June 26, 2006 Volume 15 No. 15 Update on Pest Management and Crop Development
Geneva: Spotted Tentiform Leafminer 2nd flight increasing.
Highland: Plum Curculio adult feeding damage observed.
Oriental Fruit Moth
Spotted Tentiform Leafminer
Insect model degree day accumulations:
DD43 since 1st Obliquebanded Leafroller catch (1st larval hatch @ 360, 25% egg hatch @ 450):
The Comstock mealybug poses two major concerns for the pear processing industry of New York: First, the emergence of crawlers and adult females from the calyx of pears at the packinghouse creates a nuisance to workers. Second, pears to be made into puree typically are not peeled or cored by New York processors, so infestations can potentially result in unacceptable contamination of the product.
Another problem, of concern to apple growers in the 1930s and 1940s, and again in the Hudson and Champlain Valleys in the early 1980s, is that the honeydew secreted by the crawlers is a substrate for sooty molds growing on the fruit surface. This type of damage has also been noted on peaches. These molds result in a downgrading of the fruit, and are therefore an additional cause of economic loss.
To date, the Comstock mealybug has been a problem to growers of processing pears because of the contamination and aesthetic reasons noted. An infestation generally requires one or more insecticide sprays during the growing season, directed against the migrating crawlers. Examine the terminal growth for crawler activity periodically throughout the summer. Crawler and adult female activity can also be monitored by wrapping double-sided tape such as white carpet tape around low scaffold branches and inspecting for crawlers that have been caught by the tape. They can be recognized with a hand lens or, with some experience, by the unaided eye.
Sometime in early August, we'll advise an application of a material such as Provado, Diazinon, Actara, Calypso or Assail to control this insect.
Dilute trunk applications of an insecticide with good residual activity can provide control of established infestations. Lorsban 75G, 4E or 50W may be used postbloom as a directed trunk spray in N.Y. for borer control in apples. We feel that Lorsban is the best tool we presently have for this use, and early to mid-July would be a good time to take advantage of this welcome opportunity to use it on apples to control both dogwood borer and the second generation of American plum borer. Another option at this point in the season is an application of Thiodan 50WP applied once during this first week of July, and again one month later at the beginning of August. We would also note that, in case you didn't follow the strategy of using Lorsban as a prebloom trunk spray for American plum borer, these treatments will also serve as the last opportunity for a control measure against this pest.
Flyspeck has caused more commercial losses in New York and New England over the past few years than during most of the previous decade. What has contributed to those losses? How can we prevent them from re-occurring this year? Let's start by enumerating what we know about flyspeck biology:
1 - Flyspeck infects a wide range of host plants. That means that the flyspeck fungi can grow on the waxy cuticle of most bushes and trees in orchard perimeters, and these hosts can produce inoculum that blows into orchards. (Recent work in Iowa suggests multiple species may be involved in causing flyspeck, so in this article I will refer to the causal organisms as flyspeck "fungi.")
2 - After a spore from one of the flyspeck fungi lands on an apple, the apple must be exposed to 270 hr of accumulated wetting (hrAW) before the flyspeck colonies become visible on the fruit. Brown and Sutton in North Carolina were the first to identify the incubation period for flyspeck, and they found the best correlations when they ignored wetting periods of less than 3 hr duration. I have since found good correlations with the 270 hrAW over several years in the Hudson Valley when we included all measurable wetting periods. Variability among types and locations of wetness sensors is so great that quibbling about the details of whether to include or dismiss short wetting periods is probably meaningless.
3 - In Massachusetts, Cooley and Lerner showed that ascospores for flyspeck are released around petal fall. Therefore, flyspeck colonies initiated by ascospores may begin appearing on unsprayed fruit at 270 hrAW after petal fall (hrAWPF). However, scab sprays usually control the ascospores, so ascosporic infections are not common in commercial orchards.
4 - Infections initiated by ascospores in wild hosts begin releasing conidia as soon as those infections become visible (after 270 hrAWPF). Once those infections produce conidia, orchards in the northeast are exposed to a continuous supply of conidia blowing throughout the remainder of the summer and fall. If fungicide residues on fruit drop below effective levels, then the conidia will initiate flyspeck infections on fruit.
5 - Where trees are left unsprayed after 2nd cover (i.e., they are protected from flyspeck ascospores but not from conidia), flyspeck incidence and severity on fruit increases dramatically around 540 hrAWPF. Thus, flyspeck requires 270 hrAWPF to produce conidia on wild hosts and another 270 hrAW to infect and produce visible colonies on apples.
