Vermont Apple IPM Alert
June 2, 1999
L.P.Berkett, IPM Specialist
Disease Management
Apple Scab -- The following chart contains the estimated DD accumulated as of 5/31/99.
1999 Estimated Degree-Day Accumulation (Base 32F,
from McIntosh Green Tip) for Selected Sites
| Date | So.Burlington (GT=4/18) |
Shoreham (GT=4/16) | New Haven (GT=4/18) | Bennington (GT=4/11) | Walpole, NH (GT=4/15) |
| 5/31 | 1021 |
1059 |
1003 |
962 |
1008 |
Once 900 DD have been reached, the next soaking, daytime rain is expected to release the remainder of the ascospores in the leaf litter. However, do not ease up on your fungicide schedule until enough time has elapsed for lesions to develop from the last infection period during the primary season. Currently, scab lesions have been observed on middle-age cluster leaves and on the 4-5 leaf from the base of vegetative terminals. It is suspected these lesions originated from the infection period on May 4-5 and/or May 8. What has not appeared are the lesions from the May 19 infection period -- which you may start to see on mid-terminal leaves. That infection period may turn out to be a major one since ascospores where building up in the leaf litter for 10 days and by May 19 we were around 80% mature ascospores. Also, one should not ignore the infection period we had over May 24-25. The bottom-line is that you should thoroughly scout your orchard after enough time has elapsed from the last infection period to determine your scab situation. A thorough scouting would involve looking at all the leaves on 10 clusters and 10 terminals on 10 trees per block.
If you do have scab in your orchard, a description of treatment options appears on page 15 of the 1998-1999 New England Apple Pest Management Guide. Two full-rate captan applications 5-7 days apart is an effective treatment, especially when the temperature is above 80 F.The question that is always asked at this time of year is: "How long does a scab lesion remain 'active'? " and, unfortunately, there is no precise answer. As a guideline -- consider that lesions are most productive during the first two weeks after they appear; spore production tends to decrease after about 4 weeks. Unfortunately, no 'eradication' effort is 100% effective. If you find scab in your orchard, you will need to continue to carefully monitor your orchard to determine if any new lesions develop.
Arthropod Management Update
1999 Estimated Degree-Day Accumulation
(Base 50F, from Jan. 1) for Selected Sites
| Date | So.Burlington | Shoreham | New Haven | Bennington | Walpole, NH |
5/31 |
395 |
412 |
383 |
294 |
351 |
Plum Curculio: Based on a degree model developed by H.Reissig and Glen Koehler's Orchard Radar, it is estimated that a full dose of Imidan or Guthion applied during the remainder of this week should provide protection for the remaining period of activity for this insect for the sites indicated above. It will be interesting to see what type of pressure growers around the state are experiencing. At the UVM HRC, we have seen very little damage so far on non-sprayed trees in a block that has a history of high pressure.
European Red Mites: The hot weather is not helping matters in blocks where ERM were not effectively managed pre-bloom. Bronzing has already been observed. From June 1-15, the action threshold = 45% of middle-age fruit cluster leaves with motile mites.
European Apple Sawfly: Larvae are in the stage where they are migrating from fruit to fruit and burrowing into the core. Orange colored frass can be seen on the outside of the infested fruit. Insecticides applied for PC should also manage this insect.
Leafminers: Tissue-feeding mines are becoming more prevalent. If you are at threshold for mines, it is important to treat while most of the mines are in the sap-feeding stage. Ideally, no more than 10% of the mines should be in the tissue-feeding stage when you treat.
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