As with all plants, thoroughness must be the watchword in spraying or dusting. Timeliness, choice of weather conditions so far as possible, and covering all surface lightly rather than throwing on heavy blotches of spray or dust all require careful attention.
Fusarium Wilt (Fusarium lycopersici) is perhaps the most serious of all the tomato diseases although it occasions little trouble in the more northerly states. It is troublesome as far north as New Jersey to Iowa.
The fungus winters in the soil, enters through the roots and blocks the water passages of the plant causing wilting, yellowing, and finally, death. Water vessels in the stem are discolored,—another means of identifying the disease.
Spraying or dusting are of no service since the fungus is within. Long time rotation and use of the many resistant strains are [effective means of] control.
Figure 17.—Resistance to fusarium wilt. Row on left center is planted to an ordinary variety, row on right with one of F. J. Pritchard's wilt-resistant selections. Insert shows plant attacked by wilt fungus.
Leaf Spot, Septoria Blight (Septoria lycopersici) causes heavy loss by destroying the foliage and so the fruit-making power of the plant. It also opens the fruits to sunscald. The spots appear as small dark water soaked areas which enlarge but little though they increase in number and turn brown. Tiny black dots, the fruiting bodies of the fungus, appear. The spores germinate only on moist leaves and the disease is spread by wind, rain, workers and the like. It winters on refuse of the tomato and related plants.
Fall plowing helps to control. Bordeaux spraying beginning in the seed bed and carried faithfully through the season will usually hold the trouble in check.
Late Blight (Phytophthora infestans) is the same fungus as the late blight of potatoes, affecting both foliage and fruit. It is often troublesome the first few weeks after plants are set out-of-doors. Clean soil in seed bed and bordeaux spraying are helpful.
Western Blight, Yellows, Curly Top,—cause unknown,—is prevalent in California. Leaves roll and become thickened and brittle, later turning a sulfury yellow. Veins become purplish. The trouble prevails in hot weather. The cause is likely a virus, similar to or identical with the curly top of beets. It is apparently spread by leaf hoppers. No satisfactory control has been devised though there is some promise in [resistant strains].