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Bacterial spot of Stone Fruit a Challenge in 2009

The regular rains throughout most of the season in Niagara provided optimal conditions for bacterial spot in stone fruits in 2009. The bacteria that cause this disease infect woody tissue in late fall as the leaves fall from the tree. The bacteria multiply within the twigs during periods with favorable temperature and the bacteria ooze out of natural openings in the spring during periods of wetness. The bacteria require water congestion for infection to occur so since newly emerged leaves contain no air spaces between the cells; these leaves are immune to infection. Thus, leaf and fruit infections generally do not occur until around petal fall and shuck split, respectively. Warm temperatures (2l-29°C) with light rains, heavy dews or fogs and windy weather are most conducive for disease development and spread. The disease makes little progress when weather is hot and dry. Bacteria exude from the cankers for a period of about 30 days and also exude from leaf and fruit lesions throughout the season.

On peach, nectarine and apricot, symptoms appear as sunken, tan, pinpoint spots, which can develop into larger lesions, forming pits on the surface. Fruit infected early in the season develop unsightly deep-pitted blemishes and may exhibit gumming. Since the infected areas cannot expand with increased fruit size, the spots crack as the fruit matures. Pits or cracks on the fruit surface extend into the flesh and create large brown to black depressed areas on the fruit surface. Lesions that develop during the pre-harvest period are usually superficial and give the fruit a mottled appearance.

Peach and nectarine leaves frequently show more severe spotting along the midvein and towards leaf tips because bacteria are suspended in water droplets that run down the leaves and these regions of the leaf dry more slowly. The leaf spots are always angular. Within one to two weeks, the centre of the lesion is walled off by the rest of the leaf and drops out, resulting in a "shothole" or tattered appearance. Leaves with as few as two to three lesions turn yellow and fall.

Bacterial spot symptoms are typically more severe on plums than other stone fruit, with water-soaked sunken spots, developing into brown/black necrotic lesions 1 cm in diameter mid-season. Leaf infections on plums consist of angular spots on veins or on leaf injuries; these spots are shiny black on the leaf undersurface.

Nitrogen deficiency can cause leaf symptoms that are very similar to those of bacterial spot. Care must be taken to avoid an incorrect diagnosis. In both cases, the leaves of affected trees turn yellow and drop prematurely. With bacterial spot, symptoms are most likely to appear at the tip of the leaf but with nitrogen deficiency, symptoms are usually most evident along the midrib and entire leaves may turn red. Leaf tissues surrounding shot-holes caused by a nitrogen deficiency are more likely to have a reddish color than when bacterial spot is involved. Captan spray injury may also be mistaken for bacterial spot. This injury is more circular and occurs mostly on young leaves, many of which may be damaged on only one side of the midvein.

On peaches and nectarines, two distinct types of cankers damage twigs. These cankers are different from the large perennial cankers caused by fungi. "Spring cankers" develop on young twigs produced the previous summer. Spring cankers first appear as water-soaked, slightly darkened blisters about the time the first leaves appear. If these cankers encircle the twig, it will die. When conditions are moist, the canker surface has a black, water-soaked appearance. As the season progresses, the tissues over the blister-like lesions rupture and bacteria are released. These bacteria can be spread by windblown or splashing rain and can result in new infections. In time, spring cankers heal and become inactive.

"Summer cankers" develop on green twigs of the current season's growth. They usually occur later in the summer after leaf spots are evident. At first, they are water-soaked, dark purplish spots. In time, they enlarge, turn brown to purple-black, become slightly sunken and round to elliptical with water-soaked margins.

On certain plum and apricot varieties, twig cankers may continue to develop in two- and three-year old twigs. If the cankers are deep-seated, they can deform or kill the twigs. These can be confused with hail-damaged stems. Bacterial spot cankers are generally greasy in appearance with water-soaked margins.

There are no registered pesticides for control of bacterial spot. However, research in the US showed that a fall application of copper at 50% leaf drop for peach leaf curl reduced overwintering bacterial spot pathogen populations as well. A delayed dormant application in the spring covers the tree, particularly the woody portions, with copper during the late dormant period, so that the first thing the bacteria are exposed to as they emerge in spring is the bactericide. Reduced rates of some fixed coppers have been shown to be effective and less phytotoxic to leaves when applied in the three weeks after petal fall. Babygold 5 and Springcrest are especially susceptible to bacterial spot so these varieties should be avoided.

Will next year be a bad one for bacterial spot? "One year does not predict the next. It depends on the weather conditions in late winter and early spring. If we have favourable weather conditions, then it could be a very severe year again," says David Ritchie, a North Carolina State University plant pathologist in Raleigh. "If we have dry conditions as the trees begin to come out in the spring and the fruit sets, we could expect it to be very light." Dormant sprays are cheap insurance against a disease for which we have no products later in the season.

Bacterial spot on plum. Note sunken lesions with oozing on fruit and shot-hole injury on leaves.

Bacterial spot on peach