Abstract
Department of Plant and Environmental Sciences, UMB.
In the early 1990s Norwegian lettuce production changed from mainly being a greenhouse production towards increased field production. Following the increased intensity in the lettuce production, disease problems have become more severe, and in p articular lettuce downy mildew (
Bremia lactucae) has been the cause of severe losses. This study has been part of several projects with focus on epidemiology and control of downy mildew, in addition to a survey of other fungal pathogens in the lettuce fields.
Especially for downy mildew, the risk of fungicide resistance in the pathogen and loss of resistance in new cultivars is protruding, demonstrating the need to implement integrated pest management (IPM) strategies to minimize the number of fungicide applications and optimize the timing of applications relative to infection periods. The distribution and incidence of
B. lactucae is highly variable, but this pathogen is the most important disease in Norwegian lettuce fields due to the destructive nature of uncontrolled epidemics. Other pathogens considered to be of economic importance in Norwegian field lettuce are
Septoria lactucae,
Sclerotinia sclerotiorum,
Pythium tracheiphilum,
Alternaria spp.,
Botrytis cinerea and
Rhizoctonia solani. Lettuce downy mildew appears at different times and locations every year, but the disease is more frequent in the most intensive lettuce production areas. The unpredictable outbreaks of
B. lactucae in Norway may be an indication of the possible role of long distance transport of airborne inoculum in initiation of epidemics. Also, the diversity found among isolates of
B. lactucae may contribute to increased selection and loss of resistance in the lettuce cultivars.
To achieve a better understanding of how
B. lactucae is affected by the long days and short nights during midsummer at Nordic latitudes, studies on the effects of light in sporulation of
B. lactucae were conducted. We found that sporulation is timed by a diurnal rhythm, and the direct effect of light or the absence of light is of less importance for sporulation than previously assumed.
Two forecasting models, PlantPlus (PP) and Modell-Analys (MA), have been tested in late season lettuce fields in Norway. Both models were useful alternatives to a calendar based schedule for fungicide applications in lettuce, although the number of fungicide applications was only slightly reduced for PP, while MA prompted a slight increase in number of fungicide applications compared to a 10-day schedule. However, the knowledge derived from field trials and trapping of
B. lactucae spores over lettuce fields has provided valuable information regarding possible adjustments of the MA model. Adjusted criteria for MA provided a better accuracy of the model, increasing the percentage of correctly predicted spore production events from 66 % to 78 %.
The results have contributed to a better understanding of the epidemiology of B. lactucae and to adjustments of an existing forecasting model. Other diseases may not be as devastating as downy mildew, but are nevertheless often of economic importance in the lettuce production.
List of Papers
Impact of diurnal periodicity, temperature, and light on sporulation of Bremia lactucae.Nordskog, B., Gadoury, D.M., Seem, R.C. and Hermansen A.
Phytopathology,
Accepted
Evaluation of disease forecast models in relation to airborne inoculum and fungicide control of Bremia lactucae in lettuce.Nordskog, B., Gadoury, D.M., Seem, R.C. and Hermansen A.
Manuscript
Virulence characteristics of Bremia lactucae populations present in Norway from 2001 to 2005.Nordskog, B. and Hermansen, A.
Manuscript
A survey of fungi causing diseases in Norwegian lettuce fields
Nordskog, B., Nærstad, R., Herrero, M.L., Sundheim, L. and Hermansen, A.
Manuscript
Updated: 01.08.06
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