Healthy strawberry plant (left) and strawberry plant infected with verticillium wilt (right), photo by Howard F. Schwartz, Colorado State University, Bugwood.org
Verticillium wilt is a wiltdisease affecting over 350 species of eudicotplants. It is caused by six species of Verticilliumfungi: V. dahliae, V. albo-atrum, V. longisporum, V. nubilum, V. theobromae and V. tricorpus.[1] 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 non-tarped potato production in North America . 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.
Hosts and symptoms
Verticillium dahliae infected sunflowers, photo by Howard F. Schwartz, Colorado State University, Bugwood.orgVerticillium albo-atrum infected tree crown, USDA Forest Service Archive, USDA Forest Service, Bugwood.org
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.[2] A list of known hosts is at the bottom of this page.
The symptoms are similar to most wilts with a few specifics to Verticillium. Wilt itself is the most common symptom, 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.[3] 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.[2]
In Verticillium, the symptoms 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.[2]
Disease cycle
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,[4] while microsclerotia have been observed in fields planted with non-host crops for over 10 years [5] and even 15 years has been reported.[2] 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),[6] 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.[7]
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, 115cm, 24 hours after initial conidia inoculation, so the spread throughout the plant can occur very quickly.[8] 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.[9]
A heavily infected plant can succumb to the disease and die. As this occurs, the Verticillium will form its survival structures and when the plant dies, its survival structures will be where the plant falls, releasing inoculates into the environment. The survival structures will then wait for a host plant to grow nearby and will start the cycle all over again.
Besides being long lasting in the soil, Verticillium can spread in many ways. The most common way of spreading short distances is through root to root contact within the soil. Roots in natural conditions often have small damages or openings in them that are easily colonized by Verticillium from an infected root nearby. Air borne conidia have been detected and some colonies observed, but mostly the conidia have difficulty developing above ground on healthy plants.[10] In open channel irrigation, V. dahliae have been found in the irrigation ditches up to a mile from the infected crop.
Without fungicidal seed treatments, infected seeds are easily transported and the disease spread, and Verticillium has been observed remaining viable for at least 13 months on some seeds. Planting infected seed potatoes can also be a source of inoculum to a new field. Finally, insects have also been shown to transmit the disease. Many insects including potato leaf hopper, leaf cutter bees, and aphids have been observed transmitting conidia of Verticillium and because these insects can cause damage to the plant creating an entry for the Verticillium, they can help transmit the disease.[3]
Environment
While Verticillium wilts often have the same symptoms of Fusarium wilts, Verticillium can survive cold weather and winters much better than Fusarium, which prefers warmer climates. The resting structures of Verticillium are able to survive freezing, thawing, heat shock, dehydration, and many other factors and are quite robust and difficult to get rid of. The one factor they do not tolerate well is extended periods of anaerobic conditions (such as during flooding).[3]
Verticillium will grow best between 20 and 28 degrees Celsius,[2] but germination and growth can occur well below (or above) those temperatures. Still, Verticillium will generally not survive in the branches and trunks of infected trees during hot, dry seasons in regions such as summer in southern California. This does not generally "cure" the entire tree, however, and recurrence can happen via a reinfection from the roots during winter and spring.[11] Water is necessary for resting structure germination, but is not as important for the spread of the fungus as in many other fungi. While not an environmental requirement for the fungus, stressed plants, often brought on by environmental changes, are easier to attack than healthy plants, so any conditions that will stress the plant but not directly harm the Verticillium will be beneficial for Verticillium wilt development.[3]
Management
Verticillium wilt begins as a mild, local infection, which over a few years will grow in strength as more virile strains of the fungus develop. If left unchecked the disease will become so widespread that the crop will need to be replaced with resistant varieties, or a new crop will need to be planted altogether.[2]
Control of Verticillium can be achieved by planting disease–free plants in uncontaminated soil, planting resistant varieties, and refraining from planting susceptible crops in areas that have been used repeatedly for solanaceous crops. Soil fumigation can also be used, with chloropicrin being particularly effective in reducing disease incidence in contaminated fields.
