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The first symptoms are pale chlorotic spots, with an indefinite margin, on the upper surface of the leaf. Sporulation, which occurs on the lower surface of the leaf beneath the spots, is downy, light grey but becoming buff to tawny-brown or olive green. Defoliation may occur if the infection is severe enough. The optimum temperature for disease development is 22-24°C and, therefore, in temperate regions the disease is almost invariably limited to greenhouse crops. In the tropics, the disease is most severe during cooler periods when relative humidity is high. Leaf and sepal penetration is via stomata; there are no appressoria formed. The importance of high relative humidity for penetration, lesion growth and sporulation has long been recognized.
Cultural Control and Sanitary Methods
It has been shown that in the glasshouse, disease incidence can be decreased (and yield increased) by limiting the periods of high relative humidity. There was less leaf mould at a constant 20°C compared with a 20°C (day) and 13°C (night) regime (Winspear et al., 1970). However, other means have been tested. Vakalounakis (1992) grew two tomato cultivars in a greenhouse covered with a long-wave infrared absorbing (IRA)-vinyl film, which absorbs infrared emission by soil and plants during the night, and in a control greenhouse covered with a common agricultural (CA)-polyethylene film. At the end of the crop seasons, total disease index for leaf mould, caused by P. fulva (amongst other fungi), on both cultivars was much less in the IRA-vinyl greenhouse than in the CA-vinyl greenhouse. Another method used soil sterilization by solar heating by polyethylene mulching, followed by covering the soil again with plastic and planting seedlings through holes made in the covers. Hasan (1989) found that this reduced the severity of tomato leaf mould (P. fulva) as well as tomato yellow leaf curl bigeminivirus and early blight (Alternaria solani).
Breeding work in N. America and Europe has used oligogenic resistance in a range of Lycopersicon species (such as L. chilense, L. hirsutum, L. peruvianum and L. pimpinellifolium) to introduce resistance genes into L. esculentum: for instance, the Cf-2 gene was introduced from L. pimpinellifolium and Cf-9 from L. esculentum var. cerasiforme (Dickinson et al., 1993). The highest percentage of effective sources of resistance to P. fulva and Tobacco mosaic virus were found in Lycopersicon peruvianum, L. hirsutum and L. chilense (Vlasova, 1986). For further details about the mechanism of resistance and pathogenicity see section on Biology and Ecology: Physiological races and genetics.
Apart from generating a hypersensitive response, it appears that some tomatoes can produce a glycoalkaloid, tomatine, which is toxic to P. fulva, the degree of toxicity depending on the pH and nutrient status of the assay medium. Dow and Callow (1978) found that tomatine exerted both fungistatic and fungicidal effects.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
In UK glasshouse tomatoes, P. fulva was found in 28% of crops but was severe in only 3% (Fletcher and Harris, 1979). However, a recent quantification of the impact of P. fulva in economic terms is not available. In developed countries the seriousness of P. fulva outbreaks has decreased as the means of control have been improved and new varieties introduced.