Disease update: apple scab, rusts and fire blight

In Winchester, we recorded a heavy apple scab and rust infection period May 5-7 with split wetting totaling 37 hr at 51-60° (mean 53°), with 0.95 in. rain. Also, many cedar-apple rust and quince rust spores were released during this extended wetting period, and flowers remain susceptibility for quince rust infection. In areas where there is a concern for rust infection, include an SI (DMI) fungicide for after-infection control in the next spray application. Rust galls remain active for more spore discharge with additional wetting. Cedar-apple rust lesions have been observed from the infection period Apr 24-25, and scab lesions should be expected this week raising the possibility of secondary infection where scab was not well-controlled at that time. Powdery mildew conidia have been available for infection at our AREC since Apr 10. Any “dry weather “ day above 53° is suitable for mildew infection and we have had 18 mildew infection days.

FIRE BLIGHT CAUTION: While we are past the peak of apple bloom in the Winchester area, susceptible bloom remains on many varieties. Below are the fire blight risk assessments, comparing graphics from Maryblyt and also from the Cougarblight model as shown on our NEWA site, using the same weather data from our NEWA station to make these comparisons. 

Graphic from Maryblyt 7, May 8, 2018. Click to enlarge.

Above is a cropped graphic from the Maryblyt 7 program, with first bloom open Apr 14. The temperature and rainfall data are current through Monday evening, May 7. Predicted weather conditions are shown for May 8-12. 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, dew or a spray application), and T (average daily temperature 60°F or above). For infection to be predicted, wetting must occur after the EIP (epiphytic infection potential) reaches 100 or higher, and this must coincide with an average daily temperature of 60°F or more. Based on recorded temperatures and wetting, the risk column shows that infection conditions occurred May 5 and 7, and are predicted for May 10 and 11. High risks are shown for all of the other days between May 2 and May 12 with only wetting lacking for infection May 9 and 12. Because a spray application can serve as the wetting trigger, include streptomycin with spray applications May 8-12. Remember to protect young and recently planted trees, with flowers but considered to be "non-bearing".

The BBS column in the above graphic is tracking predicted symptom development for the first infection May 5, with 64% of the degree hours required for blossom symptom prediction. The CBS column indicates progression toward the appearance of canker margin symptoms due to extension of overwintering cankers from last year, which is predicted for May 8. Canker advancement cannot be prevented by chemical treatment at this time and the presence of symptoms will signal build-up of inoculum which could become a factor in the event of a trauma blight/shoot blight situation due to hail injury, etc. To offset the potential for shoot tip infection in an active fire blight year such as this one, apply the plant growth regulator, prohexadione-calcium (Apogee, Kudos), at petal fall on king bloom. Shoot blight suppression results from hardening off of vegetative shoot growth starting about 10 days after the initial application.

Below is the graphic from the Cougarblight model as shown on our NEWA site. We selected orchard blight history option as “Fire blight occurred in your neighborhood last year” and first blossom open date as 4/14/2017. Cougarblight shows color-coded risk assessment as “Cougarblight 4-Day DH” risk is extreme for May 6, 7, and 10-12, and EIP infection conditions are shown for May 7, 10 and 11.

Graphic from Cougarblight, May 8, 2018. Click to enlarge.

Similar fire blight risks prevailed the past several days throughout areas of Virginia wherever susceptible bloom was present and wetting occurred. Although the length of wetting was more variable across the region and some areas did not receive the scab infection period, these areas still may have had enough wetting for fire blight infection.