In tomato plants, the presence of ethylene during the initial stages of infection inhibits disease development, while in later stages of disease development the same hormone will cause greater wilt. Tomato plants are available that have been engineered with resistant genes that will tolerate the fungus while showing significantly lower signs of wilting.[2]
Verticillium albo-altrum, Verticilium dahliae and V. longisporum can overwinter as melanized mycelium or microsclerotia within live vegetation or plant debris. As a result, it can be important to clear plant debris to lower the spread of disease. Verticilium dahliae and V. longisporum are able to survive as microsclerotia in soil for up to 15 years.[2]
Importance
Verticillium wilt occurs in a broad range of hosts but has similar devastating effects on many of these plants. In general, it reduces the quality and quantity of a crop by causing discoloration in tissues, stunting, and premature defoliation and death.[12] Stock from infested nurseries may be restricted. Once a plant is infected, there is no way to cure it. Verticillium wilt is especially a concern in temperate areas and areas that are irrigated. Verticllium spp. can naturally occur in forest soils and when these soils are cultivated, the pathogen will infect the crop.[2]
The Salinas Valley in California has had severe problems with Verticillium wilt since 1995, most likely due to flooding in the winter of 1995. Many areas in the Salinas and Pajaro Valleys are unable to grow lettuce due to the high levels of Verticillium dahliae in the soil.[13] Potatoes grown in Verticillium infested soils may have a reduced yield between 30–50% compared to potatoes grown in "clean" soil. Verticillium wilt has also caused a shift in peppermint cultivation from the Midwest in the mid- to late-1800s to western states such as Oregon, Washington and Idaho, to new, non-infested areas within these states now.[12]
Lists of plants susceptible or resistant
Replanting susceptible species on the site of a removed plant that has succumbed to V. albo-atrum or V. dahliae is inadvisable because of the heightened risk of infection. Instead, resistant or immune varieties should be used. The following two lists show both susceptible and resistant/immune plants by Latin name.[11][14][15][16][17][18]
(*) indicates that the plant occurs on both lists because different varieties or cultivars vary in their resistance. (#) indicates that some strains are resistant. (+) indicates susceptibility to some European strains of Verticillium albo-atrum.
Fusarium wilt is a common vascular wilt fungal disease, exhibiting symptoms similar to Verticillium wilt. This disease has been investigated extensively since the early years of this century. The pathogen that causes Fusarium wilt is Fusarium oxysporum. The species is further divided into formae speciales based on host plant.
Northern root-knot nematode is a species of vegetable pathogens which produces tiny galls on around 550 crop and weed species. They invade root tissue after birth. Females are able to lay up to 1,000 eggs at a time in a large egg mass. By surviving harsh winters, they can survive in cold climates.
Verticillium is a genus of fungi in the division Ascomycota, and are an anamorphic form of the family Plectosphaerellaceae. The genus used to include diverse groups comprising saprobes and parasites of higher plants, insects, nematodes, mollusc eggs, and other fungi, thus the genus used to have a wide-ranging group of taxa characterised by simple but ill-defined characters. The genus, currently thought to contain 51 species, may be broadly divided into three ecologically based groups - mycopathogens, entomopathogens, and plant pathogens and related saprotrophs. However, the genus has undergone recent revision into which most entomopathogenic and mycopathogenic isolates fall into a new group called Lecanicillium.
Ralstonia solanacearum is an aerobic non-spore-forming, Gram-negative, plant pathogenic bacterium. R. solanacearum is soil-borne and motile with a polar flagellar tuft. It colonises the xylem, causing bacterial wilt in a very wide range of potential host plants. It is known as Granville wilt when it occurs in tobacco. Bacterial wilts of tomato, pepper, eggplant, and Irish potato caused by R. solanacearum were among the first diseases that Erwin Frink Smith proved to be caused by a bacterial pathogen. Because of its devastating lethality, R. solanacearum is now one of the more intensively studied phytopathogenic bacteria, and bacterial wilt of tomato is a model system for investigating mechanisms of pathogenesis. Ralstonia was until recently classified as Pseudomonas, with similarity in most aspects, except that it does not produce fluorescent pigment like Pseudomonas. The genomes from different strains vary from 5.5 Mb up to 6 Mb, roughly being 3.5 Mb of a chromosome and 2 Mb of a megaplasmid. While the strain GMI1000 was one of the first phytopathogenic bacteria to have its genome completed, the strain UY031 was the first R. solanacearum to have its methylome reported. Within the R. solanacearum species complex, the four major monophyletic clusters of strains are termed phylotypes, that are geographically distinct: phylotypes I-IV are found in Asia, the Americas, Africa, and Oceania, respectively.
