Submitted to: Southeastern Professional Fruit Workers Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 3/10/2003
Publication Date: 6/6/2003
Citation: Smith, B.J. 2003. Susceptibility of southern highbush blueberry cultivars to phytophthora root rot and botryosphaeria stem bight. Southeastern Professional Fruit Workers Conference Proceedings pg.18-30. Interpretive Summary: Southern highbush blueberry cultivars are being widely planted in the southern United States. These cultivars are hybrids of the northern highbush blueberry and various blueberry species native to the south. Little is known about the susceptibility of these cultivars to important blueberry diseases such as Phytophthora root rot and Botryosphaeria stem blight. One of seven southern highbush cultivars grown in soil infested with the root rot pathogen was determined to be as resistant to Phytophthora root rot as the two rabbiteye blueberry cultivars in the trail. The other six southern highbush cultivars were very susceptible with most of their plants dying by the end of the five-year study. Traditional Phytophthora root rot control practices such as fungicide drenches and raised beds resulted in only marginal improvement over untreated plants. The finding that one southern highbush cultivar is resistant to Phytophthora root rot indicates that blueberry breeders should be screening their elite selections to identify other resistant lines. A laboratory assay was used to determine the susceptibility of 20 southern highbush blueberry cultivars to stem blight. There are no fungicides available for control of this disease; therefore, resistant cultivars and cultural practices offer the most promise for its control. Ten southern highbush, one rabbiteye, and two low chill highbush cultivars were identified as relatively resistant. Blueberry growers will benefit immediately from the knowledge of which cultivars are most disease resistant. The lack of control by the fungicides indicates that further research is needed in this area.
Technical Abstract: Acreage planted with southern highbush blueberry cultivars (hybrids between highbush and "southern" blueberry species) is steadily increasing; however, little is know of the susceptibility of these new cultivars to various diseases. Two important diseases are Phytophthora root rot (caused by Phytophthora cinnamomi) and Botryosphaeria stem blight (caused by Botryosphaeria dothidea). These studies were initiated to compare the susceptibility of southern highbush, highbush, and rabbiteye cultivars to these two diseases, and to evaluate the effect of bed height and fungicide treatments on plant vigor and root rot severity. After five years growth in Phytophthora infested soil, 100% of Premier, 90% of Gulfcoast and 76% of Tifblue plants were still alive and vigorous compared to less than 25% of the other southern highbush cultivars and 21% of plants drenched with metalaxyl twice a year for 3 years and 16% of plants grown on raised beds were very vigorous compared to 14% of plants grown in untreated soil and 9% of plants grown on flat beds. These results indicate that at least one southern highbush cultivar, Gulfcoast, has field resistance to Phytophthora root rot equivalent to rabbiteye blueberry cultivars. The susceptibility of 20 southern highbush cultivars, two rabbiteye cultivars, and two highbush cultivars were compared using a detached stem assay. Succulent, partially-hardened stems were wounded by scraping away a section of bark and inoculated by covering the wound with a 2 mm square mycelial agar block of B. dothidea and securing with parafilm wrap. The stems were ncubated at 25oC, 100 % RH for 15 days. Lesion length was measured after 15 days. Cultivars with the shortest mean lesion length were classified as relatively resistant and included Pearl River, Emerald, Star, Sharpblue, Elliott, Misty, Bluecrisp, Darrow, Southmoon, Ozarkblue, Sapphire, and Brightwell. Cultivars with the longest lesions at 15 days were classified as relatively susceptible and included Legacy, Gulfcoast, Cooper, Georgiagem, O'Neal, Reveille, Jubilee, and Magnolia.