Plant diseases are caused by three categories of pathogens. Ranked in the order of prevalence, these disease-causing agents are: fungi, bacteria and viruses.
“Fungicides may be able to stop or slow down some of the fungi, but there are just too many to defeat totally,” Dr. Daniel Egel, Extension Plant Pathologist, Southwest Purdue Agricultural Center said. “Perhaps the top reason that fungi are such good plant pathogens is there are so many propagules (spores).”
Whether fungal spores travel on wind, water, or soil residues on equipment, most spores can survive for long periods of time without a host. Spores wait patiently in soil or plant debris until conditions are right to reemerge. A susceptible host plant, a viable spore and conducive conditions for the fungal growth can add up to a lot of disease.
Fungal diseases can be foliar or soil borne in nature. Powdery mildew of cucurbits, early and late blight of tomatoes, and downy mildew are some foliar examples. Verticillium wilts — which are generalists, infecting many plants — or Fusarium wilts — which tend to be crop-specific — are examples of soil borne fungal diseases.
Soil borne fungal pathogens aren’t controllable via foliar fungicides, Egel said.
There are some fungicides that can be useful in disease suppression of these soil born fungi. However, gaining control of these diseases requires adept attention to sanitation, crop rotation, selection of resistant varieties and the use of disease-free seeds or transplants.
Tomatoes, peppers and eggplant are susceptible to infection from Fusarium oxysporum f. sp. lycopersici. This fungus is a serious concern worldwide. Three distinct races — named 1, 2, and 3 — have been identified. All occur in the United States. Plant symptoms include wilting during the daytime, with seeming recovery as temperatures cool at night. Leaves become yellow, initially on one side, but then expanding to the entire plant. Vascular discoloration also occurs.
Watermelon is susceptible to Fusarium oxysporum f. sp. niveum, an extremely important disease of the crop. Cucumbers are infected by another strain of the fungus. Both cause similar concerns, paralleling those caused by the tomato-infecting strain. Cantaloupe, okra and sweet potatoes can also be infected by their own host-specific strains of Fusarium oxysporum fungus.
Fusarium oxysporum f. sp. cepae is specific to alliums, causing basal rot in garlic and onions. Yellowing and die-off of leaves occurs, along with the potential for wilting. Bulb symptoms may not appear until storage.
Fusarium wilt is a warm soil disease. The fungus enters damaged roots, then grows until it reaches the water-conducting cells of the plant. Once in the vascular system, the fungus can propagate and spread, restricting water uptake as transpiration slows, causing wilting symptoms and possibly death. Warm weather causes more rapid deterioration.
Regulating soil pH can slow the spread of the disease. Fusarium spores prefer acidic, sandy soils. Soil moisture levels can impact pathogen survival, too.
There are some plant cultivars resistant to Fusarium wilt. Interestingly, root knot nematode infection and the resulting wounding can actually overcome any resistance bred in the plant. Planting a resistant cultivar isn’t a cure, even when root knot nematodes aren’t a problem. Even resistant plants can play host to the fungus, introducing it into a field where it is then available to remain in the soil, and infect non-resistant plants.
Fusarium wilt spores are long-lived, and remain dormant in the soil, in crop residue, or even in weeds or other non-susceptible crops — just waiting for the right conditions and a vulnerable crop — for many years. A minimum of a 7-year rotation is recommended to guard against disease problems in susceptible crops. Seeds can transmit Fusarium wilt, and a clean seed source or clean transplants are a necessity.
Growers often have difficulty diagnosing whether the crop is suffering from Verticillium wilt or Fusarium wilt. Soil temperature can provide a clue, as can the pattern of discoloration in the stem, but laboratory tests are needed to verify which fungal family is causing concern.
Verticillium albo-atrum and Verticillium dahliae are the causative agent for wilts in many crops. Tomatoes, peppers, eggplants, potatoes, broad beans, beets, cucumbers and others are impacted by one or both of these fungal pathogens. Onions are not susceptible. These pathogens act to plug the xylem, as does Fusarium species, preventing adequate water intake, causing yellowing, wilting and necrosis. Both diseases also cause vascular streaking in stems.
Microsclerotia remain in the soil, able to survive for years. Verticillium fungi spread readily when roots are stressed or wounded, and fruiting plants seem most susceptible. Because of the wide host range of Verticillium fungi, eradicating it from fields can be difficult, as it can continue to propagate, increasing the number of fungal pathogens in the soil, happily replicating and causing disease in a wide variety of crop hosts.
All infected plant material, including roots, should be removed from the soil and destroyed. Soil sanitization can be effective, but may not be cost-effective. Avoid over-fertilization, which can enhance disease incidence, and plant resistant varieties if a problem is known to exist. Heavy soils with alkaline pH support Verticillium growth. Verticillium wilt fungi thrives in cool soil temperatures. Infected seeds and transplants can spread the disease.
Fungal wilt strategies
All plants in a field may not become infected when soil borne fungi are a concern. Depending on the distribution of spores, only plants in a certain area may show signs of infection. But as the population of fungal bodies increases in the soil, the entire field can play host to spores.
If seasonal growing conditions are not conducive to the fungi, there may be little infection and crop loss. If conditions which promote infection — preferred soil moisture levels, conducive soil temperatures, root wounds — occur, infection can spread rapidly throughout the field. Depending on plant growth stage, losses can be total or partial. Changing conditions can slow or hasten the disease progress in infected plants, too.
Using cover crops has been shown to provide some defense against soil borne fungal pathogens. Biological controls such as Trichoderma or Pseudomonas species may have some fungicidal effects, and utilizing non-pathogenic strains of disease causing fungi may reduce infection via competition with the infectious agent for root colonization of the crop plant. Intercropping or companion planting studies have shown that these strategies may be effective in protecting susceptible crops from fungal wilt diseases, possibly by root exudates which offer protection from colonization.
Soil borne fungal pathogens which cause wilting diseases are difficult to eradicate, are viable for long periods of time, and can result in significant losses in numerous economically important crops. Control requires an understanding of the life cycle and environmental needs of the fungal pathogen, and a combination of approaches to prevent widespread crop losses.