Scaffolds 01 index

(Dave Rosenberger, dar22@cornell.edu, Plant Pathology, Highland)

Apple scab ascospore counts as determined from squash mounts:

Commercially significant spore releases usually begin soon after spore our counts show 15% mature spores. The spore counts shown above are consistent with the conclusion drawn from the Hudson Valley count reported last week: Spore maturity is advanced this year compared to tree phenology. Spores from Saratoga County were not yet ready to discharge on April 9, but trees in that area were not yet at green tip at that time. (Kevin Iungerman reports that they are at green-tip in Saratoga County as of today, April 16). The low number of spores in the tower shooting test for the Orleans Co. location indicates that most of the "mature" or colored spores observed in that squash mount evaluation were not quite ready to discharge. However, a day or two of warm weather is often enough to change "mature" but non-discharging spores into spores that will discharge with the next rain. The April 16 tower-shoot counts for Highland show that large numbers of ascospores are ready to go with the next rain in the Hudson Valley. Given the advanced levels of spore maturity evident in all the counts completed to date, I would advise that early infection periods will pose a greater-than-average risk of significant scab infection this year.

(Bill Turechek, wwt3@nysaes.cornell.edu, Plant Pathology, Geneva)

On your mark... GO! Apple scab ascospore maturity counts are sufficiently high to suggest primary-scab inoculum will be plentiful at green tip. There is little time to 'get set'. Growers should be prepared to apply a protectant fungicide (i.e., copper, mancozeb, Polyram, captan) at green tip. This is especially critical for those orchard blocks that have had appreciable scab in the past few years. Copper is recommended at green tip in orchard blocks with a history of fire blight (see Dave Rosenberger's article in Scaffolds Vol. 10, No. 2). The first half of the week of April 16th is forecast to be cool and wet. It will dry out toward the latter half, providing suitable conditions for your green tip application.

(Dave Rosenberger, dar22@cornell.edu, Plant Pathology, Highland)

Ascospore maturity assessments have been used for many years to predict the beginning of the apple scab season. In most years, mature ascospores of V. inaequalis are present and ready to be released soon after apple trees reach the green tip bud stage. However, in some years ascospore maturity is delayed compared to apple bud phenology. When that occurs, apple growers can omit one or more early season fungicide sprays with minimal risk of developing apple scab. Conversely, in years when ascospore maturity is advanced compared to apple bud phenology, early season sprays for apple scab may prove essential for controlling scab. Ascospore maturity assessments provide growers with one predictor for differentiating high versus low risk for early season scab infections. As noted toward the end of this article, there are numerous other factors that contribute to severity of apple scab in any given year.

For the 2001 season, the New York State IPM program is funding early season ascospore maturity counts for all of the major apple production regions in New York. Ascospore maturity counts have been conducted continuously in eastern New York for many years, but maturity counts in other regions were discontinued several years ago when the technician who performed the counts at the Geneva Experiment station retired. Conducting ascospore maturity assessments is a tedious process. Skilled and experienced observers produce more consistent results than novices. Fritz Meyer, a plant pathology technician at the Hudson Valley Lab, has been performing squash mount assessments for more than 25 years and will be performing all of the IPM-funded counts for New York State in 2001.

Assessments are done by picking 20 pseudothecia (the small fruiting structures) out of overwintering leaves and squashing them on microscope slides. Pseudothecia usually contain anywhere from 80 to 150 asci. Asci are transparent cylindrical sacs that each contain eight ascospores.

The ascospores are clear when immature, but turn an olive-green color as they mature. Maturity assessments are conducted by observing the squash mounts at 400X magnification and counting the number of asci that have clear spores (immature), colored spores (mature), or no spores. Asci in the latter category must be further differentiated between closed asci, which means the asci are so immature that no spores have yet formed, and empty asci from which spores have already been discharged.

In microscopic assessments, squashed pseudothecia appear as a jumble of overlapping asci. The technician conducting the counts must visually scan the field of view in the microscope while clicking off on a counter the maturity category for each ascus that is observed. There is no way to "mark" the asci already counted, so the observer must mentally track which asci have already been counted. The process is somewhat analogous to trying to count the number of branches in a small brush pile without touching any of the branches. Consistency in scanning and enumerating the asci as seen through the microscope is critical for generating useful squash mount information.

