Summer Disease Control

The cool wet growing season experienced in Ontario during 2004 has been ideal for the dissemination, infection and establishment of many diseases. Keeping up with the weather and finding time to spray between showers has challenged many growers this year. Unfortunately, the wet weather along with slightly below normal temperatures is also conducive for summer diseases.

In Ontario the two most common summer diseases in apples are Fly Speck (Figure 1) and sooty blotch. The fungi that cause these two diseases rarely penetrate the waxy cuticle but live on the surface of the fruit. Unfortunately, the unsightly superficial blemishes they create can make the affected apples unmarketable. The fungi also live in the buds and bark of apple trees without causing any damage, however, when wet conditions together with temperature ranging between 18-27^oC persist, the fungi produce spores that are rain splashed to developing fruit. Infections can take place as early as 2-3 weeks after petal fall. During periods of cool humid conditions the fungi colonize areas of the apple fruit surface. Symptoms often do not appear in the orchard until 3-4 weeks after the infection. Hot temperatures greater than 30^oC inhibit the growth of these fungi.

Figure 1- Fly speck symptoms appear as a colony of tiny black specks on the surface of apples.

Many of the fungicides used to control apple scab probably keep early infections under control. However, later in the season, particularly when wet cool weather persists, the fungi produce spores that cause secondary infections when scab control is no longer required.

Repeated summer sprays of Captan or Maestro applied at the full rate will reduce the severity of colonization by these superficial fungi. Flint 50 WG, a new fungicide registered this past year on apples, has also been shown to provide very good control of these summer diseases. Although this product is more costly than Captan or Maestro, an application or two in August may protect a good apple crop from becoming infected with the summer diseases this season. Flint is in the strobilurin group of fungicides along with Sovran and only four applications of strobilurin fungicides are allowed in apples per season to reduce the risk of resistance developing.