Green tip has arrived in many Vermont orchards

By Terence Bradshaw

Yesterday, April 9, we ‘called’ green tip at the UVM Horticulture Farm, orchards. This signals that buds are beginning to open, and therefore green tissue is present that may be susceptible to apple scab infection. The apple scab ascospore maturity model, which predicts the presence of mature overwintering inoculum that could infect, given appropriate weather conditions, uses ‘McIntosh’ green tip as a biofix to start accumulating degree days. It is best to record your own green tip date, and if you are using NEWA to track infections, use the station that is closest to your farm (or buy your own station, and I can help you to connect it to the NEWA network).

Presently, the orchards in the warmest parts of the state (Bennington and Putney) are predicting 2-5% of ascospores mature; in Addison, Chittenden, and Grand Isle counties, it’s more like 1-2%. Inland and upland sites likely aren’t even at green tip yet, but will likely advance once the warm weather returns. What does this mean? In warmer areas, I would get copper on as soon as you can get in- remember, after the buds reach ½” green tissue, your window for high-rate copper is closed. Wind is looking pretty fierce on Monday (rainy too) and Tuesday, and both of those days look to be in the 50s-60s. So I’d try to get some copper on this weekend. Hold off on oil, as it’s looking like frost conditions Saturday night. For orchards that are closer to the very beginning of green tip and which have less chance of advancing beyond half-inch green on Monday and Tuesday, I would wait until Wednesday or so and apply copper (and oil if you can). For inland and upland sites with no bud break- hold off.

It’s important to recognize that early season infections when <5% of ascospores are mature typically won’t cause much if any scab, especially if your inoculum is low to begin with. Some orchards had terrible scab problems last year, and if you had any noticeable scab, assume that you’re high inoculum and need to take a more conservative strategy this year. But, if you had low scab (as in, none that you know of), and/or you perform orchard sanitation, you can relax a little bit in the early season. You still have time to flail mow leaves and / or apply urea to the orchard floor to reduce scab load for this year. However, copper is important not just as a (relatively weak) scab protectant, but also as a fire blight management tool, so I strongly urge growers to get copper on before the window closes if you’ve had any whiff of that disease in the recent past.

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