Macrophomina phaseolina is a Botryosphaeriaceae plant pathogen fungus that causes damping off, seedling blight, collar rot, stem rot, charcoal rot, basal stem rot, and root rot on many plant species.
Stemphylium solani is a plant pathogen fungus in the phylum Ascomycota. It is the causal pathogen for grey leaf spot in tomatoes and leaf blight in alliums and cotton, though a wide range of additional species can serve as hosts. Symptoms include white spots on leaves and stems that progress to sunken red or purple lesions and finally leaf necrosis. S. solani reproduces and spreads through the formation of conidia on conidiophores. The teleomorph name of Stemphyllium is Pleospora though there are no naturally known occurrences of sexual reproduction. Resistant varieties of tomato and cotton are common, though the pathogen remains an important disease in Chinese garlic cultivation.
Verticillium albo-atrum is a plant pathogen with many hosts.
Verticillium dahliae is a fungal plant pathogen. It causes verticillium wilt in many plant species, causing leaves to curl and discolor. It may cause death in some plants. Over 400 plant species are affected by Verticillium complex.
Verticillium longisporum, also known as Verticillium Wilt, is a fungal plant pathogen that commonly infects canola. V. longisporum can attack other brassica plants as well as woody ornamentals. A main symptom of the infected plant is wilting. In America, V. longsiporum primarily effects eudicot plants. This pathogen can be very devastating and hard to eradicate, responding only to expensive fumigation or fungal resistant plants.
Rosellinia bunodes is a plant pathogen infecting several hosts including avocados, bananas, cacao and tea.
Pratylenchus penetrans is a species of nematode in the genus Pratylenchus, the lesion nematodes. It occurs in temperate regions worldwide, regions between the subtropics and the polar circles. It is an animal that inhabits the roots of a wide variety of plants and results in necrotic lesions on the roots. Symptoms of P. penetrans make it hard to distinguish from other plant pathogens; only an assay of soil can conclusively diagnose a nematode problem in the field. P. penetrans is physically very similar to other nematode species, but is characterized by its highly distinctive mouthpiece. P. penetrans uses its highly modified mouth organs to rupture the outer surface of subterranean plant root structures. It will then enter into the root interior and feed on the plant tissue inside. P. penetrans is considered to be a crop parasite and farmers will often treat their soil with various pesticides in an attempt to eliminate the damage caused by an infestation. In doing this, farmers will also eliminate many of the beneficial soil fauna, which will lead to an overall degradation of soil quality in the future. Alternative, more environmentally sustainable methods to control P. penetrans populations may be possible in certain regions.
A wilt disease is any number of diseases that affect the vascular system of plants. Attacks by fungi, bacteria, and nematodes can cause rapid killing of plants, large tree branches or even entire trees.
Black rot, caused by the bacterium Xanthomonas campestris pv. campestris (Xcc), is considered the most important and most destructive disease of crucifers, infecting all cultivated varieties of brassicas worldwide. This disease was first described by botanist and entomologist Harrison Garman in Lexington, Kentucky, US in 1889. Since then, it has been found in nearly every country in which vegetable brassicas are commercially cultivated.
Devendra Prasad Gupta was an Indian pre-democratic political sufferer, botanist and academician.
Helicobasidium is a genus of fungi in the subdivision Pucciniomycotina. Basidiocarps are corticioid (patch-forming) and are typically violet to purple. Microscopically they have auricularioid basidia. Asexual anamorphs, formerly referred to the genus Thanatophytum, produce sclerotia. Conidia-bearing anamorphs are parasitic on rust fungi and are currently still referred to the genus Tuberculina.
Verticillium nonalfalfae is a soilborne fungus in the order Hypocreales. It causes verticillium wilt in some plant species, particularly Ailanthus altissima. The fungus produces a resting mycelium characterized by brown-pigmented hyphae. It is most closely related to V. dahliae and V. alfalfae.
