Austin Hagen and J. Randy Akridge

Within the last decade, Phytophthora shoot blight has become a common and often devastating disease of annual vinca in both the nurseries and landscapes across Alabama. Once introduced on diseased flat or pot-grown vinca, the causal fungus Phytophthora parasitica is impossible to eradicate from landscape beds. In addition, all commercial lines of annual vinca apparently are highly susceptible to Phytophthora shoot blight. As a result, landscape contractors have been forced to replace annual vinca with other summer annuals or depend on fungicide soil treatments for protection.

Little work has been done to evaluate the effectiveness of available fungicides for the control of Phytophthora shoot blight on annual vinca. AAES field trials were established at the Brewton Experiment Field in Escambia County to assess the efficacy of selected fungicides for the protective control of Phytophthora shoot blight in a simulated landscape planting of annual vinca. The first study in 1995 showed that effective season-long control of this disease was provided by bimonthly applications of Aliette T/O at a rate of 2.5 pounds per 100 gallons of spray volume. In a subsequent study in 1998, all fungicide treatments, including Aliette T/O, failed to prevent extensive Phytophthora shoot blight-related stand losses of annual vinca.

In 1995, approximately 400 pounds of 5-10-15 fertilizer were broadcast and incorporated prior to bed establishment into a Benndale fine sandy loam. Several weeks before planting, the beds were fumigated with 250 pounds of Terr-O-Gas 98 per acre. Annual vinca cultivar Grape Cooler was planted on April, 20, 1995. A 200-parts per million solution of 20-10-20 was delivered weekly through a drip irrigation system and the plots were irrigated as needed.

In 1998, approximately 400 pounds per acre of 13-13-13 fertilizer were broadcast pre-plant over the beds and incorporated. On May 29, 1998, annual vinca cultivar Peppermint Cooler was planted on raised beds heavily infested with P. parasitica. Throughout the growing season, calcium nitrate at a rate of 10 pounds per acre was delivered at two-week intervals through the drip irrigation system and the plots were irrigated as needed.

With the exception of the monthly drenches of Subdue 2E, all other treatments were applied to runoff as directed foliar sprays at the intervals specified in the tables below. Fungicide treatments were made from May 5 to August 30, 1995, and June 15 to August 24, 1998. Plant survival was rated on August 30, 1995, and in 1998 on June 23, July 17, and August 5.

In 1995, dry, hot weather in late spring and early summer suppressed the development of Phytophthora shoot blight. Typical dieback symptoms were not seen until after several days of heavy rains occurred associated with a tropical storm. Once the onset of disease occurred, plant decline was quite rapid.

Unsprayed control plots in 1995 killed by P. parasitica.

Vinca treated in 1995 with 2.5 pounds per 100 gallons of Aliette.

Although several fungicide treatments improved plant survival compared to the nontreated control, only Aliette T/O applied at two-week intervals protected annual vinca from Phytophthora shoot blight (see Table 1). The causal fungus P. parasitica, however, did kill a small percentage of shoots on the Aliette T/O-treated vinca. Aliette T/O was considerably less effective in controlling this disease when applied at four-week intervals than at two-week intervals. Poor plant survival and generally high percentages of blighted shoots clearly highlight the ineffectiveness of the remaining fungicide treatments.

Within three weeks of planting in 1998, plants in some treatments were lost due to Phytophthora shoot blight. By mid-July, heavy stand losses were seen in all plots except those treated with either five pounds Aliette T/O or five pounds of Aliette T/O plus two pounds of Fore 80W per 100 gallons of spray volume (Table 2). Among all treatments, the lowest survival rate of 45% was noted in the nontreated controls.

Vinca treated bimonthly in 1998 with five pounds per 100 gallons of Aliette .

In early August, substantial stand losses were observed not only in all fungicide-treated plots but also the nontreated control plots (see Table 2). None of the fungicide treatments gave acceptable control of Phytophthora shoot blight. The highest survival rate of a dismal 50% was obtained with Heritage 50W. For most of the remaining treatments, percent plant survival ranged from 20% to no more than 35%. All the nontreated vinca succumbed to Phytophthora shoot blight.

In summary, fungicides probably are not the best answer for controlling Phytophthora shoot blight in landscape plantings of annual vinca. In new beds or beds where vinca has never been grown, selected fungicide treatments, such as bimonthly applications of recommended rates of Aliette T/O, should provide good preventative control of this disease. Where Phytophthora shoot blight has wiped out landscape plantings of annual vinca in previous years, foliar and soil-applied fungicides will likely prove ineffective.

In contrast, preventative fungicide treatments are critical to producing quality, disease-free flat- and pot-grown annual vinca. In production settings, bimonthly sprays of recommended rates of Aliette T/O applied throughout the production cycle should protect annual vinca from catastrophic outbreaks of Phytophthora shoot blight. Also, protective fungicide treatments will greatly reduce the risk of spreading this devastating disease from production sites to the landscape.