Vegetable Crops Fact Sheet

FL Cooperative Ext. Service / Institute of Food and Agricultural Sciences / University of Florida / J. T. Woeste, Dean

*Professor of Vegetable Crops, Gulf Coast Research & Education Center, Bradenton and University of Florida, Gainesville, respectively.

Silverleaf of squash and irregular ripening of tomato caused substantial economic losses to South Florida vegetable growers in the 1987-88 crop year. Losses were especially severe during the spring 1988 season. Irregular ripening had not occurred previously, whereas silverleaf has appeared sporadically in South Florida squash plantings for several years.

This fact sheet summarizes our current knowledge of these disorders in Florida and provides descriptions and illustrations to aid in diagnosis.

Silverleaf symptoms appear first at the leaf veins, as opposed to interveinal silvering that is genetically controlled, and is common in many zucchini cultivars. Symptoms develop in the interveinal area so that the entire upper leaf surface is distinctively silver (Figure 1). Symptoms do not occur on the leaf underside.

Silverleaf symptoms have been observed on very young plants with only three true leaves, on plants in later vegetative stages, and on plants during fruit production. All or some leaves on a plant may be affected. In severe cases, plants are stunted and fruit production is restricted. The disorder may appear on plants scattered through a field, on plants on one side of a field, or on all of the plants in a field.

Some observations suggest that symptoms disappear occasionally, and plants resume normal growth. However, silvering is permanent in any given leaf in which it occurs, but it is developmentally reversible. Normal plants can develop new silvered growth and plants having silver leaves can develop new green growth.

Fruit symptoms are not noticeable in cases of mild leaf silvering, however, when leaf silvering is severe, fruit color is lighter than normal. Yellow summer squash is very pale-colored, zucchini squash is light-green to yellowish-green, acorn squash is mottled green to yellow, and golden acorn squash is white (Figures 2 and 3). Yield reductions and poor fruit quality are usually associated with leaf silvering.

Silverleaf symptoms have been noted on all types of squash (Table 1) but have not been observed in muskmelon, cucumber, or watermelon.

Silverleaf of squash was first observed in Israel in 1963. It was defined as a distinct disorder differing from the genetically controlled leaf silvering that is common in zucchini-type summer squash (1). Further Israeli studies established a relationship between stress, especially drought conditions, and the occurrence of silverleaf (2).

Figure 2. Silverleaf-affected fruit is shown to the left of normal fruit. Compared in this photo are spaghetti squash (left), acorn squash (center), and pumpkin.

Figure 3. Jersey Golden Acorn squash fruit showing symptoms of squash silverleaf.

Previous to the 1987-88 season, silverleaf had been observed occasionally in Dade County. In the spring of 1988, however, silverleaf was observed in Palm Beach and Collier counties, and in the fall of 1988 in Manatee, DeSoto, Lake, and Alachua counties. It has not yet been reported in the state's panhandle.

Table 1. Cucurbita species and cultivars observed to be susceptible to silverleaf.
C. pepo Yellow summer squash Zucchini squash Acorn squash Pumpkin Scallop squash Spaghetti squash	C. moschata Butternut squash Calbaza		C. maxima Hubbard squash

The cause of squash silverleaf in Florida is not known. It does not appear likely that drought stress causes silverleaf here, as reported in Israel. However, stress induced by other factors may be related to the disorder. Extremely high populations of the sweetpotato whitefly are frequently noted in association with severe silverleaf. However, in recent studies where sweetpotato whiteflies were caged with squash plants, silverleaf occurred when as few as three adults per leaf were present. Therefore, it is thought that this insect may cause a physiological imbalance that leads to silverleaf symptoms by (a) direct feeding, (b) introduction of a toxin of some sort, or (c) by vectoring a heretofore unknown disease. No other biotic or environmental factor has been consistently related to the occurrence of silverleaf.

Research is currently being conducted at the University of Florida at Gainesville and at several Research and Education Centers to determine the cause of silverleaf and to develop appropriate control measures. Growers should advise their county extension agents of suspected cases of silverleaf for accurate identification and collection of crop histories.

Because of the association between the sweetpotato whitefly and the occurrence of silverleaf, growers are urged to follow the appropriate cultural management guidelines discussed under tomato irregular ripening and to control the whitefly with labeled insecticides. Asana, Safer Insecticidal Soap, Thiodan, and Vydate are effective against the sweetpotato whitefly and are approved for use on squash. Always follow label directions for use of pesticides.

