Tree Fruit & Grape News, March 1999

Where Did All the Scab Come From?

The past two years have seen an increase in apple scab damage in all parts of the Okanagan. Fruit damage even occurred last summer in the Oliver/Osoyoos area, where scab is seldom considered to be a serious problem. Where did it come from? Why all the damage in 1998?

Weather patterns are undoubtedly a major factor in the appearance of scab. The disease is always present at low levels, but usually well under control due to fungicide programs. The summer of 1997 was a very wet one. Although apple scab severity was not too much higher than normal, the continuous wet weather provided a good opportunity for the apple scab fungus to multiply and build up high inoculum levels. Sometimes late-season leaf damage can hard to recognize due to the presence of other problems like leaf miner damage.

So, we entered 1998 with high levels of inoculum. Although the summer was an overall dry one, there were numerous infection periods in the spring, many of them quite early. This, combined with a tendency to push the first sprays later and later in the spring resulted in early season infections. Many of these were soon visible as scab lesions on the calyx end of the fruit. Inadequate spray coverage, and stretching the intervals between sprays for too long can also result in more leaf and fruit infections.

Control Strategy for 1999

1. Start early.

In general, apple scab inoculum levels will be higher than ever this spring. With high inoculum, it is important NOT to delay your spray coverage. Trees become susceptible to scab at the green tip stage. However, many growers are delaying coverage until the tight cluster to pink stage. Delaying that first spray until the tight cluster stage has proven to be safe when inoculum levels are very low. But this is not a safe practice with moderate to high inoculum levels. If you had scab last year, start at green tip.

2. Keep ‘em covered

A protectant schedule is recommended from green tip until after petal fall. The blossom stage is the "peak susceptibility period". The plant tissue is highly susceptible to infection, and ascospore inoculum is peaking. In addition, plant tissue is growing rapidly, which means more frequent sprays will be needed to keep up the coverage. Frequent showers are also common in the spring.

3. After petal fall

Continue either a protectant or eradicant schedule until mid to late June when the primary scab season is finished. Then monitor leaves and fruit for apple scab lesions for a couple of weeks. If you made it through primary scab season without damage, then the spray program was successful. If you did end up with some damage, continue to monitor infection periods and protect fruit through to harvest.

4. Monitoring Infection Periods

If using an eradicant schedule, it is important to carefully monitor infection periods and have enough eradicant fungicide on hand to get a spray on quickly afterwards. Sometimes infection periods are borderline (e.g. the wet period was not quite long enough, but it was close). The production guide uses a Mills chart to determine how many hours of wetness are required at various temperatures. This is not exact, but only a guideline. If you have very little scab, it may be safe to ignore a borderline infection period. If you have abundant inoculum, a borderline infection period may result in significant damage.

Similarly, infection periods are sometimes classed as light, moderate or severe depending on how long the wet period was. Any infection period that meets the criteria for light infection is a definite infection period and will result in scab damage if inoculum is present. The extent of damage will be more for moderate or severe infection periods. Do not ignore light infection periods.

New Products

There are two new apple scab fungicides on the market this year:

Nustar is a Dupont product that is similar to Nova in its mode of action and usage pattern. It has proven to be an excellent scab and mildew fungicide in North American trials. Nustar has a 96 hour kick-back for apple scab. If using as an eradicant spray, be sure to use the higher label rate. The high rate is also required for mildew control.

Vangard is a Novartis product with a unique mode of action. It will be valuable as a resistance management tool, since no other fungicide is cross-resistant. Vangard has good activity at lower temperatures, and thus has a good fit for early season scab and blossom blight sprays. Experience in the U.S. indicates that some control may be lost under very hot and humid conditions. Vangard does not provide mildew control. 

See the Tree Fruit Production Guide for more information on these fungicides. 

For more information contact:
Gayle Jesperson, Plant Pathologist 
Crop Protection Program, Kelowna

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