Fire blight outlook

CAUTION: The observations, conditions, and recommendations reported for Winchester, VA are provided as a guide to fire blight risk assessment only for the immediate area of the Virginia Tech AREC located six miles southwest of Winchester. Use of the information reported here for making orchard management decisions outside of that area is not our intent. Fruit producers outside of that area are encouraged to consult their state extension specialists for information similar to that provided here.

FIRE BLIGHT: Above is a cropped graphic from the Maryblyt 7 program. With many cultivars with bloom open, others are at full pink, and more bloom opening gradually over the next several days, here is a prediction for trees with first bloom open Friday, March 23, 2012. The temperature and rainfall data are current through Friday morning, March 30. Predicted weather conditions are shown for March 31-April 3. The components of fire blight risk are indicated in the columns labeled B (blossoms open), H (degree hours for epiphytic bacterial populations), W (wetting by rain or dew), and T (average daily temperature 60 F or above). Based on predicted temperatures and wetting, the risk column shows that risk of fire blight infection will be mostly low or moderate through Apr 3, but higher if wetting occurs with showers Apr 1. For infection to be predicted, wetting must occur after the EIP (epiphytic infection potential) reaches 100 or higher. Note there had been an earlier threat of fire blight on apples at our AREC last weekend, but temperatures fell about 3 degrees cooler than would have been required, based on bloom first open Friday, March 23. For pears which had been in bloom earlier in the week, or apples in areas with bloom open Mar 22, infection would have been possible with wetting Saturday, Mar 24. In high-risk situations, a protective streptomycin application is recommended ahead of predicted infection.

This graphic will be updated April 2, or sooner if significant change warrants it. The weather conditions used in the predictive part of this graphic come from the Weather Channel for Winchester, supplemented by site-specific data from SkyBit Inc.

Scab and rust infection period

As predicted Saturday, extended wetting from March 24-26 resulted in scab, quince rust and cedar-apple rust infection periods at our AREC. A split wetting period was recorded with 8 hours of wetting at 61 degrees, followed by four hours of marginal wetting/drying, then 13 more hours of wetting from 55 to 48 degrees and 3 hours of intermittent wetting. An inch of rainfall, early in the wetting period, likely depleted the residual effects of any protective fungicide residues applied before these events. Cedar-apple and quince rust spores were released within four hours of the start of the wetting period Saturday. For the record, wind direction from potential cedar tree rust spore inoculum sources during the spore discharge period, was mostly from the east or northeast.

In areas where rusts are a common problem, it is advised that an SI fungicide be included in the mix to be applied this week for after-infection control of rusts; in areas where scab is resistant to the SIs, other fungicides are advised to be included for after-infection and protective control of scab and suppressive effects on powdery mildew which infected last week and will continue this week.

The threat of fire blight on apples did not materialize at our AREC, with temperatures about 3 degrees cooler than would have been required, based on bloom first open Friday, March 23. For pears which were in bloom earlier in the week, or apples in areas with bloom open Thursday, infection was possible with wetting Saturday. Cool forecasted temperatures should minimize any threat of fire blight for the rest of this week.

Threats of fire blight, rusts and scab.

Scab ascospores and rust spores are mature, and there are threats of cedar-apple and quince rusts and scab infection with the forecast of relatively warm, intermittent showers through tomorrow. Where bloom was open yesterday and there was wetting last night, there is also a threat of fire blight infection. Mildew infection occurred with moderate temperatures throughout this past week.

Scab infection period

Wetting last night at our AREC gave us less than 0.01 inch of rainfall, but it stayed wet 14 hr at 57-67 degrees, and resulted in our first apple scab infection period of 2012. As noted in yesterday's post below, the spore target was big for many varieties. Other areas of Frederick County had more rainfall. With the relatively warm temperatures, cedar-apple rust and quince rust spores matured quickly, but probably did not infect last night, at least in our situation. Blossoms beyond tight cluster with their bases exposed would be susceptible to quince rust. Also as noted yesterday, mildew spores were available and infection has already occurred. So these constitute the present apple disease control spectrum concerns. But with the way it's going this year, we may be concerned about fire blight on early bloom before we know it.

It's a new season!!

Bud development of apple trees is mostly at tight cluster stage and peaches are near full bloom.

Apple scab ascospores were first trapped last Friday, March 16, but it was not wet long enough for infection to occur. Today abundant powdery mildew spores are present on emerging infected tissue, as shown below, and conditions are favorable for mildew infection.

Here is a healthy Idared terminal bud on the left; and a mildew-infected terminal bud on right. Mildew spores on emerging leaf tissue (below) are being dispersed for infection today.