|Miller Butler, Melinda
Submitted to: Crop Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2016
Publication Date: N/A
Interpretive Summary: There are no fungicides recommended for control of Botryosphaeria stem blight of blueberry. Cultural control recommendations include pruning out infected tissue at least 15 cm below visible lesions and avoiding over fertilization and late season fertilization. The effect of two types of fertilizer applied at four rates was evaluated on rabbiteye blueberry using a detached stem assay. Stems from plants receiving no fertilizer developed shorter lesions than those receiving any rate of either fertilizer. Several related fungi have been reported to cause Botryosphaeria stem blight of blueberry. The ability of isolates of three of these genera to infect detached blueberry stems was compared at six temperatures on five blueberry cultivars. On average, the southern highbush cultivar, Star, develop longer lesions than the other cultivars; Botryosphaeria dothidea incited longer lesions than the other two fungal species; and lesions developed the fastest on stems incubated at the highest temperature, 35°C. While differences were found among the three species in their ability to infect detached blueberry stems and cause stem blight symptoms, these differences were relatively minor and in the same range as found in previous detached stem studies comparing isolates of B. dothidea. Each fungal species could be recovered from the margins of lesions on stems inoculated with that species. Only a very low percentage of each species could be recovered from stem tissue taken from 10 or 20 mm beyond the visible lesion indicating that the pathogens are not present in stem tissue a short distance from the visible lesion. Even though several genera of Botryosphaeriaceae have now been reported to cause stem blight symptoms, our findings indicate that the current control recommendation to prune out infected stems at least 15 to 20 cm beyond any visible lesion is still valid. This information will be used by extension specialist making disease control recommendation and research scientists.
Technical Abstract: Botryosphaeria stem blight is a destructive disease of highbush, rabbiteye, and southern highbush blueberries in the southeastern U.S. The disease has been observed to be more severe on vigorous plants than on slower growing plants. Historically stem blight has been reported to be caused by the fungus Botryosphaeria dothidea, but more recently, other species in the Botryosphaeriaceae family have also been identified as causal pathogens of stem blight. Detached stem assays were used to compare the effect of four rates of two types of fertilizers on lesion development following inoculation of rabbiteye cultivar Tifblue with B. dothidea and to compare the effect of temperature on infection of southern highbush and rabbiteye blueberry cultivars following inoculation by three fungal species in the Botryosphaeriaceae family, B. dothidea, Diplodia seriata, and Neofusicoccum ribis. Partially-hardened blueberry stems were wounded, inoculated with a mycelial block of the pathogens, and incubated for 21 days in a growth chamber. There were no differences in lesion length 5 and 10 days after inoculation on stems from plants receiving 1, 2, 4, and 8 times the recommended level of a liquid and a slow release fertilizer. Stems in the temperature study were incubated at 10, 15, 20, 25, 30, and 35°C, and isolations were made from five locations above and below the inoculation point on each stem. Stems incubated at the higher temperatures had the longest lesions, the fastest developmental rate, and the shortest incubation time compared to stems incubated at the lower temperatures. On average the longest lesions were on stems of the southern highbush cultivar ‘Star’. Stems inoculated with B. dothidea developed longer lesions at a faster developmental rate and shorter incubation time than stems inoculated with either of the other two species. B. dothidea had the highest recovery percentage among the three species. These results support current cultural control recommendations to prune out infected tissue at least 15 cm below visible lesions and avoid over- or late-season fertilization.