Muskmelon mosaic is another disease caused by a virus. Leaves of infected plants show a conspicuous dark-green banding about the larger veins. Later leaves usually do not show vein banding but display a severe to mild yellow and green mottle. The margins of many leaves are serrated. Some leaves may become distorted and curled. Flowers on infected plants are often deformed and generally fail to set. The pollen grains are irregular in shape and fail to germinate normally. Only rarely are there mild mosaic patterns on immature fruits. The symptom normally disappears with the development of the normal fruit net, but sometimes the virus appears to inhibit normal netting. The total soluble solids of infected fruits are lower than normal.

The muskmelon mosaic virus infects only cucurbits. It causes typical mosaic symptoms on gherkin, pumpkin, and squash and only a faint mottle on cucumber. The virus is commonly seed-transmitted, ranging from more than 95 percent in fresh seed to less than 5 percent in seed 3 years old. It has been recovered from seed stored 5 years. It is readily transmitted by aphids.

Although the planting of virus-free muskmelon seed will assure freedom from the disease in young plantings, it fails to assure continued freedom from infection as the plants mature when grown in the vicinity of other sources of the virus.

Muskmelons grown in the Southwest frequently are affected by mosaic caused by all three of the viruses we described and by a complexity of strains. Usually one virus is predominant over the others. Symptoms may vary somewhat from year to year depending on the particular virus, but the similarity of the symptoms produced by the several viruses is such that they cannot be readily distinguished in muskmelon. Crop losses due to the reduction of foliage and exposure of fruits (so that many are sunburned) usually are great. A fairly large number of the fruits fail to develop a satisfactory net and are discarded; others appear normal but are of inferior quality.

Mosaic in muskmelon has been observed in the Imperial Valley of California for many years, but not until 1946 did the disease appear in epiphytotic form. Since then it has been of considerable economic importance.

Attempts to control mosaic of muskmelon through control of insect vectors by airplane application of DDT, parathion, benzene hexachloride, and nicotine were unsuccessful.

R. C. Dickson and his associates determined that the causal viruses were spread principally by transient populations of aphids, chiefly the green peach aphid, which did not originate from melons but came mostly from sugar beets and weeds and moved across the melon fields in great swarms. The aphids usually fed for less than a minute on the melon plant and moved on in short flights to feed on a large number of plants. Such a feeding pattern is efficient in distributing the viruses and allows a large increase in the low initial incidence of the disease on weeds, escaped and cultivated melons, and squash.

In the absence of methods for reducing aphid populations at their source, intensive efforts are being made to develop muskmelons resistant to the several causal viruses.

FRUIT ROTS of cucurbits occur in the field, in transit, in storage, and in market. Many of the decays observed on harvested produce originate from infections initiated while the fruits were maturing upon the plant. For a discussion of some of these disorders, the reader is referred to our earlier discussion of angular leaf spot (also known as bacterial spot of cucumber); anthracnose, the most serious fruit decay of muskmelon, squash, and watermelon; fusarium root rot, also a rot of gourd, pumpkin, and squash fruit; and scab (sometimes referred to as cladosporium rot), a fruit rot of cucumber and muskmelon.

Fusarium rot of muskmelon and pumpkin is an important market disease. Many small, scattered lesions are found on the surface of infected fruits which are tan to light brown and often difficult to separate from healthy tissue. Later the spots become sunken, more extensive, and covered by a white or pink growth of the fungus. Infections are usually confined to the rind but may sometimes extend into the flesh and seed cavity. The seeds then appear clumped and embedded in the fungus mycelial mat. The disease is caused by Fusarium moniliforme, F. roseum, and F. solani. Those fungi can enter the fruit only through ruptures in the skin. The common occurrence of the rot on ripe muskmelons is associated with appearance of minute rifts and abrasions on the surface, many of which result from handling practices.

Fusarium rot can be fairly well controlled by reducing the frequency of handling muskmelons, shipping under refrigeration at about 45 F., and marketing promptly.

