A research proposal is presented to challenging the late blight problem in organic potato production by the better use of resistance present in potato varieties available today. It is based on an attempt to include variety rotation and/or variety mixtures in addition to the established methods of sanitation and prevention. In addition geographic isolation may also contribute to reduction of late blight impact in the Norwegian situation.
Late blight (Phytophthora infestans) is a major constraint in organic potato production and results in large losses of yield and agronomic inputs. The increased variation in the pathogen population in Europe made the situation even worse after introduction of a mating type of the fungus in the early eighties. This variability in the pathogen population may also present possibilities when the sources of resistance in the host population is exploited in a strategic way. This paper puts forward some methodological thoughts to cope with the problem of late blight.
Dealing with late blight in an organic agriculture setting requires first of all a basic knowledge of plant disease epidemiology. It gives an insight into the impact of the various management decisions that influence the development of the disease. Major parameters are initial inoculum and rate of disease development. In addition we have the genetic diversity in both host and pathogen populations. These elements dominate in the development of a strategy to prevent, as much as possible, late blight in organic potato production.
A major problem lies in the fact that an epidemic caused by Phytophthora infestans can develop rapidly under favourable weather conditions. Spores can spread over large distances, although its density will decrease with further distance from the source. It is the reduction of spore density that will delay the onset of an epidemic. This plays a role in areas with a land use pattern with little potato production, as is the situation in a large part of Norwegian agriculture. In these situations a major input in removing and destroying potential sources of inoculum may even result in the further delay of the onset of the disease. In more intensive potato cultivation, a concerted action among growers, both in commercial and home production is required, since one single inoculum source may be sufficient to nullify the efforts of many. This is not easy to establish, but positive results are documented (Førsund, 1970).
A broad range of activities can achieve a reduction of the rate of disease development. Variety choice, plant distance and plant nutrition are just some. The choice of each measure depends largely on the local situation and the consideration of the impact of each possible agricultural measure.
Increasing the host biodiversity in the field may counteract the increased genetic variation in the pathogen population and its subsequent effect on late blight disease. This increase in host biodiversity may be obtained through mixing potato varieties or mixing with other crop species in row cropping systems. This will reduce the epidemic development (Andrivon and Lucas, 1998).
Slowing down the epidemic development does not exclude the appearance of new fungal genotypes and its subsequent building up of their inoculum for the next season. New genotypes will only increase in density when a susceptible potato cultivar is present since Phytophthora is a biotrophic fungus. Since no potato cultivars are completely resistant to late blight a certain late blight development will occur if host and pathogen are present.
Epidemiological studies with fungal genotype analysis (Zwankhuizen et al., 1998) revealed that most epidemics on single field level deal with only one fungal genotype. The genotypes surviving in the field from one growing season to the next season may be dominated by those that were responsible for the late blight development in the previous season. Using another potato cultivar with another type of resistance may delay the onset of the late blight epidemic in a subsequent season. The proposed strategy therefore, includes cultivar rotation in addition to the well-established crop rotation.
The choice of the strategy depends largely on the local situation. Since the rate of a late blight epidemic is mainly determined by biotic (the presence of inoculum, susceptibility of the host to certain races of the pathogen) and abiotic factors (moisture and temperature). In the Norwegian situation, late blight comes usually later in the season. In areas with low seasonal temperatures the crop can even escape infection. In our experience we met situations in which potato varieties which are susceptible to late blight could withstand infection almost throughout the season, whilst other varieties became destroyed mid-season. This indicates the potential of the strategy.
A complete escape of the pathogen can only take place in situations with low growing temperatures using healthy seed-tubers. This is mainly restricted to mountain farms. A similar escape takes place in areas with dry summer conditions. In most areas with potato growing the weather is favourable for late blight. Thus the potential of escape is rather limited.
As stated before, potato production covers a limited area in Norwegian agriculture: cultivated fields can be far away from each other. This means that individual farmers may profit to a larger extent from the preventive measures laid out on their own farm. A delay of the epidemic at a time that the crop is fully established and that has a maximal interception on sunlight has a large impact on the efficient use of inputs. In this respect, one should also remember the potato growing in home and allotment gardens. Delay of epidemics has a potential in Norwegian conditions.
It is expected that the proposed strategies have a potential to reduce the destructive effect of late blight in organic potato production. However, a major risk lies in the dependence on the consequent implementation of the strategy.
Andrivon, D. and J.M. Lucas (1998): Performance of cultivar associations to control the potato late blight pathogen Phytophthora infestans. Book of abstracts 7th International Congress on Plant Pathology, Edinburgh, Scotland. 9-16 August 1998.
Førsund, E. (1970): Forsøk med avl av sjukdomsfrie settepoteter i Gjøvdal i Åmli herad, Aust- Agder i åra 1965-69. [Experiments with the cultivation of disease free seed potatoes in Gjøvdal in Åmli, East-Agder in the period 1965-69]. Internal Report Planteforsk.
Zwankhuizen, M.J., F. Govers and J.C. Zadoks (1998): Development of potato late blight epidemics: Disease foci, gradients and infection sources. Phytopathology 88: 754-763 pp.
