Etiology of a Tuber Rot and Foliar Blight of Potato Caused by Phytophthora nicotianae

Although Phytophthora nicotianae is not normally considered to be an important pathogen of potato (Solanum tuberosum), intermittent outbreaks of a foliar blight and tuber rot have been reported in the United States over the past 75 years. Due to the sporadic nature of these reports, little is known about the etiology of the disease in potato. However, foliar disease and tuber rots caused by this pathogen are usually centered near areas of standing water in the field and along pivot tracks. Moreover, soil particles adhering to the foliage of infected potato plants suggest that water splash is involved in P. nicotianae dissemination and infection. Soil infestation and water splash dissemination studies were conducted under greenhouse conditions to examine the role that zoospores of P. nicotianae may play in disease on potato. In the soil infestation study, inoculum of P. nicotianae was added to soil at four rates (0.0, 1.0 x 10(5), 5.0 x 10(3), and 4.0 x 10(4) zoospores/ml) and three timings (at planting and 7 and 14 days after planting). Direct infection of aboveground plant tissues was achieved via splash dissemination of inoculum onto potato foliage. All soil infestations significantly reduced emergence, with the exception of the 1.0 x 10(3) zoospores/ml treatment, and no plants emerged from soil infested with 4.0 x 10(4) zoospores/ml. Significant reductions in stem number were observed with infestations of 1.0 x 10(3) and 5.0 x 10(3) zoospores/ml at planting and 5.0 x 10(3) zoospores/ml at 7 days after planting. Progeny tuber infections were confirmed with infestations at 1.0 x 10(3) zoospores/ml at planting and 7 days after planting. Lesions developed on leaflets, petioles, leaf axils, and stems in all water splash dissemination treatments within 3 days of inoculation, significant differences in the lesion number were observed, and disease severity generally was proportional to inoculum concentration. Relative area under the disease progress curve of the 5.0 x 10(3) and 4.0 x 10(4) zoospores/ml splash dissemination treatments was significantly greater than the 0.0 zoospore and 1.0 x 10(3) zoospores/ml treatments. Progeny tuber infections were observed with all water splash dissemination treatments but infection rates did not differ significantly among treatments. These studies confirm the hypothesis that water splash dissemination of P. nicotianae inoculum is a likely means by which infections of this pathogen are initiated in potato.