Apple Scab/Mildew Threats

Monday afternoon, May 12, 2014:  According to the best apple scab models, major ascospore discharges and infection periods occurred April 29-May 2 and again May 8-11.  If current weather forecasts are accurate, we will experience the third really major apple scab infection period with the rains that are predicted for May 15-18 (the upcoming Thursday through Sunday).

Scab lesions resulting from infections that occurred April 29-May 2 should become visible tomorrow or Wednesday.  That means that any orchards where fungicide coverage was less than perfect for the April 29-May 2 infection period may have conidia available for causing secondary infections during future infection events.  Thus, rains predicted for the end of this week pose a triple threat for apple scab:
1.  High levels of ascospore discharge if the orchard had over-wintering inoculum.
2.  Potential for secondary spread if fungicide protection earlier this year was less than adequate.
3.  Apple trees at their most susceptible phenological stage (many new terminal leaves, plus significant potential for fruit infections).

It may be tempting to stretch fungicide coverage this week in hopes of delaying the next fungicide until petal fall sprays can be applied.  Given the high risks associated with the next infection period, that could prove to be a costly mistake.  To avoid problems with scab throughout summer, be sure to have fungicide coverage in place for rains predicted for later this week.

Those relying on protectant programs of mancozeb plus captan should have already applied at least one mildewcide.  Where no mildewcides have been applied so far this year, a DMI fungicide (Rally, Topguard) should be considered in the next spray because none of the other mildewcides will provide both post-infection activity against mildew and excellent activity against rust diseases that also pose a high risk for fruit infection during late bloom and petal fall.

The only situations where DMIs may not be the best choice for mildew/rust control are orchards where mildew is suspected of being resistant to DMI fungicides.  Orchards in the latter category should be protected with Flint, Merivon, or Luna Tranquility (or Luna Sensation outside of New York State).  Fontelis is a good mildewcide if applied ahead of infections, but its ability to “catch-up” where early mildew sprays have been omitted is questionable.  Unfortunately, the non-DMI alternatives for powdery mildew are generally less effective than the DMIs against rust diseases.  Most of them, including Fontelis, will protect against new rust infections, but none of them can eradicate existing rust infections like the DMIs are capable of doing.

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