Scaffolds 99 index

APPLE SCAB UPDATE

(Dave Rosenberger dar22@cornell.edu & Fritz Meyer fwm4@cornell.edu, Plant Pathology, Highland)

An apple scab ascospore maturity count on April 1 showed 5% mature spores with no discharge in the shooting tower. Economically significant spore discharges generally do not occur until the spore maturity count shows 15% mature spores with more than 60 spores per scan in our shooting tower discharge test. Thus, in a normal year, I would say that there is still no need to worry about apple scab in the lower Hudson Valley even though we are at Quarter-Inch Green bud stage on early apple varieties.

However, this is hardly a normal year because many commercial orchards have extremely high levels of inoculum in overwintering leaves. Thus, my recommendation for the lower Hudson Valley is as follows:

1. In clean orchards, scab sprays can probably be omitted until Half-Inch Green, as many growers have done in the past. This is especially true for orchards where SI fungicides will be applied within 72-96 hours after the first infection period.

2. Orchards with scab last year should be protected with fungicide before the infection period(s) predicted for later this week.

3. Caveat emptor! Effects of high inoculum levels on early season scab infections in years when spore maturity is still very low at Green Tip have not been sufficiently investigated, so those omitting early season sprays (because, of course, their personal orchards ALWAYS have low inoculum) do so at their own risk!

ERRATUM

In last week's article, "A Retrospective on Apple Scab in '98 and Suggestions for '99", one sentence in the section on reviewing what we know about contact fungicides was printed as follows:

"Full rates (Mancozeb at 2 lb/100 gallons or Captan 50W at 5-6 lb/100 gal) provide excellent scab control when used on a 7-day program."

An alert reader pointed out the error in Captan rates: The sentences should have indicated that a "full rate" of Captan 50W 5-6 lb/ACRE. I apologize for the error.

BLACK KNOT ON PLUMS

(Dave Rosenbergerdar22@cornell.edu, Plant Pathology, Highland)

If they have not already done so, plum growers should be removing all black knots from plum trees within the next week. Black knot in plums can be controlled only by using a combination of inoculum reduction and an effective fungicide program.

Knots that are pruned from trees can still release ascospores if the knots are left on the ground in the orchard. Thus, black knots pruned from plum trees should be burned if possible. If burning is not feasible, move the knots to brush piles that are a considerable distance (at least 100 yards) from the orchard.

Before spring foliage obscures the view, plum growers should also check surrounding woodlots and hedgerows for black knots that may be growing in wild plum or cherry trees.

In the Hudson Valley, we commonly find black knot in chokecherry and occasionally in black cherry. One or two knots 4-50 feet above ground in the top of a tall black cherry tree can provide inoculum that will be blown long distances in windy rain storms. Eliminating such external inoculum sources can minimize risks of black knot outbreaks in commercial plum orchards.