Lisa here. Monday morning I was so wrapped up in our Eco-Apple conference call that it slipped my mind to take a moment to leave the call and start the recording. So unfortunately there is no recording of this week’s call. I do apologize to anyone hoping to hear the call. In light of my mistake, I have attempted to do a more detailed summary of the call this week. The discussions generally revolved around monitoring for and treating scab.
John Aue’s general observations about the state of WI orchards at this time:
Across the southern 2/3 of Wisconsin trees range from dormant to silver-tip to half inch green. Although most trees have experienced very little growth the last seven days, the growers expect this to change with the warmer temperatures forecast this week. Some farmers have seen frost damage to outer leaf scales, but the buds themselves appear to be fine. While most orchards at green-tip or beyond have experienced at least one rain event since bud-break, John hasn’t been informed that any have resulted in an actual scab infection.
John also noted that there had been an error in his calculation of a Sauk County orchard’s spore maturity data that was previously posted on Apple Talk. We will revise the posting soon.
Bob Willard questioned whether we needed to begin the degree-day model for scab prior to MacIntosh green-tip, as he understood the model was actually developed with Mac green-tip as a starting point. John reiterated that, based on George Sundin (Michigan State University plant pathologist) and others finding apple scab developing on warm days in February, January seemed a more appropriate time to begin the model, although it was likely that in most years calculating degree-days from March 15 would yield very similar results. Patty McManus remarked that North Carolina uses February 1 for its start date. She also stated that it is hard to estimate the impact of warm days in January on scab spore maturity levels, but that some spores will certainly begin to mature.
Jim Lindeman asked: Since the start date is based on Macintosh, is there any information on whether or not things are different for other cultivars?
John and Patty’s answered that a Macintosh is a safe bet to use as a model tree because it’s a common variety that is highly susceptible to scab. What is more critical than the variety is the amount of inoculum in the orchard, the timing and coverage of fungicide applications, and maintaining accurate weather data (rain events, leaf wetness hours, and temperature) to gauge the relative severity of particular infection periods.
Ron Schaeffer remarked that he starts his model on the first day it hits 50 degrees, and that his data shows that he has had an infection period.
John confirmed with Ron that he had had no green tissue during the infection period, thereby negating the importance of those periods. As Ron’s infection periods were based on a wetness threshold of six, a discussion of wetness threshold levels ensued. John questioned whether a six, five, or three is best, and said he would talk to Cynthia at Spectrum Technologies about any data they may have on this topic.
John noted that as we’ve had more problems controlling scab over the last six years or so, we’ve recognized that one of our weak points may be in our control of early season infections, whether they result from an ascospore (primary) infection or (probably less likely) overwintering conidia (secondary scab spore). The ability to accurately predict ascospore maturity (chiefly that is knowing when to begin the model) could better direct costly fungicide applications. He will be comparing weather and early season scab incidence data from a number of orchards across Wisconsin that have yet to apply a fungicide this spring. He is hoping the results will, at a minimum, reflect on how important the scab model’s start date was this year.
Patty said, putting aside all discussions of when to start the degree day model, it is clear that at this point it is definitely primary scab season. She then asked John about what scab treatment regimes or materials have been losing efficacy. John replied that while it is clear that the SI’s are generally much less effective than when first introduced, some growers are having trouble keeping innoculum down even when using advanced protectant strategies. He said that some growers have perhaps been fooled by clean fruit in the fall, not realizing that their leaves were full of innoculum. He knows some growers who are attempting to reduce the inoculum level with cultural controls such as chopping the leaves very finely, or applying lime or urea. The rate of lime applied is 2.25 tons/acre, which, as a regular application, is unsustainable for growers with alkaline soils, but could, on a one time basis, be part of a more comprehensive scab reduction program.
Ron Schaeffer then remarked that when he goes through his orchard he sees very few intact leaves. They seem on the whole to be well decomposed. He doesn’t see any scab on these leaves, but he does see some black spots. He wondered what this could mean for the ability of said leaf matter to contribute to scab problems.
John replied that sometimes the scab will come even when you can’t find scabby leaves on the floor. He recommended that Ron try to incorporate his leaves into the soil. Patty remarked that scab on leaf matter is very diffuse and therefore very hard to see. It is possible that the black spots he’s seeing are scab. John and Patty agreed that an orchard’s scab innoculum is never so low that a grower won’t need to manage it.
