Cucurbit Genetics Cooperative Report 16:75-76 (article 27) 1993
Leaf Silvering of Squash: A Brief Review
Harry S. Paris
Department of Vegetable Crops, Agricultural Research Organization, Newe Ya'ar Experiment Station, P.O. Haifa, Israel
Leaf silvering is an important malady of squash and pumpkins in the Middle East, Puerto Rico, the southern United States, and possibly other regions (1, 2, 3, 9, 11, 12, 13). Leaf silvering was first recognized and reported as a serious disorder of squash in Israel (1, 2, 3, 4). The symptoms were illustrated and described by Paris et al. (10) in Israel and by Simons et al. (13) and Maynard and Cantliffe (9) in Florida. The symptoms, in mild cases, are silvering in and parallel to the veins in the upper surface of the leaves; in severe cases, the entire upper leaf surface is silvered and the petioles, stems, flowers, and fruits are pale in color (10). The rate of photosynthesis is about 30% lower in completely silvered than in green leaves (5).
Leaf silvering is induced by the sweetpotato whitefly, Bemisia tabaci Genn., especially by the nymphs (11, 14) of what is sometimes referred to as the poinsettia or IV-90 strain (6). Efforts to find a pathogenic organism such as a virus or viroid, associated with silvering have failed, leading to the conclusion that silvering is a systematic phytotoxemia (6, 14). Silvering is exacerbated by drought stress in the broad sense (i.e. plant water deficit) and its components (low soil moisture, high temperatures, high light intensity, long days, etc.) (3, 4, 10). Chemical control of the whitefly (8) and cultural practices which reduce plant water deficit (3, 4) have been reported to reduce the severity of the disorder. Silvering was reduced in a cultivar that was less susceptible to silvering when grown on reflective mulch with full irrigation (7).
Differential susceptibility to silvering occurs among cultivar groups, cultivars, and even among different strains of the same cultivar in Cucurbita pepo (H.S. Paris, P.J. Stoffella, and C.A. Powell, manuscript in preparation). Whilst genetic material immune to silvering has not been found, the cocozelle and vegetable groups of C. pepo have been found to contain some less susceptible cultivars.
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Contribution No. 3668-E from the Agricultural Research Organization, Bet Dagan, Israel.