William McPhee
Powdery Mildew
These infected buds are visible during pruning. White shoots are easy to spot during pruning and carry viable fungus in their buds, which are typically grey and distinctly pointed.
Powdery mildew on apple trees is a problem in the Okanagan because of the ideal climatic conditions for the fungus Podosphaera leucotricha. Control of the disease is poorly understood by growers, which has resulted in a lot of frustration. This article will relate the disease to conditions encountered in local orchards with an emphasis on fall-winter management.
Flower and vegetative buds become infected in the summer under moist, warm conditions. Bud infections occur within a month of bud formation and before bud scales are suberized. These infections remain quiescent until bud break the following spring. Chemical control begins in the spring, but assessment of the potential for spring problems are best carried out in late summer, fall and winter. The level of leaf infections evident during picking and the number of white shoots present during winter pruning should be a signal indicating the seriousness of the problem.
During the winter infected flower buds and white tip shoots that contain infected vegetative buds are easily identified and can be removed. If left in the tree many of these visibly infected buds will not survive; however, they indicate the presence of many other fruit and vegetative buds harbouring the live fungus that are not showing any sign of the disease outwardly. Their numbers are indicative of the potential for the primary infection level expected in spring.
These quiescent infections post a threat in the spring but can be monitored using the following simple technique:
The percentage of infected buds is determined by selecting a representative sample of bud sticks from orchard trees late in the winter. These buds can be forced to open by placing them in water in a warm room (20-25oC). The severity of the mildew expected in the spring can be estimated on the basis of the number of shoots with infected buds.
If you’re wondering whether a severe winter can kill the fungus the answer is there is some evidence that the fungal mycelium is more sensitive to cold than the buds themselves and may be reduced when temperatures reach -24oC. However, since the fungus only survives in living tissue if the bud survives, then generally, the fungus survives.
Since infected buds are more vulnerable to winter injury a severe winter will reduce the carry over by reducing the number of surviving buds that are infected. This answers the other common question, “Is it necessary to remove and destroy white tips that are pruned out?” The answer is no. The buds in pruned wood do not survive so the fungus cannot survive.
“White-tip prunings” often contain chasmothecia (formerly called cleistothecia) and these structures can survive independent of the host. However, the ascospores they contain do not pose a significant threat in apples and can be disregarded. In cherries on the other hand, the chasmothecia are the primary infection agent in the spring so in cherry blocks infected prunings should be removed if possible to reduce the potential for spring infections.
Growers are surprised to see a major blossom infection within an apple block even though they put on an early systemic, but this is simply the over-wintering fungus within the bud developing as the bud develops. The early systemic fungicide penetrates into the bud tissue and impacts the fungus’ ability to generate viable spores, but cannot suppress expression of the fungus at this early stage. However, the early sprays greatly reduces the potential for secondary infections (infections visible on leaves during early summer) to initiate and cycle throughout the summer. In other words, early systemic applications facilitate summer control by preventing the initiation of secondary summer infections.
The powdery mildew potential of apple blocks for the following spring can be estimated if the current level of infection is assessed during the late summer, fall and winter (Table 1). This can be done by observations made in the orchard in late summer and during winter pruning or by bringing in shoots and forcing them to expose mildew levels in the terminal buds.
This management effort will allow you to be as aggressive as necessary early in the spring and enhance summer control.
Tree growth within your trees will visually improve if mildew is controlled, which is so important in the new high density systems. ■
