Uncinula necator
Encyclopedia
Uncinula necator is a fungus that causes powdery mildew
of grape. It is a common pathogen of Vitis
species, including the wine grape, Vitis vinifera. The fungus is believed to have originated in North America. European varieties of Vitis vinifera are more or less susceptible to this fungus. Uncinula necator infects all green tissue on the grapevine, including leaves and young berries. It can cause crop loss and poor wine quality if untreated. The sexual stage of this pathogen requires free moisture to release ascospores from its cleistothecia in the spring. However, free moisture is not needed for secondary spread via conidia; high atmospheric humidity is sufficient. Its anamorph is called Oidium tuckeri.
This mildew can be treated with sulfur
or fungicide
s; however resistance to several chemical classes such as Benomyl
, the DMI
s, and Strobilurin
s has developed. While synthetic fungicides are often recommended as applications around bloom, it is common to include sulfur in a tank mix to help with resistance management.
The primary inoculum process begins with an ascogonium (female) and antheridium (male) joining to produce an offspring. This offspring, a young clesitothecium, is used to infect the house immediately or overwinter on the host to infect when the timing is right (typically in spring). To infect, it produces a conidiophore that then bears conidia. These conidium move along to a susceptible surface to germinate. Once these spores or conidia germinate, they produce a structure called a haustoria, capable of "sucking" nutrients from the plant cells directly under the epidermis of the leaf. At this point, the fungi can infect leaves, buds and twigs that then reinfect other plants or further infect the current host. From this point, you see more white powdery signs of powdery mildew, and these structures produce secondary inoculum to reinfect the host with mycelium and conidia, or use the mycelium to produce primary inoculum to another plant.
For germination to occur using a cleistothecium, the cleistothecium must be exposed to the right environmental conditions to rupture the structure to thereby release spores in hope that they'll germinate. Germination of conidia occurs at temperatures between 7 and 31°C and is inhibited above 33°C. Germination is greatest at 30-100% relative humidity.
Young underdeveloped tissues are most susceptible to infection, primarily leaves and fruit. Warmer weather cultivars like Vitis vinifera and French hybrids provide overwintering protection in buds and during moderate winters climates. American cultivars are generally less susceptible to infection unless an unusually warm winter does not kill the cleistothecia in buds. Most cleistothecia survive on the vine where ample protection is provided in the bark.
Current organic agricultural practices still use a sulfur formulation as a treatment for powdery mildew. However, some cultivars like Concord are susceptible to phytotoxic damage with sulfur use. Since the fungus grows on tissue surfaces rather than inside epithelial cells, topical applications of oils and other compounds are recommended. Integrated pest management
programs are utilized by organic and conventional agriculture systems, while the latter prescribes the addition of fungicides.
Typical applications of fungicides occur during prebloom and for 2–4 weeks post bloom. If the previous year was a conducive environment for infection or the current year had a warm winter, earlier sprays are recommended due to a potentially higher amount of overwintered cleistothecia. If warm and humid, conidia are produced every 5–7 days throughout the growing season. To limit powdery mildew resistance, growers alternate treatments by employing multiple modes of action.
Powdery mildew
Powdery mildew is a fungal disease that affects a wide range of plants. Powdery mildew diseases are caused by many different species of fungi in the order Erysiphales. It is one of the easier diseases to spot, as its symptoms are quite distinctive. Infected plants display white powdery spots on the...
of grape. It is a common pathogen of Vitis
Vitis
Vitis is a genus of about 60 species of vining plants in the flowering plant family Vitaceae. The genus is made up of species predominantly from the Northern hemisphere. It is economically important as the source of grapes, both for direct consumption of the fruit and for fermentation to produce...
species, including the wine grape, Vitis vinifera. The fungus is believed to have originated in North America. European varieties of Vitis vinifera are more or less susceptible to this fungus. Uncinula necator infects all green tissue on the grapevine, including leaves and young berries. It can cause crop loss and poor wine quality if untreated. The sexual stage of this pathogen requires free moisture to release ascospores from its cleistothecia in the spring. However, free moisture is not needed for secondary spread via conidia; high atmospheric humidity is sufficient. Its anamorph is called Oidium tuckeri.
This mildew can be treated with sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
or fungicide
Fungicide
Fungicides are chemical compounds or biological organisms used to kill or inhibit fungi or fungal spores. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality and profit. Fungicides are used both in agriculture and to fight fungal infections in animals...
s; however resistance to several chemical classes such as Benomyl
Benomyl
Benomyl is a fungicide which was introduced in 1968 by DuPont. It is a systemic benzimidazole fungicide that is selectively toxic to microorganisms and to invertebrates, especially earthworms. Benomyl binds to microtubules, interfering with cell functions such as meiosis and intracellular...
, the DMI
1,3-Dimethyl-2-imidazolidinone
1,3-Dimethyl-2-imidazolidinone is a cyclic urea used as a high-boiling polar aprotic solvent. It is colourless, transparent, highly polar and has high thermal and chemical stability. It is a homolog of the related solvent DMPU...
s, and Strobilurin
Strobilurin
Strobilurins are a group of chemical compounds used in agriculture as fungicides. They are part of the larger group of QoI inhibitors, which act to inhibit the respiratory chain at the level of Complex III....
s has developed. While synthetic fungicides are often recommended as applications around bloom, it is common to include sulfur in a tank mix to help with resistance management.
