Summer Diseases of Apple: To Spray or Not to Spray

To spray or not to spray, that is the question for summer disease control in apple and pear, especially during a dry year. Summer diseases of pome fruit can significantly reduce the quality and marketability of fruit. Severe disease is favoured by cool wet springs, with extended periods of wetness from summer rains, and cool weather prior to harvest. But what is the risk of these diseases causing significant damage during this dry season?

The two most important summer diseases of apple and pear in Ontario are:

• Sooty blotch caused by the fungi Gloeodes pomigena, Peltaster fructicola, Geastrumia polystigmatis, Leptodontium elatius
• Fly speck caused by the fungus Schizothyrium pomi (Zygophiala jamaicensis).

Sooty blotch appears as brown or green blotches on the outer skin of the fruit. These blotches are in fact dark, tiny fruiting structures of the fungi, all connected with a loose, profusely branched thread-like fungal growths. On the other hand, flyspeck appears as small, shiny black dots that cluster in groups of a few to about 100 (Figure 1). These dots are the sexual fruiting structures, also connected by inconspicuous thread-like growth. Neither of these fungi penetrates the epidermis, growing superficially on the surface. The result is dirty looking fruit, unappealing to consumers. Both diseases have similar biology, occur around the same time and can be managed the same way.

Both diseases overwinter on branches of wild woody plants, in addition to apple and pear. Infection and disease development are favoured by extended periods of wet, humid weather during late summer. Although wet weather has been sporadic so far, the development of these diseases can still occur quickly if favourable conditions occur.

Fly speck produces ascospores (they will be the primary inoculum released next spring from before bloom until 10 days after petal fall). This is similar timing for apple scab spore release. Fungicides such as Flint or Sovran applied for apple scab in the spring also control fly speck primary infections. Unlike apple scab, very few of the fly speck ascospores would reach the small developing fruit in the spring. Regardless, these spores do reach alternative wild hosts near orchards, and will initiate infections that produce secondary inoculum. This secondary inoculum is blown from the alternative hosts into the orchard throughout the summer. Look for these diseases on fruit in border trees in close proximity to wooded areas.

The fungi do not grow well, if at all, when temperatures reach above 29^oC. Researchers in North Carolina have shown that 270 hours of accumulated wetness either due to rains or dew is required to produce symptoms of fly speck. This was verified in trials conducted by Cornell University researchers in 2003. Because of our warm spring and early summer temperatures, any spring infections may not show up until the weather turns cool and wet in late summer.

Dr. Dave Rosenberger, plant pathologist at Cornell University, calculated that the initial infection on wild hosts requires 270 hours of leaf wetness in the spring, and that an additional 270 hours of fruit wetness in mid-summer is required for fruit infection. This means that a grand total of 540 hours of wetness after petal fall are required before symptoms actually appear in the orchard. During a dry year, fungicide sprays are not necessary for summer disease control until later in July.

To spray or not to spray for summer disease this year is still a good question. Last year was a cool, wet summer, with ideal conditions for summer diseases to buildup on alternative hosts. The overwintering inoculum on wild hosts was likely higher than in previous years. Although conditions have not been conducive for summer diseases so far this season, and symptoms may not show up until mid August, a registered fungicide at the end of July will ensure protection of fruit. Poor summer disease control is usually a result of poor spray coverage. Spray coverage for summer fungicides is better when fruit are smaller and limbs are not bent with fruit loads. A fungicide application at the end of July will also help prevent powdery mildew, black rot and white rot.

A single application in late July may provide season long control of summer disease during a dry year. However, an application at the end of July followed by another application 3 weeks later is a safer strategy. If more than 5 mm (2 inches) of rain falls after the second application, another application may be necessary in early September, particularly on varieties that won’t be harvested until late September. So keep an eye to the sky to decide if a third fungicide application is necessary.