May 31, 2001

Disease Management

 Apple Scab –  Microscopic examination of approximately 15 pseudothecia dissected out of about five scabby, overwintered leaves collected today from underneath non-sprayed McIntosh trees at the UVM HRC revealed that approximately 95% of the ascospores have matured and have been discharged.  Looking at the estimated DD accumulation at the six sites listed in the table below shows that we are close to the end of ascospore release at all sites.  As a conservative estimate, the final scab ascospore release in commercial orchards can be assumed to have occurred when 900 scab DD have accumulated before a daytime rain of at least 0.1 inch and temperatures of at least 50F during the wetting period.     Based on our estimate of DD at the 6 sites we are monitoring,   only Dummerston has reached the 900 DD mark.   In other words,   everyone should be covered during this rainy period.   Please note it will take about 10-17 days after the final release and infection period to see if any lesions develop before sprays can be extended.   Unfortunately, we have seen some disturbing levels of  fresh scab lesions from the May 12 infection period -- even in blocks that were thought to be "covered".    You may want to go in with a material such as the strobilurins or an SI/protectant combination if you have any doubt about coverage over the May 12 infection period or any infection period since then.    We have also seen some powdery mildew -- both the SIs and the strobilurins will be effective against powdery mildew.

Powdery Mildew:     Powdery mildew is now evident on the undersurface of terminal leaves.  The fungus appears as white patches.   Sprays during this time of the growing season are effective in curtailing the further spread of this disease on young leaves.   The strobilurins and SIs are effective against powdery mildew.

Arthropod Management Update:

Predicted Degree Days for Arthropod Pest “Events”:

PC- 1^st oviposition scars observed                           249-323

OBLR  - 1^st adult catch, 1^st summer brood                 484-612

Plum Curculio:   Beware of Plum Curculio --  we have seen more damage this year on non-sprayed trees than we have had in the past few years at the UVM Hort. Res. Center.   The warm weather predicted in a few days should make them more active.

Codling Moth:  A degree day model developed in Michigan for predicting CM development has been shown to be a good tool in effectively timing CM sprays in the Northeast.  In orchards where there is significant CM pressure,  degree days should be accumulated from the date of first sustained moth catch in pheromone traps (biofix date) and the first spray applied at 250 DD (base 50F), which corresponds with predicted 3% egg hatch,  with a second spray  applied 10-14 days later.  If pressure is not too severe, one spray should be sufficient, applied at 360 DD after the biofix date.   To control the second generation, an insecticide should be applied at 1260 DD after this same initial biofix date.    At the UVM HRC, we are using May 17^th as our biofix date and have accumulated 98 DD to date.

The following is an excerpt from an article written by Harvey Reissig, entomologist at Cornell University, which appeared in the May 30 issue of Scaffolds Fruit Journal.  It provides some insight into some of the newer insecticides available to manage CM:

"Insecticide trials conducted in N.Y. over a number of years in research orchards heavily infested with CM and other species of internal Lepidoptera have shown that most currently available IPM-compatible, "reduced risk" insecticides (Comply, Dipel, Confirm, and SpinTor) are slightly less effective in preventing fruit injury than are standard organophosphate insecticides, such as Guthion and Imidan.  However, it is likely that these selective materials applied on a schedule of 2-3 sprays/generation of CM, based on predictions from a CM developmental model, will provide adequate control in normal commercial apple orchards that are not located adjacent to abandoned orchards or extensive acreages of feral, unsprayed apple trees.  However, since some of these materials have limited contact activity against young CM larvae, and are only effective when ingested, they may be more effective if they are applied 5-7 days earlier than the estimated first hatching date predicted by the developmental model for each generation of CM.  This type of scheduling ensures that eggs are deposited on residues of the material so that hatching larvae are more likely to ingest a lethal dosage of the compounds before entering the fruit.”