Cucurbit Genetics Cooperative Report 6:70-71 (article 35) 1983
Overcoming the Silvering Disorder of Cucurbita
Y. Burger, H.S. Paris, H. Nerson, Z. Karchi and M. Edelstein
Division of Vegetable Crops, Agricultural Research Organization, Newe Yaar Experiment Station, P. O. Haifa, Israel
Silver mottling of leaves is a widely known characteristic of cultivated Cucurbita which is controlled oligogenically. This mottling generally occurs as patches in axils of leaf veins (2, 3, 4, 6).
Silvering of leaves, sometimes confused with silver mottling, is considered to be a serious physiological disorder affecting summer squash in Israel. The phenomenon is most common in summer and during hot spells and is associated with a lack of fruit set (1).
Silvering, which has been observed to occur as early as the third true leaf, appears in and on both sides of the upper surface of the main leaf veins in mild cases, but in more acute cases can also appear in secondary and the smallest of leaf veins. In severe instances, successively developing leaves become less green and more silvery until entirely silvered leaves appear. Concomitant with the silvering is a whitening of stems, petioles, flower buds, and ovaries. Fruit set is markedly reduced or absent. Plants less severely affected will set some fruit, but in dark green-fruited cultivars the fruit will be light in color and in precociously yellow-fruited cultivars the fruit will be streaked green or entirely green.
The silvering problem has been observed on numerous occasions across a wide spectrum of cultivars and races of C. pepo and C. moschata. It has been seen especially frequently in commercial fields of local cultivars in Israel and was serious among late summer and autumn greenhouse-grown transplants in our breeding nursery in three consecutive years. Thus, an understanding and solution of this problem became imperative.
An interesting case occurred in autumn 1981. Seedlings of a breeding line closely related to Bennings Green Tint, a scallop cultivar of C. pepo having unmottled leaves, were transplanted after expansion of the cotyledons to soil mounded on straw bales in the greenhouse. Irrigation was applied using the drip method, with 20-20-20 fertilizer plus trace elements applied through the irrigation system. As the plants grew and true leaves appeared, some began losing turgor during the heat of the day. At the third leaf stage, some plants began showing silvering with successive new leaves becoming more silvery until entirely silvered leaves appeared. These plants were observed to undergo extensive turgor loss during the day from which they did not recover overnight. A few scattered plants remained green for the first 10 leaves or so. Gradually, they began showing turgor loss and the newer leaves began showing silvering. It was at this time that the frequency and quantity of irrigation was increased several fold. Plant turgor increased noticeably, and one week later, on the slightly silvered plants, the newly developing leaves were green. On the heavily silvered plants, successive newly developing leaves were less silver and more green until finally, five to eight leaves later, completely green leaves were formed. It must be emphasized that heavily silvered plants exhibited a developmental leaf sequence of green to silver to green.
After these observations in autumn 1981, care was taken to provide enough irrigation to plants to prevent turgor loss, in both greenhouse and field-sown plants. However, in some cases, even saturation of the soil may not be enough to prevent turgor loss and leaf silvering completely. The plants usually overcome the problem eventually, probably due to the onset of cooler temperatures and, in the case of transplants, recovery of the root system.
The structural basis of silver mottling has been described by Scarchuk and Lent (3) but the physiologically caused silvering has not been studied anatomically, to the best of our knowledge. However, a preliminary check of chlorophyll content revealed no differences between green and silvery leaves.
Shifriss (5) described a line which is almost completely silvery and which he noted to be a low seed producer. This is consistent with the observations that silvery plants set few or no fruit. How silvering and impaired reproductive capability are connected is a matter for speculation. One possibility would be that silvering lowers the rate of photosynthesis in affected plants.
Summer squash in Israel is commonly grown under regimes of supplemental irrigation or early spring seeding with no irrigation (1) and no rain falls from late spring until autumn, This would explain the importance of the silvering disorder in Israel and its lack of importance in regions with frequent rainfall during the growing season, or where summer squash is grown under an intensive irrigation regime.
The ability to produce silvery foliage appears to be inherent in Cucurbita plants and is probably an adaptation allowing them to survive periods of inadequate water supply.
- Beeri, Y. and B. Kapuler. 1963. Giddul yeraqot. [Growing of vegetables.] Part B. Margalit, Y. M. (ed.) Qishu. [Summer squash.], pp. 235240. Sifriyyat Hassadeh Publ., Tel Aviv. (in Hebrew).
- Scarchuk, J. 1954. Fruit and leaf characters in summer squash. J. Hered. 45:295297.
- Scarchuk, J. and J. M. Lent. 1965. The structure of mottled-leaf summer squash. J. Hered. 56:167168.
- Scott, D. H. and M. E. Riner. 1946. A mottled-leaf character in winter squash. J. Hered. 37:2728.
- Shifriss, O. 1981. Do Cucurbita plants with silvery leaves escape virus infection? Origin and characteristics of NJ260. Cucurbit Genetics Coop. Rpt. 4:4243.
- Shifriss, O. 1982. On the silvery-leaf trait in Cucurbita pepo L. Cucurbit Genetics Coop. Rpt. 5:4849.
Contribution No. 643-E, 1982 series, from the Agricultural Research Organization, Bet Dagan, Israel.