Verticillium wilt

Verticillium wilt is a wilt disease affecting over 350 species of eudicot plants. It is caused by six species of Verticillium fungi: V. dahliae, V. albo-atrum, V. longisporum, V. nubilum, V. theobromae and V. tricorpus. (See, for example, Barbara, D.J. & Clewes, E. (2003). 'Plant pathogenic Verticillium species: how many of them are there?' Molecular Plant Pathology 4(4).297-305. Blackwell Publishing.) Many economically important plants are susceptible including cotton, tomatoes, potatoes, oilseed rape, eggplants, peppers and ornamentals, as well as others in natural vegetation communities. Many eudicot species and cultivars are resistant to the disease and all monocots, gymnosperms and ferns are immune. Signs are superficially similar to Fusarium wilts. There are no fungicides characterized for the control of this disease but soil fumigation with chloropicrin has been proven successful in dramatically reducing Verticillium wilt in diverse crops such as vegetables using plasticulture production methods, and in potato production in North America (non-tarped). Additional strategies to manage the disease include crop rotation, the use of resistant varieties and deep plowing (to accelerate the decomposition of infected plant residue). In recent years, pre-plant soil fumigation with chloropicrin in non-tarped, raised beds has proven to be economically viable and beneficial for reducing wilt disease and increasing yield and quality of potato in North America. Soil fumigation is a specialized practice requiring special permits, equipment, and expertise, so qualified personnel must be employed. Verticillium spp. attack a very large host range including more than 350 species of vegetables, fruit trees, flowers, field crops, and shade or forest trees. Most vegetable species have some susceptibility, so it has a very wide host range. A list of known hosts is at the bottom of this page. The signs are similar to most wilts with a few specifics to Verticillium. Wilt itself is the most common sign, with wilting of the stem and leaves occurring due to the blockage of the xylem vascular tissues and therefore reduced water and nutrient flow. In small plants and seedlings, Verticillium can quickly kill the plant while in larger, more developed plants the severity can vary. Some times only one side of the plant will appear infected because once in the vascular tissues, the disease migrates mostly upward and not as much radially in the stem. Other symptoms include stunting, chlorosis or yellowing of the leaves, necrosis or tissue death, and defoliation. Internal vascular tissue discoloration might be visible when the stem is cut. In Verticillium, the signs and effects will often only be on the lower or outer parts of plants or will be localized to only a few branches of a tree. In older plants, the infection can cause death, but often, especially with trees, the plant will be able to recover, or at least continue living with the infection. The severity of the infection plays a large role in how severe the signs are and how quickly they develop. While Verticillium spp. are very diverse, the basic life cycle of the pathogen is similar across species, except in their survival structures. The survival structures vary by species with V. albo-atrum forming mycelium, V. dahliae forming microsclerotia, V. nigrescens and V. nubilum forming chlamydospores, and V. tricorpus forming all three. While resting, many factors such as soil chemistry, temperature, hydration, micro fauna, and non-host crops all have an effect on the viability of the resting structure. Mycelium have been observed remaining viable for at least 4 years, while microsclerotia have been observed in fields planted with non-host crops for over 10 years and even 15 years has been reported. Viability is reduced at these extremes, but the long survivability of these structures is an important aspect for Verticillium control. When roots of a host crop come near the resting structure (about 2mm), root exudate promotes germination and the fungi grows out of the structure and toward the plant. Being a vascular wilt, it will try to get to the vascular system on the inside of the plant, and therefore must enter the plant. Natural root wounds are the easiest way to enter, and these wounds occur naturally, even in healthy plants because of soil abrasion on roots. Verticillium has also been observed entering roots directly, but these infections rarely make it to the vascular system, especially those that enter through root hairs. Once the pathogen enters the host, it makes its way to the vascular system, and specifically the xylem. The fungi can spread as hyphae through the plant, but can also spread as spores. Verticillium produce conidia on conidiophores and once conidia are released in the xylem, they can quickly colonize the plant. Conidia have been observed traveling to the top of cotton plants, 115 cm, 24 hours after initial conidia inoculation, so the spread throughout the plant can occur very quickly. Sometimes the flow of conidia will be stopped by cross sections of the xylem, and here the conidia will spawn, and the fungal hyphae can overcome the barrier, and then produce more conidia on the other side.