Weathering Diseases With Pruning
The appearance of some real July weather should help considerably with the scab epidemic in the state. As growth stops, tender new leaves which are most susceptible to scab will no longer be available. As trees mature, they become more resistant to scab.
The situation can be improved more by summer pruning. Summer pruning does two things. It improves the "weather" in the tree canopy. Canopies retain humidity and moisture, and the thicker a canopy, the longer it will stay wet and humid following rain or heavy dew. Summer pruning also improves the spray coverage and penetration in trees. Both of these factors will help eliminate scab in a block.
We know that summer pruning will also help with flyspeck and sooty blotch. These diseases have started to appear on unsprayed trees to the south of us, in Connecticut and New York. Just this week, the first flyspecks appeared in one orchard in Massachusetts.
Of course, summer pruning may help fungicide coverage and reduce disease pressure, but fungicides will need to be applied frequently in most orchards. There are other fungicide options, but probably the best program for the next few weeks will be captan (1 lb. 50WP/100 gal or equivalent) at 14 day intervals or a benzimidazole (Benlate or Topsin M) plus captan at 21 day intervals. If scab is active, and needs more aggressive treatment, cut the interval in half and/or increase the captan rate.
Switch to Guthion or Imidan for Larger Invasions of Maggot Fly
For reasons laid out in the July 1 issue of Healthy Fruit, this season has the potential for development of a large population of apple maggot flies. Along with the potential for high populations, trap captures in late June suggested that AMF would be active very early compared to recent years. Samples taken this past week seem to indicate that the AMF population may in fact become quite large, while activity is certainly early.
Well developed ovipositions were observed last week in early, susceptible varieties such as Red Free and Jersey Mac, but no significant damage has been recorded in any mid- to late-season cultivars. Captures on unbaited monitoring spheres are consistent with threshold levels; growers who applied Guthion or Imidan in the last week of June would be wise to check trap captures to determine when the next application is necessary.
Some growers have chosen to use Lorsban for early-season AMF control. This material does offer some control of early, light populations, and has the secondary benefit of knocking down woolly apple aphids and leafhoppers. However, with the potential for large influxes of AMF, we recommend switching to Guthion or Imidan for effective AMF control.
Summer Mite Management
As has been the case for the past month, orchard blocks which received an early-season miticide treatment (Savey, Apollo or Agrimek) remain relatively clean, harboring ERM populations of 0 to .5 ERM per leaf. Blocks which received a double-oil program are beginning to show higher levels of ERM infestation, from 1 to 3 ERM per leaf. Logically, blocks which received no oil or miticide have the highest ERM populations (up to 20 per leaf) along with pockets of significant bronzing. This week will be a real test for the early-season miticide treatments, with forecasts of sustained hot, dry weather conditions which are ideal for rapid development of mite problems.
Last year, some orchards found that treatments of Kelthane against ERM were almost totally ineffective. This ineffectiveness is not particularly surprising, as similar reports are received each season. Even if a grower has not used Kelthane for many years, loss of effectiveness is generally attributed to development of resistance to the material, which chemicals of this nature (chlorinated hydrocarbons) are notoriously prone to.
Development of resistance was enhanced by widespread use of these chemicals in the past, and the trait for such resistance can be carried in the genome and passed from generation to generation over the course of decades. A grower should not expect optimal performance from a Kelthane treatment unless no related chemicals have been used in the past 40 years. If a mite problem develops which warrants treatment, we recommend an application of Pyramite, but the potential for development of resistance should be a consideration regardless of the material of choice.
