September 2, 2003 Volume 12 No. 25 Update on Pest Management and Crop Development
(Art Agnello, Entomology, Geneva)
No one is very likely to confuse this growing season with last year's, but continuity does tend to play a role in the occurrence of certain insect problems, and we have been most mindful this year of the possibility that the 2002 internal worm infestations might recur if the sun kept rising in the east and made the trees grow upward. Naturally, nothing ensures a problem's elusiveness more than paying some attention to it, and so far there have been very few signs that such infestations have been occurring on anything like the same scale this year. Weekly examinations of thousands of fruits in problem sites have failed to turn up much more than nominal damage, and Harvey Reissig's bioassays of CM and OFM adults for insecticide resistance have thus far revealed no smoking gun. Still, this has been a fairly delayed season, and we have noted a recent upturn in moth catches (as would be expected about now), with perhaps a hint of an increase in fruit damage levels here and there.
So, to be cautious, we're not ruling out the possibility that blocks with a history of internal worm problems might need a last-minute application of a short-PHI material to help stave off the final feeding injury caused by young larvae. Before the harvest period begins in earnest, a fruit examination (we've been inspecting 20 fruits each on a number of trees throughout a given orchard) could help determine whether the last brood of any of the likely species needs a final deterrent before the sprayer is put away. Some thought could be given to using an alternative material such as a B.t., a pyrethroid, Spintor, or a sprayable pheromone, as appropriate.
PEARLEAF BLISTER MITE
Another season-end problem that may deserve attention now is this sporadic pest of pears that shows up in a limited number of commercial pear orchards and is a fairly common problem in home plantings. The adults are very small and cannot be seen without a hand lens; the body is white and elongate oval in shape, like a tiny sausage. The mite causes three distinct types of damage. During winter, the feeding of the mites under the bud scales is believed to cause the bud to dry and fail to develop. This type of damage is similar to and may be confused with bud injury from insufficient winter chilling. Fruit damage is the most serious aspect of blister mite attack. It occurs as a result of mites feeding on the developing pears, from the green-tip stage through bloom, causing russet spots. These spots, which are often oval in shape, are usually depressed with a surrounding halo of clear tissue. They are 1/4-1/2 inch in diameter and frequently run together. A third type of injury is the blistering of leaves; blisters are 1/8-1/4 inch across and, if numerous, can blacken most of the leaf surface. Although defoliation does not occur, leaf function can be seriously impaired by a heavy infestation.
The mite begins overwintering as an adult beneath bud scales of fruit and leaf buds, with fruit buds preferred. When buds start to grow in the spring, the mites attack developing fruit and emerging leaves. This produces red blisters in which female blister mites then lay eggs. These resulting new colonies of mites feed on the tissue within the protection of the blister, but they can move in and out through a small hole in its center. The mites pass through several generations on the leaves but their activity slows during the warm summer months. The red color of the blisters fades and eventually blackens. Before leaf fall, the mites leave the blisters and migrate to the buds for the winter.
A fall spray is recommended sometime in early October, when there is no danger of frost for at least 24-48 hr after the spray. Use Sevin 50 WP (2 lb/100), or 1-1.5% oil plus either Diazinon 50WP (1 lb/100 gal) or Thiodan 50WP (1/2-1 lb/100 gal). A second spray of oil plus Diazinon or Thiodan, in the spring, just before the green tissue begins to show, will improve the control.
MANAGING LATE SEASON FRUIT SCAB
As apple harvest approaches, growers should be concerned about managing "pin-point" scab. Pin-point scab refers to infections of the fruit that occur late in the season and are usually detected in late August and September as fruit matures. Pin-point scab lesions are usually quite small and may appear as black "pin-points" or as larger dots on the apple skin. The term storage scab refers to incipient infections that were too small to see prior to fruit storage or may be the result of infections during storage that occur as a result of sporulation from older, undetected scab lesions.
Simply put, orchards with many actively sporulating lesions are at the greatest risk of developing pin-point and storage scab. Aging fruit and foliar lesions exposed to hot summer temperatures (>86F) or lesions exposed to a strict fungicide program have far fewer viable conidia and a considerably reduced capacity to produce new spores, thus reducing orchard risk for pin-point scab. None of the currently available fungicides seem to "burn out" lesions the way that dodine and the benzimidazoles did before scab became resistant to those fungicides. Given extended periods of cool and damp weather, many lesions that appeared during summer may still produce viable conidia during the fall. Determining whether or not viable conidia will go on to infect maturing fruit is largely dependent on the availability of moisture and, to a lesser degree, the susceptibility of the variety.
