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Intermittent Wetting Periods - Growers who rely on the Mills Table to predict apple scab infections often ask, "How should I handle intermittent wetting periods?" Several studies have been conducted in an attempt to answer this question, but none of the studies provided answers for all of the various combinations of temperatures and wetting and drying intervals. Yet, results from the experiments that have been conducted DO provide us with enough information to derive a good rule of thumb.

The most detailed study to date was conducted by Chris Becker and Tom Burr in the early 1990's. In their approach, they asked if apple scab conidia could cause disease after exposure to various wet-dry-wet intervals at either 50, 59, 68, or 77 F. Three initial wet intervals were tested, either: (1) 15 min., (2) the time at each temperature required for ~50% of conidia to germinate, which turned out to be 7, 5, 4, and 5 hours at 50, 59, 68, and 77 F, respectively, or (3) the time at each temperature required for ~20% of the conidia to also form an appresorium (i.e., 20% of the spores penetrated the host) which was 12, 8, 7, and 8 hours at 50, 59, 68, and 77 F, respectively. An appresorium is the structure the fungus produces to penetrate the host plant. After exposure to the initial wet interval, plants were exposed to 0, 0.25, 6, 12, 24, or 96 hours of drying at either 60% (low) or 90% (high) relative humidity. This was followed by a final wet interval of 24 hours. After exposure to the final wet period, they assessed the proportion of ungerminated conidia and germlings (i.e., germinated conidia) with or without an appresorium that were killed.

Results of this study showed that ungerminated conidia were not killed by exposure to dry intervals until drying exceeded 96 hours within the range of temperatures and relative humidities studied. Germlings with or without an appresorium were more sensitive to drying than ungerminated conidia. Twenty percent of germlings were killed after the first 15 min. of drying and an additional 10-30% after 96 hours. Germlings with appresoria were killed after 24 and 96 hours, too, but the attrition rate was lower than for germlings without appresoria. Even after 96 hours of drying, over 75% of ungerminated conidia and germlings were still able to penetrate the apple leaf during the second 24-hour-long wet interval.

Becker and Burr proposed the following rule based on their results: "If the interval of drying is less than 48 hours in length, the initial and subsequent intervals of wetting should be summed to calculate Mills infection periods." This rule is more conservative than the "typical" rule of 'summing wetting periods separated by less than either 8 hours of sunny weather or 12 hours of cloudy weather.' Where did this rule come from? In a review of the scientific literature MacHardy found NO scientific basis for the establishment of this rule. In fact, nearly all the research that has been conducted shows that a high proportion of both ascospores and conidia survive drying periods of 24 hours or more whether it is sunny or not. Has this rule worked in the field? Perhaps. If it has, though, it is not because spores have died after only 12 hours of drying.

Several other factors affect the amount of disease that develops after a predicted infection event. These include: (1) the amount of primary inoculum in an orchard, assessed via PAD counts in the fall; (2) the stage of development of ascosporic inoculum in the spring, assessed via squash mounts, spore traps, and degree day model calculations; (3) the time of day and season when rain and leaf wetness occurs because ascospore discharge occurs during daylight whereas conidia can be disseminated anytime by splashing rain; and (4) whether the principal source of inoculum is ascospores or conidia.

MacHardy suggests a less conservative rule than Becker and Burr's to follow for combining successive wetting periods: "two successive wetting periods, the first started by rain, should be considered a single, uninterrupted wet period if the intervening dry period is less than 24 h, regardless of weather conditions (sunshine, temperature, and RH) during the intervening dry period." This rule, in our opinion, should be the rule adopted by NY growers. This rule is easier rule to apply, slightly more conservative, and, most importantly, consistent with the results of research.

Day vs. Night Release of Ascospores - This issue is separate from the use of the revised table, but is related to its application in low-inoculum orchards. Extensive research has shown that under most circumstances, over 95% of the available ascospores are not released until after sunrise when rain begins after sunset. Thus, in low inoculum orchards, it is often safe to assume that primary infection periods begin at dawn when rain begins at night. This assumes that the low percentage of night-released ascospores, coupled with the already low seasonal “crop” of ascospores at low-inoculum sites, is insignificant. However, secondary spores (conidia) are not affected by light or darkness. Therefore, DO NOT ignore wetting hours during darkness if scab infections have already been observed in the orchard or if you are not certain of excellent control for all previous infection periods.

From: B. Turechek, J. Carroll and the Cornell Tree Fruit Guidelines

References:

Becker, C.M., and Burr, T.J. 1994. Discontinuous wetting and survival of conidia of Venturia inaequalis on apple leaves. Phytopathology 84:372-378.

MacHardy, W.E. 1996. Apple Scab: Biology, Epidemiology, and Management, APS Press, St. Paul, Minnesota, pp. 545.