Black dot disease is caused by the plant pathogen known as Colletotrichum coccodes. It affects all underground parts of the potato, including the roots, tuber, and stolon. C. coccodes is also able to infect the stem and foliage of the potato plant. It can be introduced into an area by planting an infected tuber, and then spreading to other plants. A sign of black dot disease is black microsclerotia that are produced by the pathogen, and can be found on the roots, the tuber, the stems, and the leaves. This can be used to diagnose black dot. Symptoms of black dot disease include silvery lesions on the surface of the tuber, brown or black lesions on the leaves, leaf wilting, and chlorosis.
Alternaria brassicicola is a fungal necrotrophic plant pathogen that causes black spot disease on a wide range of hosts, particularly in the genus of Brassica, including a number of economically important crops such as cabbage, Chinese cabbage, cauliflower, oilseeds, broccoli and canola. Although mainly known as a significant plant pathogen, it also contributes to various respiratory allergic conditions such as asthma and rhinoconjunctivitis. Despite the presence of mating genes, no sexual reproductive stage has been reported for this fungus. In terms of geography, it is most likely to be found in tropical and sub-tropical regions, but also in places with high rain and humidity such as Poland. It has also been found in Taiwan and Israel. Its main mode of propagation is vegetative. The resulting conidia reside in the soil, air and water. These spores are extremely resilient and can overwinter on crop debris and overwintering herbaceous plants.
Māori potatoes or taewa are varieties of potato cultivated by Māori people, especially those grown before New Zealand was colonised by the British.
Alternaria leaf spot or Alternaria leaf blight are a group of fungal diseases in plants, that have a variety of hosts. The diseases infects common garden plants, such as cabbage, and are caused by several closely related species of fungi. Some of these fungal species target specific plants, while others have been known to target plant families. One commercially relevant plant genus that can be affected by Alternaria Leaf Spot is Brassica, as the cosmetic issues caused by symptomatic lesions can lead to rejection of crops by distributors and buyers. When certain crops such as cauliflower and broccoli are infected, the heads deteriorate and there is a complete loss of marketability. Secondary soft-rotting organisms can infect stored cabbage that has been affected by Alternaria Leaf Spot by entering through symptomatic lesions. Alternaria Leaf Spot diseases that affect Brassica species are caused by the pathogens Alternaria brassicae and Alternaria brassicicola.
1 2 3 4 Pegg, G.F., Brady, B.L. (2002) Verticillium Wilts, CABI Publishing, New York, NY.
↑ Luck, J.V. (1954) Studies on the Verticillium wilt of Mentha piperita L. with special emphasis on the causal organism, Verticillium albo-atrum R. &B. Dissertation Abstracts 14, 916-917.
↑ Wilhelm, S. (1955). "Longevity of the Verticillium wilt fungus in the laboratory and field". Phytopathology. 45 (3): 180–181.
↑ Sewell, G.W.F. (1959). "Direct observation of Verticillium albo-atrum in soil". Transactions of the British Mycological Society. 42 (3): 312–IN9. doi:10.1016/S0007-1536(56)80039-9.
↑ Garber, R.H. (1973) United States Department of Agriculture Publication 1. ARS-S-19. pp. 69-77.
↑ Presley, J. T.; Carns, H.R.; Taylor, E.E.; Schnathorst, W.C. (1966). "Movement of conidia of Verticillium albo-atrum in cotton plants". Phytopathology. 56: 375.
↑ Easton, G.D.; Nagle, M.E.; Bailey, D.L. (1969). "A method of estimating Verticillium albo-atrum propagules in field soil and irrigation waste water". Phytopathology. 59: 1171–1172.
1 2 Sinclair, Wayne A.; Lyon, Howard H. (2005). Diseases of Trees and Shrubs (Seconded.). Cornell University Press. pp.242–245. ISBN978-0-8014-4371-8. See also W. A. Sinclair and G. W. Hudler, "Cornell Tree Pest Leaflet A-3 (Revised), 12/84."
↑ R. J. Stipes, Professor of Plant Pathology, Virginia Tech and Mary Ann Hansen, Extension Plant Pathologist, Virginia Tech, "Verticillium Wilt of Shade TreesArchived 2009-01-17 at the Wayback Machine ", Publication Number: 450-619, Posted May 2000"
↑ Verticillium wilt of vegetables and herbaceous ornamentals,2011-3-20
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