Contributed byT. A. ZitterDept. of Plant PathologyCornell UniversityIthaca, NY 14853-4203
Septoria leaf spot is present in the Northeast and the Midwest of the United States and in eastern Europe, the former USSR, Africa, and Australia on summer and winter squash, pumpkin, melon, cucumber, and bitter melon. The disease has occurred in New York State since the early 1900s.
Fig. 1. Leaf lesions with pycnidia. Septoria leaf spot of pumpkin, caused by Septoria cucurbitacearum. Courtesy T. A. Zitter. (Click image for larger view)
The disease causes similar symptoms on all cucurbit hosts. Under moist conditions, lesions first appear as small, dark brown, water-soaked spots, measuring 1–2 mm in diameter. Under dry conditions, they are circular or occasionally irregular in shape and beige to nearly white. A narrow brown border surrounds the lesion and, with age, the tissue may crack (Fig. 1). Lesions on melon may reach 3–5 mm in diameter. On older lesions, small black pycnidia are formed, embedded in the tissue. Up to eight or more pycnidia may be present per lesion. Lesions are very abundant on melon, pumpkin, and winter squash leaves; they are less common on summer squash. Small, erumpent, whitish spots (1–2 mm in diameter) appear as a "rash" on the surface of infected butternut and acorn squash and pumpkin fruits (Figs. 2 and 3). Unless other diseases, such as gummy stem blight or anthracnose, are involved in fruit infection, fruit rots caused by Septoria are not likely to occur. Heavy occurrence of the small white speckles may lead to unmarketable fruit.
Fig. 4. Release of thin, needle like conidia from a ruptured pycnidium. Courtesy of T.A. Zitter. (Click image for larger view)
Septoria cucurbitacearum Sacc. produces globular, black pycnidia filled with very narrow and long conidia (Fig. 4). The conidia are hyaline, straight or slightly curved, and one- to five-septate; they vary in length from 36 to 70 µm or longer and are 1.5–3 µm in diameter. Published data suggest the occurrence of several Septoria spp., based upon the occurrence of shorter and broader conidia, but no detailed studies have been conducted with these taxa.
S. cucurbitacearum overseasons on crop debris and can survive for more than 1 year. The fungus most likely survives as dormant mycelium. In the spring, pycnidia are produced. They give rise to conidia, which serve as the primary inoculum. The needle-shaped conidia are expelled in long, mucilaginous tendrils and are readily disseminated by splashing or windborne rain. High humidity and rainfall and temperatures between 16 and 19°C are ideal for infection and further disease development. If favorable weather conditions persist, the organism can produce additional inoculum and infections in repeated secondary cycles. Although disease development slows with warmer summer weather and lack of rainfall, recurrence in the fall is likely unless early controls are practiced. It is not known when initial fruit infection occurs, but it is likely to be in the early stages of development, before the outer rind becomes fully mature. Infected fruit can be found in the field before the fall harvest.
Because the pathogen can survive on infected crop debris, a minimum 2-year rotation out of all cucurbits should be followed. If cool and moist conditions persist into June, lesions will probably appear. Scouting at this time will allow for early detection and the scheduling of protectant fungicide sprays to prevent further disease spread. Early control may preclude the need for additional sprays in the fall.
Eastburn, D. M. 1991. First report of Septoria fruit and leaf spot, caused by Septoria cucurbitacearum, on Cucurbita moschata in Illinois. Plant Dis. 75:1286.
Punithalingam, E. 1982. Septoria cucurbitacearum. Descriptions of Pathogenic Fungi and Bacteria, no. 740. Commonwealth Mycological Institute, Kew, England.
Stewart, F. C. 1910. Notes on New York plant diseases. I. Pages 364-365 in: N.Y. Agric. Exp. Stn. (Geneva) Bull. 328.
Zitter, T. A. 1992. Diseases of cucurbits: Septoria leaf and fruit spot of cucurbits. Cornell Coop. Ext. Veg. Crops Fact Sheet 732.80.