Apple Powdery Mildew
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. The disease can stunt tree growth and weaken buds, and is found on blossoms, leaves, twigs, buds and fruit.
Management guidelines were given in the 2012 year end issue of Orchard and Vine for a winter strategy in powdery mildew management. This article is a detailed strategy for summer control.
Most varieties grown locally, such as Gala, Fuji, Ambrosia and Spartan can become seriously infected if not properly managed, but Sunrise, Honeycrisp and McIntosh are most susceptible and more difficult to control.
Newly planted nursery trees should be considered a special case because the level of mildew cannot be detected at the time of planting. A prevention program is recommended in all new plantings for the first year starting with an early systemic fungicide.
The Summer Cycle
Blossom buds infected the previous season will appear white in spring. This mildew that over-wintered in fruit buds is called a primary infection. As active bud growth is initiated the fungus rapidly develops along with the new plant tissue, generating masses of conidia on all surfaces it covers (Figure 1).
More primary mildew becomes visible as infected vegetative shoots emerge.
The surface of leaves or blossoms, covered with a powdery film, actually consists of chains and chains of spores (Figure 2). These spores will spread in the wind and, under ideal conditions, will germinate on new tissue causing secondary infections.
There is a delay between the spread of spores, infection, and the visual expression of the secondary fungus. This delay of 10 to 14 days can be very misleading. Generally the first realization a significant secondary infection has occurred is in the first week in June. Growers interpret this as being an early June infection when the infection probably occurred two weeks earlier.
As is usual for fungi the number of spores produced are astronomical and the level of secondary infections can be extremely high under good environmental conditions. Optimum conditions for the fungus are high humidity (80-100%) and temperatures between 20 to 22ºC. However the fungus is a threat at a much wider range of temperatures (10 to 25ºC), but very little spore germination occurs in free water or at temperatures of 30ºC or above.
The table on page 32 summarizes summer control strategies and is based on the previous fall-winter infection level estimates as discussed in the previous issue of Orchard and Vine. Also, as outlined at that time, any carry-over infections will become visible as soon as blossom occurs. The fungus that has over wintered in the buds will begin actively growing when the host tissue begins to grow.
It is important to understand spring carry-over in relation to tree development in our area, and that the fungus is alive and well within these buds before the first mildew sprays go on.
Without the early, systemic fungicide, blossom infections will result in a major summer leaf infection, especially in susceptible varieties, which can be almost uncontrollable (Figure 3). In such a case an extensive protectant program throughout the summer becomes necessary.
A protectant summer program can be difficult once the initial secondary infection takes hold. The extent of the cyclic re-infection of secondary mildew throughout the summer will depend on all the factors discussed above, i.e., the susceptibility of the variety, the level of the initial secondary infection, the micro-environmental conditions of the block, etc. However, once a secondary infection is established early in the season (usually the first week in June in the Okanagan Similkameen), the protectant program becomes a calendar spray that attempts to maintain a permanent fungicidal cover on newly developing tissues.
The fungus does not survive well in rain, but can be very aggressive in warm, overcast, and high humidity conditions, particularly with the more susceptible varieties.
Under ideal conditions a block can become heavily infected very quickly. This ability of the mildew to spread rapidly causes a real problem for growers and is why a proactive and aggressive early control strategy is so important. Early sprays, where infection levels are significant, are the key to summer control and suppression of mildew carry over to the next season.
With the newer varieties now common in our orchards there seems to be more late-season pressure causing late season infections. I have not personally experienced this, but growers indicate “blocks that are apparently clean mid summer, suddenly become infected as harvest approaches.”
This needs to be investigated, but the likely explanation is that early season control was weak. Mild secondary infections occurring in early summer would be visible by late June, but not obvious. Ideal infection period conditions in July would start a secondary cycle. Although the trees would be less susceptible then, they would not be immune. The mildew build-up, if untreated, would continue until an apparent heavy infection appears to occur in mid- to late-August.
Russet, the appearance of corky, roughened, brownish or grey areas on apples, may result from a number of causes. These include cool wet weather, frost, sprays, some viruses or powdery mildew. The pattern of the russet can indicate the cause. Mildew infection will be tan to grey with a net like appearance.
Fruit russet can occur from about three weeks pre-bloom to three weeks after bloom after which surface wax production makes the fruit resistant to the russeting.
Pruning style can have a significant impact on powdery mildew levels. Shady blocks encourage mildew development when environmental conditions are met. Open trees with good light penetration and air circulation will reduce humidity within the tree and increase temperatures. Both these will discourage mildew development.
Over watering should be avoided. When trees are not well opened the heavy canopy can keep the humidity high in the block.
Overhead irrigation should not be a problem in well open blocks. Mildew spores do not germinate in free water. In open trees the water from overhead irrigation should dry off quickly without increasing the relative humidity.
Chemical control timing is outlined in the table on page 32. This table outlined the strategy variations that are acceptable for different infection potentials within the trees. When choosing a fungicide keep in mind that the early sprays should be with a systemic product that will penetrate the infected buds. This is critical to minimize or eliminate the occurrence of secondary infections.
Is mildew killed by low winter temperature? Mildew infected buds are more susceptible to winter injury than healthy buds. When temperatures dip to minus 20ºC or below, infected buds do not survive. Since the mildew is an obligate parasite, i.e. it cannot survive without living tissue, the mildew does not survive. A reduction in spring mildew after a severe winter simply relates to the fact that the buds harbouring the mildew did not survive.
Follow these guidelines, be proactive, and mildew will be controlled.