Tomato plants are susceptible to virus, fungus and bacteria. Disease can affect the fruit, roots, stems or leaves and can progress to consume the entire plant. Symptoms of different diseases often appear very similar and can also have the appearance of improper plant care, such as overwatering or poor nutrient content in the soil.
Blight refers to several types of fungi that cause leaf spots. Signs of early blight include brown or black spots on the leaves that form concentric rings like a target. These spots have a yellow border. Leaves may turn yellow and fall off.
Blight may also cause black spots on stems. Late blight starts near the edges of the leaves and appears as water soaked spots that can quickly grow and turn dark brown or purplish black. White mould may be visible on the spots in humid climates. If humidity continues, the blight can spread to the entire plant, according to tomatoguru.com.
Septoria leaf spot also begins as water soaked spots on lower leaves. Spots may appear white or grey, surrounded by dark brown or black borders, according to the Colorado State University Extension.
If a tomato plant appears wilted but watering does not help, the plant could have wilt, a term used to describe several fungal or bacterial problems. Fusarium wilt is prevalent in hot climates and is evident when the leaves begin to yellow, beginning at the base of the plant and continuing up. The leaves droop and the plant may fall over. Brown discolouration along the length of the inside of the stem also indicates fusarium wilt.
Fusarium crown and root rot is a problem during cooler periods. The leaves turn brown and the lower portion of the stem can rot and crack. You may also see a brown growth on the root. Brown discolouration sometimes appears inside the first 12 inches of stem.
If the plant begins to wilt in the middle of the day but appears to rebound at night, it may be stricken with verticillium wilt, which occurs in northern climates. Other symptoms include yellowing leaves that display a "V" pattern along the edges. The disease can lead to leaf drop.
Bacterial canker causes leaves to brown and curl downward. Brown streaks may appear along the ribs of the leaf, and fruit may develop small, white, raised spots.
Yellow, curled leaves could be a sign of tobacco mosaic virus, cucumber mosaic virus or curly top virus. New growth appears narrow and twisted, and the leaves are a mottled yellow, according to gardeners.com. The leaves of a plant infected with curly top virus may also appear purplish in colour.
Tomato spotted wilt/impatiens necrotic spot tospovirus begins with dark brown or purple leaf spots that spread to the stems. As the disease progresses the plant begins to wilt, but the leaf tissue stays stiff. This is followed by yellow rings or spots on the fruit.
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