Researchers have long known that, relative to foliar infections, much longer wetting periods are required for mature fruit infection. At the optimum temperature for scab development (68F), the minimum wetting period for "light" fruit infection is about 30 hours with high inoculum pressure; however, economic damage is not likely unless wetting periods exceed 48 hours. Very severe infections could be expected if we should encounter continuous wetting periods of more than 96 hours. However, unlike foliar infections, discontinuous wetting (dry periods between any two individual wetting events) substantially reduces the level of infection. In fact, a drying period of as little as 1 or 2 hours can greatly reduce pin-point scab. In another study, it was shown that interrupting a wet period with a 24 h dry period begun at 24, 48, or 72 h after inoculation at 68F reduced the amount of scab by 67, 87, and 36%, respectively. Note that the longer the initial wetting period, the smaller the effect the drying had.
The most detailed study relating the effect of temperature on mature fruit infection was done with a single (susceptible) variety, exposed to a single (long) wetting period, and inoculated with a single (high) inoculum concentration. Unfortunately, none of the factors that combine to reduce infection in the field were considered. The general consensus among apple researchers is that, practically speaking, the temperature that trees are exposed to in late August and September has far less impact on the development of pin-point scab than does the length of the wetting period and the severity of scab in the orchard.
Varieties differ in their susceptibility to developing pin-point and storage scab. Although no comprehensive cultivar evaluations have been conducted, it appears that the relative susceptibility of cultivars follows field-based ratings, although the absolute susceptibility is dependent on inoculum concentration and environmental conditions as described above.
Infections occurring two weeks prior to harvest have a low probability of developing in storage. Thus, there is probably no reason to spray trees that will be harvested within several weeks. Delays in cooling fruit after harvest might increase the possibility that late-season infections will develop symptoms during storage, so harvested fruit should be cooled quickly.
How does one decide which blocks need a September fungicide spray? We can only guess because there are too many unanswered questions, especially if Flint or Sovran were applied during August. We know that Flint and Sovran are effective anti-sporulants, but we do not know how long that anti-sporulant activity is effective following applications in late summer. We know that Flint and Sovran are absorbed into the apple cuticle, but we don't know how quickly these fungicides dissipate when fruit are exposed to extended periods of heavy rain. Flint provides only 48-72 hours of post-infection activity against scab in spring, but we don't know if it has more extended "kick-back" activity against pin-point scab.
Because we lack the detailed information needed to accurately predict development of pin-point scab, a conservative approach is warranted for protecting the apple crop from now until harvest. We know that postharvest drench treatments will not control pin-point scab infections. Topsin M generally has no activity against scab because of resistance. Where a captan/Topsin M combination was applied in the last spray in August, only captan will contribute to scab control. Where captan was applied at one-half pound of active ingredient per 100 gallons (i.e., 1 lb of Captan 50W/100 gal.), fruit should be protected against pin-point scab through 1.0-1.5 inches of rain. If captan was applied at the full label rate of 1 lb a.i./100 gallons, then residual activity on fruit in the fall might survive through 2 inches of rain. Our guess is Flint or Sovran may remain active against scab through at least 3 inches of rain, but that is only a guess. Orchards that are still at least 14 days away from harvest could be covered with Flint, but Captan is probably a more cost-effective option for September sprays. Sovran cannot be applied within 30 days of harvest and therefore is not useful for September applications.
If weather conditions favor late-season scab development and high-inoculum blocks are not protected with fungicide, then the best solution will be to sell the potentially affected fruit as soon as possible after harvest. In experiments conducted in South Africa, the first symptoms of pin-point scab appeared on Granny Smith after 80 days at 34-36F as compared with 35-45 days for fruit at 68F. However, scab lesions might appear in less than 80 days at 34-36F on fruit that were infected earlier during the preharvest interval. Other studies have shown that new lesions appear more quickly at higher temperatures, but the total number of lesions is ultimately the same if apples are stored long enough to allow symptoms to develop at cold temperatures. Storing apples under reduced relative humidity can minimize lesion size because lesions develop greater size when fruit are held under high-humidity conditions, especially if the latter results in condensation developing on the fruit surface during storage.
The bottom line is that active lesions in combination with long wetting periods create conditions favorable for mature fruit infection. Orchards with very low levels of scab shouldn't expect pin-point or storage scab to develop unless exposed to 4 or 5 days of continuous wetting. For orchards at risk, captan 50W at 2 lb/100 gal (or equivalent formulation) should be applied prior to any wetting event or, in problem orchards, captan should be applied on a protective schedule until 2 weeks before harvest. The DMI fungicides (Nova, Procure, and Rubigan) are ineffective against fruit scab.