Occurrence of Leaf Blight on Petunia Caused by Phytophthora infestans in Maryland

In April 2001, late blight was detected on petunia (Petunia × hybrida Hort.) in a glasshouse in central Maryland. Symptomatic plants had extensive light gray, irregularly shaped, slightly sunken lesions up to 3 cm long on upper leaves and branches. Stems were distorted or dead above points where lesions had coalesced to cause stem girdling. Disease initially occurred as scattered foci but rapidly spread through the crop causing significant loss. The disease reappeared in the same and adjacent greenhouses in 2002. Phytophthora infestans Mont deBary was observed sporulating on leaves and stems of plants in both years. Isolates PE-01, PE-02, and PE-03 were isolated on rye medium from symptomatic stem and leaf tissue (1). To confirm pathogenicity, leaves and stems of 6-wk-old petunia (cv. White Madness) were sprayed with water or with aqueous suspensions of 10⁵ sporangia per ml. Plants were placed in plastic bags in a growth chamber at 20 to 22°C. Bags were removed after 3 days. Five to seven days after spraying, necrotic leaf lesions similar to those on naturally infected plants developed on inoculated plants only. Lesions occurred at leaf margins or near midribs and were 1 to 6 mm, irregular in shape, slightly sunken, grayish, and accompanied by white, fluffy mycelial and sporangial growth. P. infestans was reisolated onto rye agar from lesions from all inoculated plants, completing Koch's postulates. Lesions contained abundant, nonseptate sporangia on obvious sporangiophore structures whose morphology matched that of P. infestans (2). Sporangia from these lesions were 39 to 50 μm (average 45 μm) long × 26 to 28 μm (average 27 μm) wide with a length/breadth ratio of 1.66. Sporangia were caducous and limoniform to ovoid in shape. Oospores were not observed. The fungus grew more slowly on potato dextrose agar (PDA) than on rye medium at 18°C (2.7 versus 9.8 mm per day) and 25°C (1.6 versus 3.0 mm per day). All three isolates mated with an A1 type isolate of P. infestans forming oogonia (36.14 μm in diameter), amphigynous antheridia (22.39 μm × 17.00 μm wide), and aplerotic oospores (16.91 μm in diameter), indicating that the petunia isolates were the A2 mating type. These characteristics were consistent with a heterothallic species such as P. infestans (3). Tomato and potato plants were susceptible to PE-01. The three petunia isolates were US-8 multilocus genotype, resistant to Ridomil and mitochondrial haplotype Ia, characteristics typical of P. infestans from potato in North America. All isolates were monomorphic and homozygous for glucose-6-phosphate isomerase and peptidase (Gpi 100/100; Pep 100/100). Restriction fragment length polymorphism (RG-57) analysis confirmed the close similarity of the petunia and potato isolates. Greenhouse growers who cultivate more than one Solanaceous species should be aware that petunia transplants may have incipient Phytophthora spp. infections serving as reservoirs of inoculum, and that some strains are resistant to metalaxyl-based fungicides. P. infestans has been reported to cause leaf blight on petunia in the northeastern United States (2,4). To our knowledge, this is the first report of natural infection of P. infestans on petunia in Maryland. References: (1) C. E. Caten and J. L. Jinks. Can. J. Bot. 46:329, 1968. (2) M. Daughtrey and M. Becktell. Grower Talk Mag. 66:90, 2002. (3) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. The American Phytopathological Society St. Paul, MN, 1996. (4) M. C. Rathbone et al. (Abstr.) Phytopathology 92 (Suppl.):S145, 2003.