HUDSON VALLEY DISEASE UPDATE
(Dave Rosenberger dar22@cornell.edu,
Plant Pathology, Highland)
Apple Scab
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 10—15-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 80—85 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
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