Sooty Blotch and Flyspeck (SBFS) are caused by a group of fungi which grow superficially on waxy cuticle of apple fruit. Usually, flyspecks caused by Zygophiala jamaicensis show first and then the sooty blotches which are caused by a complex of species including Schizothyrium spp. These symptoms reduce fruit quality as surface blemishes that reduce market value (Figures 1 & 2).
Dr. Kari Peter also reports that sooty blotch can shorten fruit storage life because it allows increased water loss. In most years, according to Dr. Rosenberger we have operated under the assumption that there is reduced risk from this disease between PF and the time until an orchard accumulates 190 hr of wetting (counting from FC onward and using NEWA data for your specific location). Looking at the HVRL’s NEWA, leaf wetting data in May summed up to 195 h, then knowing we had lingering PF during 2.5 first weeks of May, and then we got 108 h of wetting in June, we surely did surpass 190 hr of wetting to allow abundant flow of SBFS spores into apple orchards. The spores are coming from near-orchard vegetation in hedgerows and forest trees lots where the SBFS fungi overwinter on the waxy cuticle of canes, shoots, twigs, fruit and leaves of large number of inoculum reservoir plant hosts. From there, SBFS spores are spread to pome fruit by wind or wind borne rain aerosol drops. In worst cases, such the 2017 seems to be due to so much rain we had, SBFS infections can start on fruit as early as 2-3 weeks after petal fall. Symptom occurrence after infections are initiated is of course all dependent on environmental factors and can take place for several weeks to months later. I think we are up against the former i.e. harder scenario in 2017, where moderate temperatures at and after PF with prolonged leaf wetness periods and high RH will trigger symptoms to appear sooner than later if cover sprays with fungicides were not applied on a tighter schedule than usual 14-21 days we are all used to do.
Now, 2017 so far was very wet in terms of wetting hours with a lot events with 2″ or near-that amount of rainfall. We probably already passed 190 hr of accumulated wetting from PF for most farms. So if on your farm you reached 190-hr of accumulated leaf wetting from PF, which I assume most of you did, SBFS spray program should have started. After you start(ed) your SBFS program, then you should respray at 14-21 day intervals, with the 14-day interval to be used if by those 14 days you have already accumulated 2 inches of rain since the last spray. The longer interval of 21 days should be used for dryer periods with less than 2-inches of rain. If you reach 2-2.5 inches of rain before the 14-day mark from the last spray – you should re-spray.
In NY and New England, we almost never see sooty blotch until after flyspeck has been visible on fruit for a week or more. If you control flyspeck (FS) well, you will also control sooty blotch. This is because sooty blotch (SB) is controlled by lower amounts of fungicide than the flyspeck (Dr. Rosenberger, personal communication). The one exception, when SB can show up before FS is if heavy rains late in summer remove all fungicide residue and the orchard owner does not respray before harvest. This happens if 2.5-3.5 inches of rain fall in usually singe storm event. In commercial orchards, we have to make sure that there is almost always enough fungicide residue to slow down SB at some point during the 190 hr incubation period for FS (NEWA). If rain reduces fungicide residues, FS will show up first because it can grow through a bit more fungicide residue than SB.
In the lower Hudson Valley, if by any chance we had a drier spring and early summer (which we did not), start of your SBFS spray program must not be delayed after July because you need to spray starting no later than July 10 to prevent summer fruit rot infections. From 10 July, infection pressure from summer rots gets very high.
In our unsprayed blocks at HVRL the SBFS symptoms have not expressed yet. We will keep you posted when they do.