Rhizopus soft rot may affect all cucurbits but is of most importance on muskmelon, pumpkin, and squash. The disease appears first as soft, water-soaked spots on the rind. As the affected area increases in size, it becomes soft, sunken, and easily broken. The fungus Rhizopus appears after the skin has been broken and develops a profuse mold growth when the temperature ranges from 80 to 90 . Decay may be controlled by avoiding skin ruptures, shipping under refrigeration at 40 to 45 , and by prompt disposal of ripe fruit.

Soft rot may also be caused by bacteria, such as Erwinia aroideae and E. carotovora, which usually gain entrance through breaks in the skin of fruits and cause a putrid collapse of tissue. Bacterial soft rots are not common and are usually unimportant. Soft rot due to fungus infections are more common and of economic importance. Perhaps the most important of the fungal decays are those caused by Phytophthora and Pythium.

Phytophthora rot occurs naturally on muskmelon and watermelon. It affects cucumber and squash by artificial inoculation. The diseased tissue is first water-soaked, then turns brown, and becomes soft; the result is a sunken and wrinkled lesion. Infections generally occur in the field and are caused by Phytophthora capsici, P. drechsleri, and P. parasitica. Decay of fruit can be arrested by shipping under refrigeration at 45 .

Pythium rot is similar to phytophthora rot. It naturally affects muskmelon, watermelon, cucumber, pumpkin, and squash. Diseased cucumber and squash fruits are conspicuously water-soaked; the tissue is soft and flaccid and ruptures readily under pressure. In cucumber and squash the disease is sometimes referred to as leak. It is incited by Pythium aphanidermatum during hot weather and by P. irregulars and P. ultimum in cooler weather. The fungi infect fruits in the field and serve as a source of further infections during transit and storage. Quite often a luxuriant white fungus growth covers the affected tissue. Losses may be reduced by careful sorting of field-picked fruit, shipping under refrigeration at 45 to 50 , and rapid distribution to retail market.

Muskmelon, pumpkin, and watermelon fruits affected by pythium rot are water-soaked and soft, but are also discolored a light tan to brown. As the infections increase, the lesions become sunken and wrinkled, breaking open easily and discharging quantities of watery fluid. Primary infections are initiated in the field, but secondary infections result from contact of healthy fruits with diseased ones. The rot in muskmelon is usually incited by Pythium aphanidermatum and P. ultimum; in pumpkin by P. ultimum; and in watermelon by P. acanthicum, P. myrioylum, and P. periplocum. Careful sorting of fruit to exclude infected merchandise in the packed container and shipping under refrigeration at 45 to 50 will reduce transit losses.

Stem-end rot of watermelon, induced by the fungus Diplodia natalensis, is an important disease and is responsible for a high percentage of the watermelons lost in transit. The fungus infects the stem end of the fruit following its cutting from the vine. The fungus does not infect fruits other than through cuts or other wounds. Infected tissues turn brown and shrivel, and the fungus may produce pycnidia and a grayish-white mycelium on the fruit surface. The disease develops most rapidly between 85 and 90 . Prompt coating of the freshly made stern cut with bordeaux paste containing no less than 6 percent copper sulfate prevents its development. Field cutters should be careful to avoid contaminating the cutting knife by faulty harvest of an infected fruit. All harvesting and packing equipment should be washed down thoroughly each day and disinfected with phenol or formaldehyde. Shipment of watermelon at 50 will deter development of the disease in transit and assure satisfactory arrival condition. The disease sometimes affects muskmelon; it is not restricted to stem end but occurs over the entire fruit surface, making surface sterilization impracticable. Temperature control and careful field selection offer some relief from losses.

JOHN T. MIDDLETON is associate plant Pathologist, University of California, Riverside. He holds degrees from the University of California and the University of Missouri. He has investigated vegetable crop diseases, especially root rots and their control and the biology of root-rotting fungi.

GUY WESTON BOHN is a pathologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering at the United States Horticultural Field Station, La Jolla, Calf. He has conducted research on the causes and control of diseases of crop plants and on the breeding of vegetable crop plants for disease resistance and other economic characters.