Scaffolds 00 index

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

Apple scab ascospore counts as determined from squash mounts:

The scab squash mount assessment completed on the morning of April 6 showed the percentage of mature spores had increased from 2% on Monday to 9% on Thursday. With the warm weather that persisted through Saturday evening, maturity will by now (April 10) have reached the threshold where significant infections can be expected from the next infection period.

(Wayne Wilcox, wfw1@nysaes.cornell.edu Plant Pathology, Geneva)

Before stone fruits are in bloom and the rain comes, it's time for the annual review of brown rot control for blossoms and fruit. Remember that brown rot likes it warm and wet. Ideal temperature is about 70 to 77°F. If inoculum of the brown rot fungus is plentiful, significant infection can occur after only a few hours of wetness at these temps, but as it gets colder and/or inoculum gets scarcer, progressively longer periods of wetness are required. For instance, we've used 12 hr of wetness at 60F and 24 hr of wetness at 50°F as "ballpark estimates" of what's needed for significant infection of tart cherry blossoms under commercial conditions, but these figures are merely educated guesses based on the results of greenhouse experiments. We also have found that blossom blight is much more common when humidity is high during the next few days following a rain, whereas low humidity during this period reduces disease development.

Where It Comes From. In relative order of importance, the main sources of blossom blight inoculum are mummies retained in the tree, mummies on the orchard floor, or nearby wild or abandoned stone fruit trees. On peaches and nectarines, twig cankers that developed from infected fruits last summer also can be very important. As a general rule, if you had more brown rot than usual last summer, expect to have more inoculum than usual this spring. Conversely, orchards and districts that were unusually clean due to drought conditions may experience less inoculum pressure than average.

Spores for fruit rot can come from these same sources, in addition to blighted blossoms, other diseased fruit in the tree, or thinned fruitlets on the orchard floor (peaches thinned before pit hardening cause less problems than those thinned afterwards).

Fruit Species Susceptibility. Apricots and sweet cherries seem to be the most susceptible to blossom blight, sour cherries the least. Peaches and plums are somewhere in the middle. Remember that the very young fruit (before pit hardening) of all species except sour cherries are fairly susceptible to infection during wet weather conditions, and the usual influences of temperature and inoculum pressure should be considered when assessing the need for control. Sometimes these early infections show up immediately, but often they remain latent until the preharvest period, when they suddenly seem to appear from nowhere.

Thus, the first few weeks after petal fall can be an important time for protecting young fruitlets. However, many labels make no provision for brown rot control between petal fall and preharvest, although some do allow sprays for control of other diseases (peach scab, cherry leaf spot) during this time. Read the labels carefully and treat as needed.

Fungicide Choices. There are several general categories of fungicides now available for brown rot control:

• Protectants (captan, Bravo, sulfur). These must be present before a wetting period occurs and need to be reapplied fairly regularly (depending on the material) if they are washed off by rain. Captan is not the best material for blossom blight, but is often good enough unless disease pressure is high. It's also one of the more economical choices. The drawbacks to captan are: the 4-day REI; it's phytotoxic to some sweet cherry and plum varieties; and new rates established in the '90s are not always effective. The 4 lb./Acre rate for cherries is inadequate for large trees, especially for highly susceptible sweet cherries.

Bravo has several positive and negative properties. It is less prone to wash-off than captan, but it's more expensive and can't be used beyond shuck split. It is also the best material available for control of black knot on sour cherries and plums. If weather is wet at petal fall/shuck split and disease pressure is high, a Bravo spray should provide good residual protection against fruitlet infection and black knot.

• Dicarboximides (Rovral and Ronilan). Although once the Cadillac materials for brown rot control, these fungicides are in their golden years. Ronilan is no longer allowed on stone fruits, and Rovral use is limted to the blossom period only. Rovral provides both protectant and limited postinfection activity (about 48 hr at 68F). Significantly, it also interferes with the production of brown rot spores from infected blossoms, thus slowing the spread of an epidemic. However, it's rather expensive. Although it's nice to have alternatives to the SI fungicides for resistance management purposes, cheaper alternatives (e.g. captan) are more attractive unless disease pressure is high.

• Sterol inhibitors (Elite, Indar, Orbit). Elite is labeled only on cherries, peaches, and nectarines. Indar is labeled on these crops plus apricots. Orbit is labeled on all all of the major stone fruits; i.e., apricots, cherries, nectarines, peaches, and plums (except "Stanley type" plums). All three are labeled for brown rot control during bloom and again 2—3 weeks before harvest, but not during the potentially important period between petal fall and pit hardening. However, during this period Elite may be used on cherries to control powdery mildew and leaf spot; Indar may be used on cherries for leaf spot and on peaches for scab; and Orbit may be used on any listed stone fruit for powdery mildew and leaf spot. All three fungicides may be applied up to the day of harvest.

In my trials over the years, each of these SI's has provided excellent control when sprayed fruit have been challenged with brown rot spores shortly after the final application of the season. However, Indar has consistently shown the best residual activity. In one typical trial, when sour cherries were inoculated with brown rot spores 9 days after the final spray, disease incidences were: unsprayed = 72%; Elite = 24%; Orbit = 17%; and Indar = 1%.

For resistance management purposes, make an effort to rotate the SI's with a non-SI during the season. Using SI's exclusively won't burn them out over a single year, but it all adds up after awhile. Researchers in South Carolina recently showed that populations of the brown rot fungus are becoming less sensitive to these materials in some of their orchards, and experience in some N.Y. apple orchards shows that SI fungicides can start losing their effectiveness over time. So, as with most pesticides, don't rely on just one class of chemistry to do the job.

• New fungicides. Vangard (cyprodinil) is a member of a new group of fungicides (the analinopyrimadines) not currently represented by any other products in North America. It's labeled on all of the major stone fruits EXCEPT sweet cherries (phytotoxicity to the fruit stems has occurred), but only for control of blossom blight. It is NOT labeled for fruit rot control, except in California. We have not been able to adequately evaluate its control of blossom blight in our trials, but have not been impressed with its activity against fruit rot. (In fairness, we've evaluated other materials that are much more effective against blossom infections than they are against fruit infections). Elevate (fenhexamid) is the first representative of yet another new chemical class. Federal approval of its proposed label for brown rot control is still pending. It has consistently provided poor control of fruit brown rot in our sour cherry trials.

> 4.10 Insects