Producers should watch plants carefully for Botryosphaeria in the spring and summer, and if additional pruning is conducted under warm conditions, a fungicide application is generally recommended after each day of pruning, and the fungicides are often also of benefit for fruit rots and leaf spots (killing two birds with the same stone). Also, do not push plants with early nitrogen this spring, as this might further exacerbate the situation with Botryosphaeria.

Pruning out damaged plant tissue will prevent Botryosphaeria stem blight from exploding in the spring. This is especially true for young plants (1 to 3 years old), and fertigated plants, so if one has to prioritize, one might prune these first. Generally, we have more problems with Botryosphaeria on southern highbush, but we have had tremendous problems with Botryosphaeria on young fertigated rabbiteyes in the last 2 to 3 years as well.

Once the extent of the damage is observed, there will be an advantage to pruning out dead tissue, and there is research information from North Carolina to back this up.

Cline reported a similar situation in 1992. A "Murphy" planting in Bladen County was examined bi-weekly or monthly for the first 3 years to determine conditions associated with high plant mortality in young bushes. Plants grew profusely and did not become completely dormant in 1992, and 139 of 500 bushes were cold injured at first frost in November.

Cold-injured stems developed a characteristic dead, hook-shaped tip that persisted throughout the following growing season. The presence of Botryosphaeria stem blight was confirmed on these tips, and in 1993, the incidence of Botryosphaeria stem blight in stems injured by cold the previous November was 19, 39, and 88 percent for March, May, and June, respectively.

Widespread infection by Botryosphaeria stem blight following cold injury was reported to account for past observations of field epidemics 1 to 2 years after planting. The take home message from this is that injured stems are colonized early, disease incidence increases with time and temperature, and the later you wait past February to prune, the more disease you are likely to see.

In cases such as this, Lockwood has generally advised growers to delay pruning until late winter/early spring so that they can feel relatively sure that the potential for additional cold injury is past.

He also advises holding off until one can easily see, based on bud swell or early shoot growth, where the strong, new growth will originate. At that time, he advises pruning back to healthy wood.

Cline suggests that “it is worth a special effort to remove cold-injured stems, especially on young bushes. With cold-injured basal shoots (suckers that emerge from the crown), we snap them off by hand right down at the crown, since the brown pith often goes all the way to the crown. In controlled experiments this significantly reduced disease incidence. For cold-injured shoots higher up on older canes, prune them back to healthy green tissue.”

Clearly, the consensus opinion is gathering for pruning away injured tissue in late January to early or mid-February. It is desirable that producers review the weather forecast, and attempt pruning when 3 to 4 days of dry weather (no overhead frost protection as well) are predicted following pruning. This will help to reduce infections on new pruning cuts. If it is warm during this timeframe, a fungicide application is generally recommend after each day of pruning.

Again, there will be added expense for these pruning efforts, but they should be seriously considered this year.

Reference:

Cline, W.O. Infection of Cold-Injured Blueberry Stems by Botryosphaeria dothidea. 1994. Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Plant Dis. 78:1010.

 

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blueberries, blueberry diseases