Key Issues for managing apple scab in 2008

Even if your crop didn't have significant scab at harvest, it's still possible to have a high inoculum load for the following spring. This is because the fungus that causes apple scab, Venturia inaequalis, continues to develop in the leaves after harvest if the post-harvest period is warm and wet, resulting in inoculum build up for the following spring. At the Ontario Fruit and Vegetable Convention (OFVC), Cathy McKay, a grower/consultant from Nature's Bounty, showed that, from the Ontario perspective, we have been seeing an increase in apple scab in recent years. Harvest assessments from the Northumberland Durham apple growers show that the amount of scab in orchards and the number of orchard blocks infected with scab at harvest have been increasing in recent years. I have also seen this trend across the province, to the extent that scab was even a problem last year in several orchards, despite the hot, dry conditions, which were not conducive to the spread of the disease. So what do growers need to remember when developing a scab management program in 2008?

Fungicide resistance

Resistance is not a concern for mancozeb, polyram, captan, ferbam, thiram, sulphur or copper. For all other apple fungicides (Nova, Nustar, Sovran, Flint, Scala) resistance has already developed or can be expected to develop. New York and Michigan have documented resistance to sterol inhibitors (SI's) such as Nustar and Nova. Research conducted by Dr. Wolfram Koeller of Cornell University estimates that resistance to SI's can occur after approximately 60 applications. This assumes label rates, complete coverage and no extended spray intervals. If any of these are compromised, so is the "life" of the fungicide. There is also some evidence that shows that resistance to strobilurins such as Sovran and Flint is developing in some areas. Research conducted by Dr. Wendy McFadden-Smith in 2006-2007 in Ontario found that SI resistance is starting to appear in a few Ontario orchards that have been reporting field failures in managing apple scab. Unfortunately resistance in apple scab is not reversible and the resistance to fungicides lasts forever. To prevent the development of fungicide resistance never use fungicides as eradicants, it is better to apply products preventatively. If you do get caught with an infection period, go in as soon as possible after infection: do not rely on 96-hr kick-back activity. If scab appears in your orchard, don't try to "burn it out" as that strategy will only encourage the development of scab resistance. Instead rely on the use of protectants to prevent the spread of the disease. Always use a ½ rate EBDC fungicide with SI insecticides, and don't use SI's before tight cluster or after fruit are present on the tree.

Fungicide Timing

While the majority of ascospores are released during bloom, some are present and ready to infect green tissue as soon as it emerges. Dr. David Rosenberger of the Hudson Valley Lab, Cornell University, has shown that the risk of economic loss from a scab control failure is highest at green tip. These green tip infections produce conidia at bloom when the fruit and leaves are the most susceptible to scab. If leaves and fruit are infected early in the season, there are more scab generations of scab before the summer heat shuts down the scab epidemic, resulting in increased damage to the crop. Early season scab infections (blossoms, and leaves) often causes secondary infections on fruit. As a result the crop should be covered from silver tip or green tip. In New York a green tip copper spray is considered to be just as good as a mancozeb spray (3.5 kg/ha). During or after half-inch green mancozeb-captan combinations are frequently used (avoid using captan where oil has been used) at a rate of 3.5 kg/ha mancozeb plus 1 to 2 kg/ha captan/Maestro. The rationale behind this is that mancozeb has better rententiveness and rain-fastness than does captan. However, by virtue of that characteristic, captan may redistribute better to newly expanding leaves. In a very heavy rain, captan may disappear more quickly than mancozeb, whereas in repeated light rains the captan may provide better protection of new leaves via redistribution.

Alternate row spraying

Growers will often chose to use alternate row middle spraying as a means to reduce spray time early in the growing season. Skipping alternate rows will lead to a reduced degree of coverage but more rapid treatment of the orchard. In this case, spray the skipped middles as soon as possible after completing the first spray. Be wary of alternate row spraying on an extended interval. For good resistance management, it is imperative that all leaves be protected every 7 days. If you have been experiencing problems with scab control, it is a good idea to spray every row to ensure good coverage and protection from early-season infections. Do not use alternate row spraying for SI's or strobilurins.

High inoculum orchards

Another important component of managing scab is reducing primary inoculum. In research trials conducted by Dr. Bill MacHardy's lab at the University of New Hampshire, urea (45 kg of agricultural urea per 1000 L of water/ha) was applied to the orchard floor after about 95% of the leaves had fallen (November) or in the spring (April) before bud break. The urea works in two ways: it directly inhibits the development of ascospores and it stimulates the growth of naturally occurring organisms that are antagonistic against V. inaequalis. Both treatments reduced the number of ascospores, but the spring treatment was more effective and resulted in fewer leaf and fruit infections in most years. In the years when it didn't work, the snow cover remained almost until bud break so there was not a lot of time for the urea to work.

Some growers have expressed concerns about applying N to trees as they are going into dormancy. Dr. Wendy McFadden-Smith did a trial in which shoots from trees treated sprayed with urea in late October and those that were not treated were frozen to temperatures as low as -40 C in a freezer. There was no difference in the cold-hardiness of buds from trees treated with urea. Donna Speranzini, Nutrient Management Planning Specialist with OMAFRA, also does not think that this small amount of N will cause excessive growth.

In summary, the best way of managing scab in your orchard is to prevent infection in the first place.

• Use urea ground sprays to reduce inoculum in orchards with high scab pressure.
• Start your spray program no later than green tip.
  
• Use SI (Nova, Nustar) and strobilurins (Flint, Sovran) conservatively -- no more than 2 sprays per group per year.
  
• Do not rely on post infection spraying
  
• Use full label rates and get complete coverage.
  
• Maintain tight spray intervals from green tip through petal fall.