First report of powdery mildew caused by Podosphaera fusca on Lactuca sativa in Korea

Leafy lettuce (Lactuca sativa) is one of the most important crops in Korea, cultivated throughout the year in plastic houses. During autumn 2005, severe outbreaks of powdery mildew occurred in plantings in plastic houses near Suwon in central Korea. Further surveying of commercial fields in central and southern Korea revealed widespread occurrence and severe losses (Fig. 1). Infections often caused leaf distortions, withering and reduced growth. No head lettuce was found to be infected.

White superficial colonies developed amphigenously on leaves and stems. Conidiophores were unbranched, cylindrical, 100-240 µm, composed of a basal cell with a slightly swollen base and three to nine barrel-shaped conidia formed in chains (Fig. 2A). Conidia were ellipsoid or doliiform, 24-34 x 15-22 µm (Fig. 2B), with distinct fibrosin bodies. Dark brown chasmothecia (ascomata) were found on stems, embedded in the mycelial felt, spherical, gregarious to subscattered, 70-96 µm in diameter, each containing a single ascus with small oculus (apical thin zone), 12-18 µm wide. Asci contained eight ascospores, 18-26 x 16-20 µm (Fig. 2C). The fungus was thus identified as Podosphaera fusca (syn. Sphaerotheca fusca) (Braun, 1987; Shin, 2000).

To confirm pathogenicity, inoculum from infected leaves was dusted onto leaves of potted leafy lettuce plants. Plants were then maintained in a greenhouse (16-24°C). After five to seven days, powdery mildew developed on inoculated plants; the morphology matching that of the original pathogen. Noninoculated control plants did not develop powdery mildew.

Previously, Golovinomyces cichoracearum (syn. Erysiphe cichoracearum) has been recorded on head lettuce (L. sativa) in the USA, South Africa and nearly all European countries. Podosphaera fusca has been recorded on wild Lactuca species in several countries of North America, Europe and Asia (Amano, 1986; Shin, 2000). To our knowledge, this is the first report of powdery mildew caused by Podosphaera fusca on Lactuca sativa in Korea as well as in the world. Voucher specimens were deposited in SMK (Korea University) and in the division collection at NIAST.