2016 was the first year in a long time to have apple scab (Venturia inaequalis) arriving so late. This fungus affects apples as a skin blemish and reduces crop by as much as 50%. The scab has 2 parts to it’s infection-one: the overwintering leaves on the ground and two: the leaves and fruit on the tree. The 1st part-leaves on the ground can be reduced by sanitation or decay (by Urea fertilizer to enhance decay). So what happened in 2016? The stage 1-leaves on the ground- disseminated 100% of the ‘ascospores’ by April 23rd – based on the biofix model using 900 growing degree days (base 32° F.). During the infection period of “1/4 inch green”, before blossom, to waxy cuticle (when the leaves resist infection, about July) the leaves/flowers/fruit need to be wet for a certain period of time at a certain temperature – higher T.= shorter wetting period (WP) to germinate the ascospores on the fruit/leaves. April 23rd we had such a WP, but it was very close to failure due to drying conditions. Only interior foliage (slow-drying) was affected on susceptible varieties. Bloom was complete, and fruit 1/4 inch.
Ok, my question, now that all the ascospores have been disseminated and are floating around, I am to assume an air sample in the orchard would be higher a month ago and that today the ascospore count in the atmosphere around the orchard is going to be quite low due to the air exchange/breezes. So far we have had a short, late wetting period (based on the Mills Chart) that offered little germination on the fruit. Cool. Clean, clean, clean! But what if there is another WP? Is the ascospore count in the air we breath high enough to perpetuate the (now thought to be insignificant) scab spore? I would love to see an air sample!
Today, May 28th, about a month after the WP, we have another WP-through the last week mostly-rainy. I will be looking for increasd infection on fruit/leaves. Lime Sulfur spray will interfere with ascospore and conidia germination as much as 36 hours after a WP. Applied at 1.25 to 1.5 gallons per 50 gallons (my sprayer capacity) is the safe rate but don’t get it in your eyes as it’s caustic and smells of sulfur (bad eggs).
Avoiding Lime Sulfur altogether would be my aspiration as it is hard on beneficial soil fungi. The quest is to use compost tea with sufficient beneficial microbes to outnumber the ascospores-known as “competitive colonization”.
==========================MODEL INPUTS================================ Model species/general links: apple maggot % emerge [cherry, apple] Type: insect Model source/other links: Jones etal 89 Calculation method: single sine Lower threshold: 44 degrees Fahrenheit Upper threshold: 113 degrees Fahrenheit Directions for starting/BIOFIX: initialize at first emergence from traps No starting/BIOFIX date, set to: default date 6 15 No ending date, set to: default date 12 31 Model validation status: testing Region of known use: UT and OR ==============================EVENTS TABLE================================ 1. 318 DD after 1st emerge 5% fly emerge 2. 541 DD after 1st emerge 25% fly emerge 3. 797 DD after 1st emerge 50% fly emerge 4. 1046 DD after 1st emerge 75% fly emerge 5. 1252 DD after 1st emerge 95% fly emerge 6. 1694 DD after 1st emerge 99% fly emerge