(4) _Seed selection._--Work done at the Florida experiment station indicates that resistant varieties may be secured, but there are as yet none in commercial use. This is an important line for experimenters to follow up. There is no proof that the disease is spread through seed from diseased plants.
=Fusarium wilt.=--This disease and the one following resemble the bacterial wilt so closely, as far as external characters go, that they are difficult to tell apart. The parasites, however, differ so materially in their nature and life history that the field treatment is quite different. There are also differences in geographical distribution that are important, for while the Fusarium wilt occurs occasionally throughout the southern states, it is known to be of general commercial importance only in southern Florida and southern California.
_The symptoms of the disease_ are a gradual wilting and dying of the plants, usually in the later stages of their development. Young plants die, however, when the soil infection is severe. There is a browning of the woody portions of the stem, and when a section of this is examined under a compound microscope the vessels are found to be filled with fungous threads, which shut off the water supply.
_The infection_ in the Fusarium wilt appears to come entirely from the soil. Little is known of its manner of spread, except that the cultivation of a tomato crop in certain districts appears to leave the soil infected so that a crop planted the next year will be injured or destroyed. The fungus does not remain in the soil for a very long time in sufficient abundance to cause serious harm. A rotation of crops that will bring tomatoes on the land once in three years has been found in Florida to prevent loss from Fusarium wilt.
_This fungus does not attack any other crop_ than tomatoes, so far as known, though it is very closely related to species of Fusarium producing similar diseases in cotton, melon, cowpea, flax, etc. Fusarium wilt has not been found in fields and gardens in the northern states, but tomatoes in greenhouses there are sometimes attacked by it or a related Fusarium, which also occurs in England. When greenhouse beds are infected the soil for the next crop should be thoroughly sterilized by steam under pressure.
=Sclerotium wilt.=--This disease resembles the two preceding in its effect on the plant, which wilts at the tip first, and gradually dies.
Its geographical range is more restricted. It seems to be confined to northern Florida and the southern part of Georgia and Alabama, where it occurs in gardens and old cultivated fields. The fungus causing this wilt attacks the root and the stem near the ground, working in from the outside. There is not the browning of the wood vessels characteristic of the two preceding diseases. If an affected stem is put in a moist chamber made from a covered or inverted dish, there will develop an exceedingly vigorous growth of snow-white fungous mycelium which, after a few days, bears numerous round shot-like bodies, at first light-colored, then becoming smaller and dark-brown. These are the sclerotia or resting bodies of the fungus. This fungus, called _Sclerotium_ sp., or "Rolf's Sclerotium," is noteworthy because it attacks potatoes, squash, cowpea, and a long list of other garden vegetables and ornamental plants. The only satisfactory means of control is rotation of crops, using corn, small grain, and the Iron cowpea, a variety immune to this and other diseases. Susceptible crops should be kept from infected fields for two or three years.
=Root-knot= (_Heterodera radicicola_ (Greef) Mul.) attacks tomatoes in greenhouses and is in some cases an important factor in southern field culture. It is caused by a parasitic eelworm or nematode, of minute size, which penetrates the roots and induces the formation of numerous irregular swellings or galls, in which are bred great numbers of young worms. The effect on the plant is to check growth and diminish fruitfulness, in advanced cases even resulting in death.
_The remedy in greenhouse culture_ is thorough soil sterilization. In the open field this is impracticable and recourse must be had to a rotation with immune crops, which will starve out the root-knot. It must now be borne in mind that the root-knot worm can attack cotton, cowpea, okra, melons and a very large number of other plants. The only common crops safe to use in such a rotation in the South are corn, oats, velvet beans, beggar weed, peanuts, and the Iron cowpea. The use of other varieties of cowpea than the Iron is particularly to be avoided, on account of the danger of stocking the land with root-knot.
Fortunately, the disease is serious only in sandy or light soils.
=Rosette= (_Corticium vagum_ (B. & C.) var. _solani_ Burt.) is a disease of minor importance, which occurs in Ohio, Michigan, and scatteringly in other states. The fungus causing it (_Rhizoctonia_) attacks the roots and base of the stem, forming dark cankers. The effect on the plant is to dwarf and curl the leaves and to restrict productiveness. The potato suffers more severely from the same trouble. Rotation of crops and liberal application of lime to the soil are advised for the control of rosette in tomatoes.