks after Petal Fall poses a much-diminished risk for fruit infection because fruit are gradually increasing in resistance to infection. Also, the fully expanded leaf canopy present at this time of year can hold large amounts of fungicide for subsequent redistribution to fruit during rainy periods, and hot weather will reduce viability of scab conidia. Furthermore, if the weather to date provides any indication of the kind of summer we can expect, then the summer will be wet and dry and scab will be the least of our concerns.
Because we cannot foresee weather conditions for the remainder of the year, there is still a possibility that the primary scab appearing now will return to haunt us. Any active scab in the tree could provide inoculum for fruit infections should heavy and extended rains move into the area sometime during the summer. For this reason, most growers would like a foolproof way to inactivate the scab that they are now seeing.
Unfortunately, there is no way to completely eradicate scab from primary infections. The SI fungicides (Nova, Rubigan, Procure) reduce sporulation and may provide some pre-symptom activity against incubating lesions that have not yet appeared. Benlate, Topsin M, and dodine used to be effective antisporulants, but their effectiveness in many orchards has been compromised by the presence of fungicide-resistant strains of apple scab. The combination of an SI fungicide plus dodine has proven very effective for arresting scab epidemics so long as dodine-resistant scab is not present in the orchard. However, the SI/dodine combination is a high-risk strategy because this combination leaves the fruit virtually unprotected in orchards with dodine-resistant scab. (The SI fungicides are not very effective for protecting fruit.)
Regular cover sprays of captan at 1015-day intervals will provide excellent protection of fruit against apple scab, provided that sprays are applied in a manner that ensures good spray coverage. The rate for captan should be 4.5 to 6 lb/A for the 50W formulation or 2.8 to 3.75 lb/A for the 80W formulation. The activity of captan seems to be enhanced when temperatures exceed 8085 degrees. If the summer weather pattern is predominantly hot and dry, then captan rates can be reduced after trees stop growing and terminal buds are set.
Those who want maximum antisporulant activity against the primary scab lesions that appeared last week should probably consider captan (at the rates noted above) in combination an SI fungicide. However, the economics of using SI fungicides this late in the season are questionable except where an SI is also needed to control mildew.
Fabraea Leaf Spot
Fabraea leaf spot is the second most important disease of Bosc pears in New York State, second only to fire blight. In the Hudson Valley, Fabraea leaf spot affects more acres of pears annually than does fire blight. Fabraea leaf spot does not kill trees as fire blight frequently does. However, Fabraea can cause early defoliation and total loss of the crop in orchards that are left unprotected during summer.
Many pear growers face an increased risk of Fabraea this year because Bosc pear orchards were left unprotected through much of 1998 after the pear crop was damaged by spring frosts. Some orchards had relatively high levels of Fabraea leaf spot in September of 1998. Leaves infected last year may provide unusually high levels of inoculum for the 1999 season. Unlike apple scab, where disease risk decreases in June, the disease risk for Fabraea leaf spot is greatest during June and early July.
Fabraea leaf spot is caused by a fungus, Fabraea maculata. The fungus overwinters either in fallen leaves on the orchard floor or in small (<one-quarter inch) indistinct cankers on pear twigs. We do not know which of the two methods of overwintering is most common in New York. Studies conducted in New York have shown that ascospores from last year's leaf litter can mature anytime from mid-May through early July. No one has determined why the time of ascospore maturation is so variable and so much delayed as compared with maturation of apple scab ascospores.
Fabraea first appears on new leaves as small, round, purple spots that are visible on both the upper and lower surfaces of the leaves. Similar leaf spots can be caused by many other fungi or by phytotoxocity from pesticide sprays. Therefore, the only way to determine if leaf spots are actually caused by Fabraea is to check under a microscope to determine if Fabraea conidia are present in the leaf spots. Fabraea produces distinctive four-celled conidia with two hair-like setae that make the conidia look like microscopic insects.
Each leaf spot produces millions of slimy conidia that are disseminated by splashing rain or by rust mites, pear psylla, or other insects. A minimum of eight hours of wetting are required for infection. If spores are disseminated by insects, infection can occur during long dew periods in the absence of rain.
Economic damage is usually caused by the rapid development of secondary infections in orchards where primary infections became established in late May or during June. If fungicide protection is inadequate during June or early July, a few primary infections can provide enough inoculum for an explosive epidemic. Secondary infections can appear almost simultaneously on most leaves throughout the tree canopy.
Fabraea can build up more quickly than apple scab because older apple leaves gradually become resistant to infection by the apple scab fungus, whereas leaf age does not affect susceptibility to Fabraea. All leaves and fruit remain susceptible to Fabraea right up until harvest. Thus, when a Fabraea epidemic develops in early summer, all of the existing leaves can become infected over a short period of time. Where the disease is severe, fruit become severely spotted and are unmarketable.
Fabraea is relatively easy to control with fungicides if primary infections are prevented during the period from Petal Fall through early July. Mancozeb is most the effective fungicide for controlling Fabraea. However, mancozeb cannot be applied within 77 days of harvest and its use is limited to a maximum 21 lb per acre per year. Ziram is the best choice for controlling spread of Fabraea during summer. Ziram applied on a three-week interval usually provides adequate protection, except where heavy rains remove fungicide residues or where the disease is well established on leaves before the first spray is applied. Where disease pressure is very high (i.e., early infections were not controlled), ziram sprays should be applied on a 14-day interval.
Ferbam is an alternative to ziram and will provide better residual protection through extended wetting periods. However, ferbam leaves a black residue on fruit, slows ripening (yellow color development) of Bartlett pears, and can cause a rougher fruit finish. Benlate is not labeled for Fabraea leaf spot and has only marginal effectiveness against this disease. Benlate may help to suppress Fabraea if it is applied to control sooty blotch at the maximum label rate of 6 oz per 100 gallons of dilute spray, but lower rates are ineffective against Fabraea. If Benlate is used, it should be used in combination with ziram or ferbam to ensure adequate protection against leaf spot.
Next in this issue: 6/7 Insects