Summary
Powdery mildews are generally host-specific, and powdery mildew of grape is caused by a host-specific pathogen named Uncinula necator. Powdery mildew is a polycylic disease that thrives in warm, moist environments. Its symptoms are widely recognizable and include gray-white fungal growth on the surface of infected plants. A sulfur formulation, fungicides, and limiting the environmental factors that favor the growth of powdery mildews are all practices that can stall and/or halt its growth.Hosts and symptoms
Powdery mildews are generally host-specific. Uncinula necator is the pathogen that causes powdery mildew on grape and linden. The most susceptible hosts of this pathogen are members of the species Vitis. The signs of powdery mildews are widely recognizable and easily identifiable. The majority of them can be found on the upper sides of the leaves; however, it can also infect the bottom sides, buds, flowers, young fruit, and young stems. A gray-white, dusty, fungal growth consisting of mycelia, conidia and conidiophores coat much of the infected plant. Cleistothecia, which are the overwintering structures, present themselves as tiny, spherical fruiting structures that go from white, to yellowish-brown to black in color, and are about the size of the head of a pin. Symptoms that occur as a result of the infection include necrosis, stunting, leaf curling, and a decrease in quality of the fruit produced.Disease cycle
Powdery mildew is a polycyclic disease (one which produces a secondary inoculum) that initially infects the leaf surface with primary inoculum, which is conidia from mycelium, or secondary inoculum, which is an overwintering structure called a cleistothecium. When the disease begins to develop, it looks like a white powdery substance.The primary inoculum process begins with an ascogonium (female) and antheridium (male) joining to produce an offspring. This offspring, a young clesitothecium, is used to infect the house immediately or overwinter on the host to infect when the timing is right (typically in spring). To infect, it produces a conidiophore that then bears conidia. These conidium move along to a susceptible surface to germinate. Once these spores or conidia germinate, they produce a structure called a haustoria, capable of "sucking" nutrients from the plant cells directly under the epidermis of the leaf. At this point, the fungi can infect leaves, buds and twigs that then reinfect other plants or further infect the current host. From this point, you see more white powdery signs of powdery mildew, and these structures produce secondary inoculum to reinfect the host with mycelium and conidia, or use the mycelium to produce primary inoculum to another plant.
For germination to occur using a cleistothecium, the cleistothecium must be exposed to the right environmental conditions to rupture the structure to thereby release spores in hope that they'll germinate. Germination of conidia occurs at temperatures between 7 and 31°C and is inhibited above 33°C. Germination is greatest at 30-100% relative humidity.
Environment
Powdery Mildew thrives in warm, moist environments and infects younger plant tissues like fruit, leaves, and green stems and buds. Free water can disrupt conidia and only requires a humid microclimate for infection. Most infection begins when spring rain (2.5mm) falls and temperatures are approximately 15°C or higher. Rates of infection decline at temperatures higher than 30°C, since the evaporation of water occurs readily. Cooler conditions, such as shading and poor aeration, promote infection due to a higher relative humidity, optimally 85% or greater. However, sporulation does occur at levels as low as 40%. Spores are dispersed mostly by wind and rain splash.Young underdeveloped tissues are most susceptible to infection, primarily leaves and fruit. Warmer weather cultivars like Vitis vinifera and French hybrids provide overwintering protection in buds and during moderate winters climates. American cultivars are generally less susceptible to infection unless an unusually warm winter does not kill the cleistothecia in buds. Most cleistothecia survive on the vine where ample protection is provided in the bark.
Management
First and foremost, limiting environmental factors that promote infection are key to managing powdery mildew on grapes. Optimal sites feature full sun on all grape structures and ample aeration to reduce humid microclimates under shading leaves. Pruning vines and clusters and planting on a gentle slope and orienting in rows running North and South promote full sun and aeration. Dusting leaves and berries with lime and sulfur was effective in 1850’s during the epidemic Europe.Current organic agricultural practices still use a sulfur formulation as a treatment for powdery mildew. However, some cultivars like Concord are susceptible to phytotoxic damage with sulfur use. Since the fungus grows on tissue surfaces rather than inside epithelial cells, topical applications of oils and other compounds are recommended. Integrated pest management
Integrated Pest Management
Integrated pest management is an ecological approach to agricultural pest control that integrates pesticides/herbicides into a management system incorporating a range of practices for economic control of a pest...
programs are utilized by organic and conventional agriculture systems, while the latter prescribes the addition of fungicides.
Typical applications of fungicides occur during prebloom and for 2–4 weeks post bloom. If the previous year was a conducive environment for infection or the current year had a warm winter, earlier sprays are recommended due to a potentially higher amount of overwintered cleistothecia. If warm and humid, conidia are produced every 5–7 days throughout the growing season. To limit powdery mildew resistance, growers alternate treatments by employing multiple modes of action.
Importance
The disease effects grapes worldwide, leaving all agricultural grape businesses at risk of Uncinula necator. Powdery mildew of grape affects the size of the vines, the total yield of fruit, as well as affecting the taste of wine produced from infected grapes. The disease can also cause the blossoms to fall and result in failure to produce fruit.External links
- WD Gubler, MR Rademacher, SJ Vasquez, CS Thomas. 1999. Control of powdery mildew using the UC Davis Powdery Mildew Risk Index. APSnet Feature.
- http://www.agf.gov.bc.ca/cropprot/grapeipm/mildew.htm