RI Apple IPM Newsletter

From Heather Faubert and Steven Alm

We have had three apple scab infection periods since the last newsletter: May 3, 6, and 9. Apple scab lesions take 9 to 17 days to appear after an infection period, depending on the temperature. Many scab lesions appeared this week in insufficiently sprayed orchards. These lesions are probably from the April 28th infection period or the May 3rd infection period. Check now for the velvety, dark lesions primarily on the underside of leaves. Scab lesions were found on 2nd, 3rd, 4th, and 5th cluster leaves.

If you find scab lesions during the first two weeks after petal fall, the safest recommendation is to apply Rubigan or Nova plus a full rate of a protectant fungicide, through second cover. This protects against the development of new lesions and suppresses further development of secondary inoculum on the lesions. Read about eradicating scab in the 1996 – 1997 New England Apple Pest Management Guide on page 15.

If scab lesions are hard to find, (Cornell says if it takes more than three minutes of scouting to find each lesion), a weekly application of Captan or mancozeb fungicide may be adequate for keeping scab off the fruit given average weather conditions for the year. Dave Rosenberger, from Cornell, believes that Captan is better against a scab epidemic than the mancozeb fungicides. Also, remember we are trying to limit the amount of mancozeb fungicides applied to the trees because these fungicides are believed to be harmful to predatory mites. We have seen quite a few predators already this year, let’s protect those predators!

Many orchards have high numbers of apple blotch leafminers this year. If you look on the underside of the oldest fruit cluster leaves with a hand lens you may be able to spot the small, round, pale, gelatinous eggs. Some eggs have started to hatch into larvae that will be inside sap feeding mines. The small sap feeding mines are only visible from the lower leaf surface. By early June the larvae will have grown large enough to cause visible damage from the upper leaf surface. This is when we can see the mines as densely spotted mines, called tissue feeding mines.

Just how much damage trees can tolerate from apple blotch leafminers is not clear . It depends on the variety and what other stresses the trees are under. For example, a tree suffering from drought or red mites can tolerate less stress from apple blotch leafminer. Generally we use a threshold of 13 mines per 100 fruit cluster leaves. Without using a hand lens you can’t accurately determine the number of mines until about 7 -10 days after petal fall. Seven to ten days after petal fall is too late to use Provado or Agrimek against leafminer. The only material that would work at that time is Lannate. The problem with using Lannate is it is very harmful to predator mites. If you do use Lannate it should be applied as soon as the mines start advancing to the tissue feeding stage. If you wait for too many mines to advance the Lannate application will be less effective.

Leafminers can also be controlled with Provado during the second generation but this often requires two applications because the second generation is more spread out than the first generation.

Agrimek must be mixed with something to help it penetrate the leaf surface. This can be accomplished by mixing it with oil or mixing it with a spreader- sticker such as Silwet or LI 7000.

European red mite nymphs are easy to find now in some orchards. They generally complete hatching from overwintering eggs by petal fall. Luckily it looks like we will have a new miticide to use this summer, Pyramite from BASF. It is suppose to give good mite control for 45 – 50 days. Hopefully everyone’s prebloom treatment will last until July and then Pyramite could be used to control mites for the rest of the season.

European apple sawfly trap captures are lower this year than the last several years. Some orchards are still above the threshold of 5 – 9 per trap by petal fall. Being above the threshold indicates a need to spray Imidan or Guthion right at petal fall. Sawfly numbers below the threshold means that you can delay applying a petal fall insecticide until plum curculio are more likely to be active. With the cool temperatures we are experiencing this spring, it isn’t likely that plum curculio will be active right at petal fall. So in locations where sawfly numbers are low, this would be a good year to delay a petal fall insecticide application.

Plum curculio adults overwinter in leaf debris in nearby woods and hedgerows, and in the orchard. They may begin migrating to apple trees during bloom, but peak migration usually occurs from petal fall to 14 days after petal fall. Egglaying and feeding damage can occur as soon as the fruit begins to form. The risk of plum curculio damage increases after there have been 3 – 4 days ofaverage temperature of 55 – 60 degrees, or 2 days with maximum temperatures above 75 degrees. Humid, calm, warm evenings pose the greatest risk. If you delay your petal fall insecticide application be sure to scout for plum curculio scars and watch the forcast.

To monitor for plum curculio, check fruit on border row trees (especially near woods) for the crescent-shaped egglaying scar. Large fruit usually get attacked first – we usually find the first damage on Lodi, Idared, or Liberty fruit. If hot, humid weather is predicted, put on an insecticide before it arrives. Extensive damage can occur in a single warm night. Use Imidan or Guthion when you first find damage and again about 10 days later. Occasionally a third spray is needed against plum curculio, if the plum curculio migration is slowed down by cool weather. The only way to know is to continue scouting orchards through June for fresh egglaying scars.

White apple leafhopper nymphs begin hatching at pink and have completed hatching by petal fall. The small, pale nymphs can be found on the underside of older leaves. White apple leafhopper feeding removes sap from leaves, causing stippling that may coalesce into silvery, white patches. Early in the season, extensive leaf damage may affect bud formation. White apple leafhoppers are easiest to control when they are small, first generation nymphs from petal fall to early June. White apple leafhoppers have developed resistance to several organophosphate insecticides so Guthion or Imidan will not control them. They can be controlled with Thiodan up until early June. Provado or Agrimek applied at petal fall against leafminer will also control leafhoppers. Sevin used as a thinner at 1 lb./100 gallons of water will control leafhoppers as well.

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