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Healthy Fruit 1997 Vol. 5:8

May 28, 1997

Tree Development

In the most advanced orchards, McIntosh trees are at petal fall to 1/4" fruit. In later-developing areas, McIntosh are at late pink to early bloom.

Development of peaches ranges from shuck split to early shuck fall.

Wind, Water, Heat and Scab

As petal fall happens or passes, depending on where you are, the next big scab question is "When does this all stop?" Good question. Used to be we would run to the lab with bags of scabby leaves that had overwintered in different parts of the state, and look at the spores in them, or see how many were released when we got them wet. When we didn't see any mature spores, or there weren't any released, then we would say the season was over.

As it turns out, we were doing a lot of extra work and not being as accurate as we could. Extensive research at UNH and Cornell shows that we can get a much better estimate of the end of scab season using a degree day model, accumulating degree days from green tip until we hit 860_(F). (The base temperature is 32_F.) It turns out the old method of looking at spores under a microscope consistently says that there is more scab inoculum than there really is. When spores are sampled from the air in and around orchards, the end of primary scab season is always a week or two earlier than the lab exams indicate it is. The orchard air samples are, of course, a more accurate picture of inoculum.

We can also use the degree day model to estimate where scab maturity is for the season at any given point. Depending on location in the state, right now areas are between 650 DD and 800 DD. By the model, that means the latest areas have about 35% of the season's spore load to go, and the earliest areas have only about 5% to go. In most parts of the state, about 10 to 15% of the spores are yet to be released.

By the end of the week, the primary season will be done in many parts of the state, including southwestern, southeastern, Nashoba Valley and North Shore areas. Growers should maintain fungicide protection for about 10 days following the end of the primary season. Then, as long as they don't see any, they can be assured that scab has been controlled for the year.

Some of our readers may be skeptical. "What do you really see if you do the squashes?" Fair enough. In Amherst, we see 12% of the spores are either immature or mature and yet to be released. We would estimate only 5% to be left. Since the squash mounts consistently overestimate the number of spores yet to be released at the end of the season, the degree day estimate looks good. The temperature model says the inoculum in eastern Mass should be down to around 5 to 10% left. New England Fruit Consultants say "Spore squash from eastern Mass. 5/26 revealed approximately 10 - 20% immature or not yet discharged spores."

If you had a problem spraying this spring, what with the wind, you should be seeing lesions from early primary infections and even secondary infections. Incidentally, Polaris Consulting pointed out that "The National Weather Service data for May through the 21 showed only 4 days averaging less than 10 mph winds, and 12 days had peak winds of over 30 mph!" If you look at the calm days, most of them had significant rain.

Too Cool for Curc'

At present, we still have not had what we would consider good weather for plum curculio immigration and egglaying. Over the past couple of weeks, we have been sampling in many commercial and unmanaged orchards; thus far we have seen no egglaying scars in apples at any of the commercial sites. Nor have we seen any egglaying scars in apples at the abandoned sites; only a handful of egglaying scars have been observed in wild plums in an early-developing PC hot spot. In one unmanaged site, captures on transparent, sticky flight interception traps showed a small to moderate influx of PC on Saturday and Tuesday (5/24 and 5/27). While there is some incoming PC flight activity, it has been too cool to give rise to any significant egglaying. Orchards will need to experience a couple of days in a row with temperatures in the mid-70s or higher to be susceptible to PC egglaying activity.

If there is little sign of significant feeding and egglaying by PC within the orchard, then a border row spray is recommended as the first treatment. This allows the grower to wait for warmer weather, which will lead to a more substantial invasion of PC and an increase of activity within the canopies. At that point, a whole-orchard spray is recommended, if the PC population in the orchard warrants any treatment. Of course, a whole orchard spray is recommended at an earlier post-petal fall interval than may be optimal for PC control if Provado or Agrimek are being applied against leafminer or if an OP is being applied against sawfly.

Check Leafminer Traps

The development of leafminer eggs and larvae has certainly been slowed by the cool weather, and only a small number of sap feeding mines have been observed in the earliest-developing areas of the state. In orchards which exceeded the trap capture thresholds, the post-petal fall application of either Provado or Agrimek will be in order this week.

European Red Mites Out and About

In blocks that received a pre-bloom treatment of Apollo or Savey, it is very difficult to find any red mites on blossom cluster leaves, suggesting the good effectiveness of these materials. In some blocks which received only a pre-bloom oil program, a few mites are present on blossom cluster leaves. Regardless of the pre-bloom management tactics, monitoring of leaves should continue in accordance with the recommendations outlined in the 1997 March Message.

Peach Plant Bugs

Shuck fall is the time to pay particular attention to signs of attack by tarnished plant bug, oak plant bug and hickory plant bug. Feeding by these insects peaks between shuck fall and when the fruit reach 1/2"-3/4" in diameter. It is important to protect fruit at this stage against 'catfacing' injury caused by plant bugs. Research has shown that either Guthion or Imidan alone are at best 50% effective in preventing plant bug injury. Therefore, we suggest use of a full rate of Guthion or Imidan in conjunction with a one-third labeled rate of a pyrethroid such as Asana, Ambush or Pounce. We feel that because of the tendency of pyrethroids toward high absorption into and slow release out of the bark of the trees, the one-third rate of pyrethroid offers a good compromise; extending the residual effects of the chemical while limiting the well-known destructive effects against mite predators. If mowing is planned in blocks of peaches, it is important that an active protective insecticide cover be in place at the time of the mowing. This will limit the effect of plant bugs, which are numerous in the groundcover, when they are pushed into the canopies by mowing.