Scab In The Time of Copper April 9, 2016

Scab In The Time of Copper                             

Across much of the region silver and green-tip in early breaking varieties are one week old, yet most growers are still waiting to apply their first fungicide.  Sub-freezing lows have prevented the application of copper and oil.  Looking ahead, there are similar freezing temps predicted through next Tuesday, April 12.  Some growers are considering a possible spray window for copper and oil this Sunday.  Predicted weather is not likely to present conditions favorable to spraying until this Wednesday or Thursday.

This situation has increased the concern over scab.  If the first fungicide (copper) is not applied until mid-week, many orchards will have had unprotected green tissue for two weeks on early breaking cultivars.  Several of these are highly susceptible to scab.  Blocks at highest risk for a scab infection are those where there was visible leaf or fruit lesions last year.  Orchards with low-scab inoculum at the end of last year have a low probability of having significant ascospore maturity or overwintered conidia within buds.  Consider an alternative to waiting for suitable weather in blocks that have a problematic-scab history should, but you know your orchard best.

Applying a half-rate of a standard protectant (without oil) can be done despite freezing temperatures prior to or after application.  An application of three pounds of a mancozeb product or two pounds of any captan 80 formulation would best be concentrated, e.g., 50 gallons per acre or less, as we are not concerned with saturating all parts of the tree as when applying copper and oil.  This initial application of protectant would not preclude the application of copper and oil later in the week if fire blight protection is also desired.

Growers have inquired whether copper and oil could be tank-mixed with a mancozeb, when conditions allow.  Captan cannot be applied within 10 days of an oil application.  This is an option, and the copper formulation must have rates for an extended spray schedule.  In most instances, this tank mix is unnecessary.  The copper products recommended to manage fire blight will also control scab.  Once growth resumes, neither a copper or EBDC will offer protection for more than five or six days.  Some copper products, e.g., Badge SC (copper hydroxide, copper oxychloride) allow an extended spray schedule at significantly lower rates.  At max rate, Badge SC only allows one application per year.  This copper + oil + EBDC tank-mix could be beneficial when tweaking a copper application after half-inch green [see below] where the amount of copper is reduced, or when rapid leaf growth is anticipated.)

Mitigating risks with copper (if applied after ST / GT / half-inch green)

  • Apply when drying conditions are good (low humidity)
  • Eliminate the oil from the application
  • Reduce the rate of Cu per acre (but not the total gallons per acre) to one pound of actual copper per acre

Freeze night of April 8 and morning April 9, 2016

How much fruit-bud damage last night’s lows produce?  I compared predicted hourly lows and wind speeds for Saturday morning from northern Illinois to Chippewa Falls and southeastern Minnesota.  With the exception of warmer air near Lake Michigan and a pocket of same near Beloit most areas will be below 22F for two to seven hours, with varying wind speeds between 2-4 and 5-6 mph.  I am unsure if the predicted wind speeds will obviate the advantages of orchard sites with good air drainage.  If so, the uniform low temperatures are likely to produce bud damage in cultivars that are well into green-tip or beyond.

Links to freeze/frost information from Michigan State University:

April 4, 2016 note on this week’s freeze concerns:

 Article from 2009 on frosts vs. freezes:

Table of critical temperatures for tree fruit:

Picture table of above: