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Disease Cycle and Epidemiology

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Epidemiology

The opening blossoms are the first emerging susceptible tissue in the spring (Figures 6 and 7). Sources of blossom blight inoculum are mummies (Figure 22), infected peduncles, and cankers. Conidia from these sources are disseminated by splashing or wind-blown rain.

Figure 6

Figure 7

Figure 22

Infrequently, apothecia (Figure 20) develop from mummies on the orchard floor or beneath secondary hosts such as wild plums and other Prunus spp. surrounding the orchard. Ascospores are released during rainfall and are carried by wind to the blossoms (Figure 23).

Figure 20

Figure 23

Blossom infection is highly dependent on wetness duration and temperature. For blossom infection to occur at 10ºC (50º F), 18 hours of wetting are necessary; in contrast, at 24ºC (77º F) only 5 hours are necessary. The time required for symptoms of blossom blight to develop may be only a few days to 1 or 2 weeks depending on the temperature. Blighted blossoms often are obscured as new flushes of leaf growth occur. Thus, the first evidence that infection has occurred may be yellow and wilting leaves on branches or twigs (Figure 24). It takes very few blighted blossoms (Figure 25) to cause severe fruit rot if environmental conditions are optimal as fruit ripen.

Figure 24

Figure 25

As fruit ripen and the sugar content increases, they become increasingly susceptible to infection. In contrast, green, immature fruit are less prone to infection unless they are injured (Figure 26). Inoculum sources for fruit infection include blighted blossoms, cankers, mummies from the previous year, and diseased fruit in the tree or on the orchard floor from thinning practices. The amount of inoculum is very important in determining the severity of brown rot. Warm, wet or humid weather during the 2 to 3 week period prior to harvest increases disease severity because it increases both the level of inoculum and the amount of infection. If wet weather extends into the harvest period, fruit loss can be severe (Figures 12 and 27). Insects, such as June beetles, which are attracted to overripe fruit, can increase disease severity not only by carrying the fungal conidia but also by creating wounds as they feed. Wet, warm conditions also increase overwintering inoculum available for blossom infections the following spring.

Figure 26

Figure 12

Figure 27

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