Tomato plants are popular among home gardeners because of their ease of care and low maintenance. These plants tend to do well with little attention, except when they develop a problem. A common disorder among tomato plants is yellowing leaves that develop brown spots, usually caused by a fungal infection.
The two most common fungi that cause discolouration or wilt are verticillium and fusarium, according to the University of Illinois.
When verticillium or fusarium attack tomato plants, the fungus works its way up the plant to the leaves. Most people don't discover the infection until a plant's leaves show changes because initially the plant seems healthy.
According to Cornell University's College of Agriculture and Life Sciences, both types of fungi can live dormant in soil for up to 10 years and can infect any tomato plants with which they come into contact.
Verticillium wilt or fusarium wilt eventually kill the plants they infect. Some plants might bear fruit that ripens, but the tomatoes will be small with poor flavour, according to Cornell University's College of Agriculture and Life Sciences.
Once the fungi take hold, the best way to avoid future infections is to plant varieties of tomatoes that are resistant to verticillium and fusarium. A tomato plant's name indicates resistance if it is followed by V, F or VFN.