Review on Distribuction, Bilology and Management of Tomato Powdery Mildew (Oldium Nelycopersici)

Abstract

Oidium neolycopersi is powdery white lesions found on leaves, petioles; the stem and the calyx characterize the causal agent of tomato powdery mildew. This disease severe infections lead to marked reductions in fruit size and quality, and to leaf mortality. Environmental conditions influence the different stages of the disease cycle. Severe disease outbreaks develop under moderate temperature and moderate relative humidity conditions (22-25ºC, 70-85% RH. Widespread leaf disease that affects several vegetable crops, in the field and in the greenhouse , but this pathogen affects numerous plants, especially solanaceous crops (pepper, eggplant, potato and tobacco) and weeds (nightshade). Infection begins when a conidium lands on the leaf surface and germinates forming a germ tube, a process which takes between three and nine hours to occur and is induced by contact with a hard surface. The smooth-surfaced germ tube elongates and locates a suitable penetration site, often at the intersection of three epidermal cells, and forms a lobed or ‘clover-leaf’ appressorium. Once completed, a haustorium is formed within the host cell, a lobed structure from which the pathogen takes up nutrients. Secondary hyphae emerge from the conidium and appressorium to produce further appressoria, resulting in rapid colonisation of the host. The cycle is complete when after a latent period of around five days conidia are produced on conidiophores positioned perpendicular to the leaf surfaceand are dispersed by wind. Oidium neolycopersici is a highly poly phagous powdery mildew fungus which infects tomatoes. It causes powdery white lesions on the adaxial tomato leaf surface. The fungus can also infect abaxial surfaces, petioles and the calyx but the fruit remains un infected. Severe infections lead to leaf chlorosis, premature senescence and a marked reduction in fruit size and quality. The disease may reduce the yields to 60-80% of the normal harvest and in general processing tomatoes are harvested much later and the harvest of processing tomatoes is more affected by powdery mildew. Fresh tomatoes are harvested earlier and it is possible to have high levels of infection in the field but no yield reduction. However, it is the fruit borer activity that has the major economic impact and the most dramatic damage is produced in the fruit itself, Fruits can be attacked as soon as they are formed and the galleries bored inside them can be invaded by secondary pathogens leading to fruit rot. Finally, an important additional problem is that the disease directly feeds on the growing tip, thereby halting plant development. Avoid high density stands, and promote air circulation through the canopy. Avoid high nitrogen rates. Scout regularly. Eliminate infected plants particularly in greenhouse production. The control was primarily through the use of chemical fungicides, although with some variability in efficacy. The need for more sustainable control methods has been recognized and a number of alternative control approaches have been investigated, including cultural and physical controls such as electrostatic spore precipitators, electrostatic discharge generators, climate management, sunflower oil and the addition of silicates to hydroponic systems.

Keywords:Tomato, powdery mildew and management