Results of two recent trials that indicate limitations of current fungicides:
1 - Two inches of heavy rain may be enough to eliminate fungicide residues. In an experiment at the Hudson Valley Lab in 2004, we applied all of the common summer fungicides to test plots on 17 Aug. We received 2.15 inches of rain on 20-22 Aug. We had 270 hr of accumulated wetting between 22 Aug and 26 Sept. Incidence of flyspeck on Golden Delicious fruit on 27 Sep was 64, 50, 31, 27 and 8%, respectively, for plots treated with Captan alone (30 oz of 80W/A), Flint, Sovran, Topsin-plus-Captan, and Pristine. Pristine had the best residual activity, but none of the fungicides had adequate residue to completely protect against flyspeck after 2.15 inches rain.
2 - Fungicides applied after flyspeck infections have been initiated can arrest growth of the flyspeck fungus temporarily, but they do NOT eradicate the infections. In a 2005 experiment, summer fungicide sprays were initiated at either 337 or 450 hrAWPF to determine if these fungicides could provide post-infection activity that would reach back through either 67 hrAW (i.e., 337 hr minus the 270 hr threshold for conidial infections) or through 180 hrAW from the start of infections. None of the postinfection treatments provided satisfactory disease control. By 26 Sep, flyspeck incidence exceeded 19% in all treatments, even though we maintained fungicide coverage up until harvest (i.e., <2 inches of rain between sprays and between the last spray and harvest). Sovran was significantly better than Pristine or Flint for suppressing infections but it was not significantly better than the Topsin-plus-Captan standard. Thus, Pristine provides the best residual protection, but Sovran and Topsin M provide the best post-infection activity, even though the post-infection suppression is less than we had hoped for.
Results from these recent trials have caused me to re-evaluate earlier hypotheses. The long incubation period required for flyspeck coupled with our inability to accurately predict or monitor fungicide residues on fruits makes it difficult to interpret results of fungicide trials in dry years. Did fungicides applied in July or August really eradicate earlier infections, or did they just slow fungal growth enough to allow fruit to be harvested before flyspeck appeared on fruit? Wet years such as we have had in the Hudson Valley in 2004 and 2005 provided more definitive evidence concerning the limitations of our fungicides.
Combining all that we know about flyspeck along with some working hypotheses, I've compiled the following statements to help formulate options for controlling flyspeck.
1 - The period of least risk for significant flyspeck infection occurs between petal fall and 270 hrAWPF for reasons noted above.
2 - After 270 hrAWPF, fruit should be continuously protected with fungicides. Any gaps in protection after 270 hrAWPF may allow flyspeck infections to be initiated.
3 - Two inches of rain can remove virtually all fungicide protection.
4 - Fungicides applied after infections are initiated do not eradicate all infections. Post-infection sprays will arrest incubating infections for varying (and at this point, unpredictable) periods of time. When the fungicide residues drop below inhibitory levels, the surviving flyspeck infections begin growing again. Predicting when suppressed lesions resume growth is difficult because we can't accurately predict when fungicide residues are exhausted.
5 - The 270-hrAW incubation period for flyspeck can perhaps be viewed as a "grace period" for lapses in fungicide coverage. If apples are consistently protected from infection during summer and fungicide residues are removed by heavy rains on September 1st, then flyspeck will not appear on fruit so long as fruit are harvested AND COOLED before they are exposed to 270 hr of wetting. However, if apples are left unprotected through 90 hr of wetting in July and/or August after conidia are being released, then part of the grace period will have been used in July-August and flyspeck may appear on fruit more quickly than otherwise expected in September.
6 - In real life, the total grace period for lack of fungicide protection during the growing season is probably less than 270 hrAW because flyspeck can continue to grow on wet fruit surfaces after harvest until fruit are cooled below roughly 45 F. Fluctuations in air temperatures as storage rooms are filled can cause condensation on surfaces of cold fruit already in the room, and that moisture can allow continued growth of flyspeck. I don't know how much of a 270 hr incubation period can be completed after harvest, but I suspect that up to 70 hr of the required 270 hr incubation period could occur after harvest if fruit a are not cooled rapidly. Application of a postharvest fungicide drench might suppress growth during the cool-down period after harvest, but I am not aware of any data that addresses this question.
7 - Given all of the above, the safest approach for controlling flyspeck will be to maintain fungicide coverage throughout summer after the 270-hrAWPF threshold has been reached. If extended rainy periods preclude timely respraying of blocks after heavy rains, then that lapse in coverage may use up part of the preharvest "grace period."
8 - Wet autumn weather such as we have had in recent years may be contributing to elevated inoculum levels in hedgerows and woodlots. Thus, extra caution (i.e., extra sprays in September and perhaps even in early October for late varieties) may be warranted until we get a dry summer-fall combination to break the current high inoculum cycle.
9 - Late summer sprays for flyspeck can be compromised by incomplete coverage of fruit surfaces. Including a surfactant with the fungicide during late summer may be helpful, but an excess of surfactant will only cause excessive run-off, thereby leaving less residue on fruit than a spray applied with no surfactant. Probably the best way to improve coverage in late summer sprays is to reduce tractor speed and increase the volume of water applied per acre.