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Healthy Fruit 1997 Vol. 5:6

May 14, 1997

Apple Development

While McIntosh in the early-developing areas of the state have progressed into early bloom, the later-developing areas remain at tight cluster. Last year at this time, McIntosh trees were at late pink in the later-developing areas, with those earlier orchards moving into petal fall. Overall, tree development continues to progress extremely slowly throughout the state.

Scab Symptoms Seen

Symptoms of scab are now visible on leaves in some abandoned orchards, as well as at least one commercial orchard. The most likely cause of these lesions is the infection of April 28. Check the leaves which emerged earliest if you are looking. They are not very easy to see, so check carefully, starting in those places where scab is most likely, like the corner near the abandoned trees or that place which has had some scab the last two seasons.

The development of these infections makes scab management more difficult. For the next few weeks, they will be producing inoculum, in addition to the primary ascospore inoculum that is available. Inside a well-managed commercial orchard, this isn't an issue, but in abandoned trees on the border, or in orchards where the early scab management wasn't entirely successful, scab pressure will be intense. The problem is particularly nasty because this comes when new fruit are most susceptible to scab. For 3 to 4 weeks after petal fall, fruit are highly susceptible to scab.

Remember that the SI fungicides (Rubigan, Nova and Procure) are not as effective against fruit scab as they are against leaf scab. If you are using these materials, make sure that you include at least a half rate of a good protectant (captan, mancozeb, etc.).

What if scab lesions have appeared in an orchard? For several reasons, I feel captan should be part of the eradicant fungicide program used against existing scab. For a few years, New England pathologists have pointed out that 2 lb./100 gal. of Captan 50W will suppress spore development, that is, burn out existing scab. This eradicant action is best at temperatures above 80 F. Since we do not have obvious cases of Benlate, Topsin-M or Syllit resistance in Massachusetts, either of these materials may be combined with captan to improve burn out. The SI's will also suppress spore development, and help burn out lesions. When eradicating infections, the interval between sprays should be kept short, at about 7 days. Spray intervals should be kept short, at about 7 days, until lesions stop growing, usually after 2 to 4 applications.

As noted last week, even if you don't have to fight established lesions in the orchard, we have reached the point in the season where primary innoculum is abundant. This is a critical period for scab managment.

Tarnished Plant Bugs Scarce

Mite egg hatch is nearly complete in orchards that are in early bloom and is ongoing in the later-developing orchards

Tarnished plant bug captures on white sticky rectangle monitoring traps remain low in most orchards across the state. Few orchards have exceeded trap capture thresholds. Because of the low level of TPB activity to date, many growers have decided to omit the pre-bloom insecticide treatment for TPB. However, as mentioned last week, a few warm days in a row can still initiate increases in TPB activity, causing damage to the developing buds during bloom and into petal fall. If this is the case, the only option for treatment is a petal fall application of either Guthion or Imidan. Whether such an application against TPB is warranted depends, in part, on what else you might be spraying. First, the timing of a petal fall spray against plum curculio may in fact cover the need for a petal fall TPB spray. Second, if there is a damaging population level of European apple sawfly present, then a petal fall spray geared toward sawfly may also take care of the active TPB population.

Leafminer Abundant

Large flights of leafminer were observed yesterday (5/13) in some orchards and many sites are now exceeding 100 LM/trap. This is in fact turning out to be a relatively large year on the LM front, for reasons outlined in the April 30th issue of Healthy Fruit. Irrespective of the reason for the high LM populations, many growers, especially those exceeding threshold trap captures, would do well to consider a petal fall application of either Provado or Agrimek for LM control. If Agrimek (in conjunction with horticultural spray oil) is the material of choice, a first-generation LM treatment is also the optimal timing for mite control.

Mites Might Be Abundant or Scarce So Check

First, the good news. Pyrimite (also known as Oracle or Sanmite) has received federal approval for use against mites in apple orchards in 1997. Before the material can be used in Massachusetts, though, the state Pesticide Bureau will also have to approve the material for use. The Pesticide Bureau meets monthly, but the docket appears full for the next two months. However, there may be a way around the necessity of a full-committee approval; use of the material in Massachusetts may be approved through a conference call between sessions. If this can proceed, we anticipate state approval sometime in June. We will keep you posted on the developments of the pending approval.

Pyrimite can be viewed as a substitute for Omite in the sense that it can knock down a building population of mites during the summer. It is harsher on predator mites than Omite was, but is less toxic to the predators than Carzol. It also carries the added benefit that it is effective against rust mites.

The more sobering news is that mite egg hatch is nearly complete in orchards that are in early bloom and is ongoing in the later-developing orchards. Soon it will be time to assess mite abundance on fruit cluster leaves. As pointed out in the April 23rd issue of Healthy Fruit, mite populations exceeding 1 motile adult per leaf at pink or bloom merit strong consideration for treatment with a post-bloom miticide. Choices for post-bloom treatment include Agrimek with horticultural (refined) spray oil and Vendex in combination with horticultural oil. A single application of Agrimek will generally suffice, but for maximum effectiveness when using Vendex, 2-3 applications should be made about 10 days apart, beginning at petal fall. Hopefully, few orchards will have early mite populations exceeding 1 motile adult per leaf, but lower populations at bloom do not always mean that the mites cannot develop to a damaging level later in the year. It is very difficult to accurately determine the potential for damaging populations this early in the year, and the best remedy is consistent monitoring of the leaves.

Too Cool for Sawfly

In most monitored orchards which are between late pink and early bloom, the numbers of European apple sawfly are relatively low. We have not yet had the warm, sunny days (temperatures in the high 70s to low 80s) on which EAS thrive. EAS exhibit little activity when temperatures are any lower. There have been isolated incidents of EAS captures of 20+ per trap, but these captures may be less indicative of high populations than they are of limited opportunities for activity. It is still too soon to make judgments regarding the necessity of petal fall treatments against EAS.

In Pursuit of the Plum Curculio

There is no substitute for each grower going into PC 'hotspots' and daily examining fruit for signs of PC feeding and egglaying, particularly king fruit"

We have yet to detect any active plum curculios, but we are engaged in a major effort across the state to monitor the buildup of PC adult populations. This study involves the use of black pyramid monitoring traps and regular assessment of damage to fruit by PC feeding and egglaying. We will have a good handle on the development of PC populations over the next month.

Last year, there were 3 successive days in late bloom with temperatures in the high 80s and low 90s. This extended spell of hot weather stimulated a concentrated emergence of PC. The weather pattern of the past 2-3 months has not indicated that such an extended heat wave will occur any time soon. If this pattern holds true, the emergence of overwintering PC adults should be much more strung out compared to last year. What does this mean as far as control is concerned? First, there is no substitute for each grower going into PC 'hotspots' and daily examining fruit for signs of PC feeding and egglaying, particularly king fruit, as these are most likely to receive the earliest injury. At the first sign of PC activity in the orchard, growers should be prepared to move into action, if the weather is favorable for continued PC activity. Little injury is caused by PC when temperatures are below 60o, but look out if temperatures remain at 70o or greater.

There are two basic approaches to the 1st application of pesticide against PC; border row and whole-orchard spraying. There is no sure-fire way to decide which will be the most efficient. In general, if the weather is conducive to high levels of PC activity (2-3 days above 70o, or 2 days above 80o) then a whole orchard spray is probably in order. If the weather after the first detection of PC feeding or egglaying in the orchard is cooler, then the invasion is slower, and a border row spray is probably sufficient. In deciding which strategy to employ, one also needs to consider the status of both tarnished plant bug and European apple sawfly populations in the orchard.