Color development during ripening is frequently lacking in one or more areas of individual fruit affected by irregular ripening. Color develops along locule septa and intermediate areas remain green or yellow (Figure 4). Overall, this pattern of ripening produces a star-burst appearance (Figure 5). With sufficient time, sometimes two to four weeks, nearly normal external color develops on most fruit. When cut, however, internal flesh is white or yellow (Figure 6). Since green fruit appears normal, the first indication of irregular ripening occurs when the fruit is removed from ripening rooms. After ethylene treatment, there is a vivid checkerboard appearance in the carton caused by some of the fruit that has colored normally and other fruit in which the color development is incomplete (Figure 7). Plant vigor and foliage appear normal, but occasionally may have virus-like symptoms. Irregular ripening has been observed on several cultivars of tomatoes as well as plum and cherry tomatoes.

Figure 4. External symptoms of tomato irregular
ripening.

Figure 5. Normal tomato fruit (left) and fruit affected with irregular ripening. Starburst effect is evident on the tomatoes in the center.

Ripening disorders such as graywall or blotchy ripening occur from time to time in Florida tomatoes. However, the incidence of these disorders is usually low because of the use of resistant cultivars, and can often be traced to faulty fertilizer application or some unusual weather situation.

Irregular ripening occurred on tomatoes grown on the east coast, in the Homestead and southwest Florida (Immokalee-Naples) areas in the 1987?88 season. The occurrence of the disorder was much more severe in the Immokalee-Naples area than in the other two areas. No verified report of the disorder came from the Palmetto-Ruskin or Quincy production areas.

In southwest Florida, irregular ripening was first noted in mid-March, and was initially thought to be graywall. The disorder was soon evident throughout the area, and was recognized as being distinct from graywall. No reports of the occurrence of irregular ripening in other parts of the world could be found.

The first appearance of irregular ripening was noted in some crops at first harvest, whereas in other crops it was not observed until the third harvest. Usually, there was an association between sweetpotato whitefly populations and the severity of irregular ripening. Because of the strong association between high populations of the sweetpotato whitefly and the occurrence of irregular ripening, it is thought that damage may occur from direct feeding of the insect, by introduction of a toxin which may cause physiological imbalances resulting in irregular ripening, or by introduction of a virus or some other pathogen. At this time the cause of irregular ripening is not known.

Research is currently being conducted at the University of Florida at Gainesville and at several Research & Education Centers to determine the cause of irregular ripening and to develop appropriate control measures. Growers should advise their county extension agents of suspected cases of irregular ripening for accurate identification and collection of crop histories.

Figure 6. External and internal symptoms of tomato irregular ripening.

Figure 7. Tomato cartons in New York City market showing fruit affected with irregular ripening and normal fruit.

Because of the association between the sweetpotato whitefly and the occurrence of irregular ripening, growers are urged to manage the sweetpotato whitefly as recommended by Price, Schuster and Kring (3).

The best sweetpotato whitefly management begins with sound sanitation and other cultural practices that may avoid, delay or lessen the severity of the problem. The following are some important points of cultural management that can have an important impact on the management of sweetpotato whiteflies in tomato crops:

Where sweetpotato whiteflies have become a problem in a tomato crop, crop management practices will differ from those previously used. Such changes, especially new insecticides chosen and new insecticide schedules, can have an important effect on other insects and mites. In view of these effects, tomatoes should be scouted very carefully for various other pests and plans should be developed to respond to additional pest problems.

Burger, Y, H. S. Paris, H. Nerson, Z. Karchi, and M. Edelstein. 1983. Overcoming the silvering disorder of Cucurbita. Curcurbit Genetics Coop. 6:70-71.

Paris, H. S., H. Nerson, and Y Burger. 1987. Leaf silvering of Cucurbita. Can. J. Plant Sci. 67:593-598.

Price, J. F, D. J. Schuster, and J. B. Kring. 1988. Management of the sweetpotato whitefly on tomato crops in South Florida. GCREC (University of Florida) Res. Rept. BRA1988-15.

Mention of brand names does not constitute a guarantee by IFAS or the University of Florida of said product, nor does it imply its approval to the exclusion of other producers or vendors of the same chemical compound.