Powdery Mildew: What Happened? What's Next?

Ontario apple growers probably noticed more powdery mildew in their orchards last fall than in previous years. The apple powdery mildew fungus, Podosphaera leucotricha, infects leaves, shoots, flowers and fruit of apple, pear and quince. Infected leaves appear narrow and tend to curl upward exposing a pinkish red underside along the margins (Figure 1). Often colonies or patches of a white powdery fungal growth can be observed on the underside and occasionally on the upper side of infected leaves. Eventually severely infected leaves dry up, turn brown and fall prematurely (Figure 2). Powdery mildew causes a net-like russetting on fruit, which may be confused with damage caused by spring frost injury.

What happened in 2002?

Powdery mildew occurs every year wherever apples are grown. However, the increase of powdery mildew observed last fall could be blamed, at least in part, on the weather conditions experienced during 2002. Due to the unusually warm winter in 2002, a larger population of the apple powdery mildew fungus probably survived in infected terminal buds than in previous years. When spring came along, although frost was a serious problem in some areas, the event was not cold enough to kill the fungus that had over-wintered in buds.

Furthermore, infected flower and shoot buds are more susceptible to cold and frost injury which, may have contributed to the severity of damage observed last spring. The cool wet 2002 spring conditions probably slowed down the disease temporarily.

When summer finally arrived in 2002, the weather quickly turned hot and dry. Unlike other disease causing pathogens, the powdery mildew fungus prefers warm and dry but humid conditions. In fact, rain can wash the conidia off leaves and conidia will not germinate in freestanding water. Due to the dry weather last year, fungicide applications for scab control, which also keeps mildew in check, were reduced. The high humidity experienced last summer was also ideal for the powdery mildew fungus to produce chains of conidia responsible for multiple infection cycles last year. Consequently, powdery mildew took off in apple orchards last year.

What's next in 2003?

With all of the powdery mildew around last year, many buds probably became infected which may result in a serious powdery mildew year in 2003. However, the cold spell that occurred during the middle two weeks of January 2003 may be a blessing in disguise. While infected buds are more susceptible to cold injury, the fungus is often killed when temperatures drop to -12oC without killing the bud. However, when temperatures drop below -24oC both the fungus and infected bud are highly vulnerable to killing. This is about 2-10oC higher than the lethal temperature required to kill non-infected buds.

Management strategy

Growers who noticed a lot of powdery mildew in their orchards last fall should scout their orchards for powdery mildew infected buds during the dormant season. Severely infected buds will appear white, flattened and pointy. Pruning out and removing infected buds during the winter or early spring will significantly reduce the inoculum level in the orchard. Since the fungus only survives in living tissue, it is not necessary to remove the infected pruned buds from the orchard unless other diseases are also present.

In the spring, powdery mildew infected buds open 5 to 8 days later than non-infected buds. Infected flower buds and cluster leaves that survive the winter will often become covered with a white fungal growth and spores, which is the primary source of inoculum. These should be pruned out if possible. Leaves tend to be susceptible to infection for only a few days after emergence, but once infected, it becomes a source of inoculum for other susceptible tissue. Although fruit symptoms are not noticed until harvest, they actually become infected during a period of 3 weeks before and after bloom. Lateral and flower buds are susceptible to infection for about a month after they are formed.

If powdery mildew was prevalent in 2003, apply a fungicide such as Nova 40 W or Nustar again at tight cluster to pink. To prevent the potential build up of resistant powdery mildew populations in the orchard, do not apply more than four applications of Nova 40 W or Nustar in a single growing season. Always rotate fungicides with others that have a different mode of action (refer to OMAF publication 360). When spraying for Scab during green tip to half-inch green, growers may consider applying a fungicide that will protect leaves and buds from both scab and powdery mildew such as Dikar. Continue to scout for powdery mildew and other diseases at least every week. If hot, humid but dry weather persists during the 2003 growing season and disease pressure remains high, additional fungicide applications may be necessary beyond the first summer spray, particularly on very susceptible varieties such as Jonagold, Cortland, Idared, Crispin and Paulared.

Figure 1. Infected leaves don't always show up as "whitish". Red stippling on the leaf margins is a sure sign of mildew problems on less susceptible cultivars.

Figure 2. Defoliation of shoot growth can take place in early September from mildew infection. Also, notice the stippling at the leaf margin and the cupping of leaves.