Scab Spotty But Tough
Most parts of most orchards don't have scab to worry about. That's the good news.
But where scab has started, this past week only made things worse. Heavy rains washed fungicide from leaves and spread the spores around nicely. Third generation scab, that is, scab lesions that started from infections in the tree, are easily visible now. The humid weather will only keep scab pressure high where there are active lesions.
Here are some guidelines on how to deal with these lesions and keep fruit clean.
First, don't use an SI fungicide. An SI will suppress spore production in lesions, and will do a good job of protecting foliage from new infections, but won't do very well at all on fruit. Another little known problem comes with resistance to SI fungicides. In Nova Scotia, scab which was somewhat resistant to SI fungicides could be stopped under cool conditions, but became quite resistant to the SI when temperatures got into the 80 degree range. Much of the effectiveness of these relatively expensive fungicide is lost in summer.
Second, captan will be relatively well retained on foliage. Bill MacHardy showed that no amount of rain could completely wash a full rate application of captan from foliage within 7 days after the application. Captan is a good protectant, even when it rains a lot.
Third, use the heat which is predicted. Don't actually apply captan, or other pesticides, in the heat. But if the temperature is going into the 80's, an early morning or previous evening application of captan will be a more effective scab eradicant than it would be under cooler temperatures.
Fourth, remember that the benzimidazoles, Benlate and Topsin M, and dodine (Syllit) are somewhat systemmic. They are locally absorded by tissue, and hence they are very good at stopping new infections. Of course, there is also a lot of scab in different parts of the world which has become resistant to these fungicides. As yet, we in Massachusetts seem to have little resistance, and so a combination of captan with any of the systemmics is probably the most effective treatment for both eradicating scab and protecting against secondary infections.
Summer Diseases Simmering Too
The wet weather has set up the potential for a lot of summer diseases, in particular flyspeck. With all the thunderstorm activity, the number of hours leaves are wet varies from place to place, but across the state we are already in excess of 300 hours since 10 days after petal fall. In a normal year, we would be closer to 100 or less by now.
Fortunately the same captan or captan plus benzimidazoles used to eradicate or protect against scab spread will also work well against flyspeck. In scab-free blocks, keep the intervals between fungicide applications tight. Pay attention to the wash-off information in Healthy Fruit no. 10. For flyspeck and sooty blotch management, it takes 2 inches of rain to wash off effective levels of captan 50W applied at 1 lb per 100 gal; it takes 2.5 in. to take off 2 lb. Benlate plus captan can take 3.5 in. of rain.
Late Leafminers
Even in the latest monitored orchards, emergence of second-generation leafminer adults is nearly complete. In some earlier-developing orchards, sap-feeding mines have become apparent, indicating that the time to treat this generation, if necessary, will soon be upon us. As a guide for determining whether a treatment is necessary, we recommend a random sample of middle-aged fruit cluster leaves from the interior of the canopy for the remains of first-generation tissue-feeding mines. If more than 10-15 mines are found on 100 leaves, treatment is recommended, as leafminer populations generally increase 8-12 times from the first generation to the second. If such a treatment is necessary, Provado, Vydate and Lannate are all available for use at this point in the season.
Mites Might Need Treatment Later
Thus far, we have not seen any significant buildup of mites in blocks treated with an early season miticide. However, some blocks which received only oil have begun to develop moderate mite populations, and may warrant treatment later in the season.
Development of mites has become increasingly spread out over the past week. Nymphs of the first summer generation are still present in some areas, while egglaying adults have been observed in other, earlier-developing areas. This year's mite crop certainly got an early start, and has the potential to develop rapidly given hot, dry conditions. Dry weather, which we haven't had much of lately, inspires mites to feed more aggressively, causing damage much more quickly. Hot, dry conditions also suppress growth of fungi, some of which are marginally effective pathogens against red mites. From now until harvest, Pyramite is the best available alternative for treatment of mite outbreaks.
Woolly Apple Aphid More Common This Year
Many reports have been received this year of large infestations of woolly apple aphids. WAA are most frequently found on watersprouts, pruning cuts and cankers; the most common consequence of an infestation is development of black sooty mold on leaves and fruit which are exposed to WAA excrement. If the population appears troublesome, treatment with a half-rate of Thiodan may be in order. If the population is extremely bad, then a full rate should keep them at bay. Provado (for second-generation leafminer) or Sevin (for leafhopper) may offer some control.
Apple Maggot Fly Arrives
The season's first apple maggot fly was captured this week, making it a good time for placement of red sticky monitoring spheres in orchards. The following information detail placement of spheres and treatment thresholds.
The key to accurate monitoring of AMF populations is accurate trap positioning. It is critical that traps be positioned with abundant fruit and foliage underneath and to all sides of them. However, light is also a necessary component of effective placement of spheres as it enhances their visibility to flies, so the most effective positioning is one-third of the distance into the canopy from the perimeter of the limbs. Dwarf trees are the exception to this rule as their low foliar density permits light entry deeper into the canopy. Foliage and fruit should not be within 6" of traps in order to keep the sticky surface free of interference and scraping by limbs and leaves. Traps should be placed at least at head height, if not slightly higher. Results from many years of data show that the capturing power of traps decreases for every week of exposure, so for accurate results traps should be examined at least once a week and should be cleaned of all flies of AMF size and larger at least every 3 weeks. Cleaning is best accomplished by picking the flies off the traps with a small, sharpened twig (removing as little sticky as possible) while holding the trap by the hook with a pair of vise grips.
Traps should be placed in susceptible varieties in order to watch areas where trouble will arise. The susceptible varieties are as follows:
Early season: Early McIntosh, Gravenstein, Jersey Mac
Mid season: Cortland
Late season: Red and Golden Delicious
A farm of 10 acres should have 10 traps placed, while a farm of 100 acres should have 20 traps placed. The treatment threshold for unbaited traps is a cumulative average of 2 AMF per sphere. When this threshold is reached, Guthion or Imidan should proved effective control. If the AMF population is low or moderate, a half or partial rate of either material should be sufficient, while a high population should be well controlled by a full rate.