The tower shooting test (or ascospore discharge test) is usually conducted at the same time as squash mount assessments and provides an independent evaluation of spore maturity. In the tower shoot test, leaves are wetted and placed on a screen about 16 inches above a plenum through which air is drawn by a vacuum pump. Spores discharged from the wet leaves are trapped on greased slides just below the holes in the plenum. Counting the number of spores trapped provides an estimate of whether or not leaves are actually discharging spores. However, these counts are of limited utility for several reasons. First, there is no quantitative way to assess how many pseudothecia are present in the leaves used for the tower shoot. A sample of heavily scabbed leaves will always produce more spores than a sample of lightly scabbed leaves, so there is no way to establish an action threshold using tower shoot counts. With moderately scabby leaves, counts in the tower shoot as we conduct them at the Hudson Valley Lab must usually exceed 40 to 50 spores before significant infections can be expected in commercial orchards. However, that threshold is very much a "fuzzy logic" rule of thumb. Another problem with tower shoot counts is that leaf samples collected during or shortly after a rain will discharge relatively few spores in the tower because all mature spores will have been released during the rain. For these reasons, we use the tower shoot only as a supplement to squash mount assessments.

An alternative approach for assessing ascospores involves a degree-day model that was developed by Drs. David Gadoury and Bill MacHardy (1982). Using this model, degree-day accumulations are initiated when 50% of McIntosh fruit buds are at green tip. The model is useful for predicting the period of peak ascospore discharge and the end of the ascospore production season. However, reliability of the model for predicting spore maturity during the first week or two after green tip is questionable. The model uses the green tip bud stage as a biofix, so it cannot compensate for years when ascospore maturity is unusually advanced at the time that trees reach the green tip bud stage. Because years with advanced spore maturity pose the greatest scab risks for apple growers, we decided to initiate statewide squash mount assessments in 2001.

Ascospore maturity is only one of many factors that determine when and where apple scab will cause commercial problems. The amount of overwintering inoculum in an orchard is far more important than relative maturity of ascospores during any given infection period. If ascospore production and release is compared to water flow through a hose, then the amount of overwintering inoculum could be compared to the size of the hose and relative maturity of ascospores could be compared to the time that the valve feeding the hose is turned on. Obviously, the volume coming from a 3-inch fire hose (i.e., a high inoculum orchard) will be much greater than the volume from a half-inch garden hose (i.e., a low-inoculum orchard). Furthermore, spore concentrations will reach economic thresholds earlier in the season in high inoculum than in low inoculum orchards, just as a 3-inch hose can deliver 100 gal of water more quickly than a half-inch hose when both hoses are activated at the same time. Thus, growers with high-inoculum orchards must always be more conservative with early season sprays (i.e., start earlier and keep tighter prebloom spray intervals) than growers with low-inoculum orchards.

Other factors that affect scab severity in any given year include the timing of the first rains that produce an infection period, the weather during bloom and petal fall, and weather during the early part of summer. The timing of rains during the prebloom period may be the most critical determinant in scab development because every day of dry weather after bud break delays scab development in trees by one more day. In years when no infection periods occur prior to tight cluster or pink, scab is often easy to control, even in high-inoculum orchards, because the earliest primary infections occur too late in the season to allow multiple cycles of secondary inoculum before fruit begin to lose susceptibility to scab. Dry weather during bloom and petal fall can similarly delay secondary infections and decrease the number of scab cycles that occur in a year. Hot, dry weather during early summer reduces viability of conidia and speeds terminal bud set on trees. Older leaves are relatively resistant to scab until autumn, so scab epidemics usually end when terminal buds are set.

None of the factors in the preceding paragraph are predictable when growers must make decisions about spraying fungicides at green tip. The squash mount assessments of spore maturity, coupled with a grower's or fieldman's assessment of inoculum levels in the orchard, provide the only two parameters for assessing potential risks of early season scab infections. The wet summer last year contributed to high levels of carry-over inoculum, and scab counts completed to date show scab maturity is ahead of tree phenology compared with an average year. Thus, this may not be a good year to gamble on omitting early season scab sprays.

Literature Cited

Gadoury, D. M., and MacHardy, W. E. 1982. A model to estimate the maturity of ascospores of Venturia inaequalis. Phytopathology 72:901-904.

More information: See apple scab disease cycle at: http://www.nysaes.cornell.edu:80/ipmnet/ny/fruits/FruitFS/tree.fr.dis/as/as.cycle.html