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Fifty-seven isolates of Phytophthora infestans from blighted potato foliage were collected in 1995 in southern Germany and analysed for mating type and sensitivity to metalaxyl. Fifty-six of them were characterised as A1 and one as A2 mating types. Resistance to metalaxyl was observed frequently: 53 isolates were resistant, three were partially sensitive, and one was sensitive. In a subsequent field study in 1999, 84 isolates collected from blighted potato and tomato foliage were analysed for mating type. Seventy-two were characterised as A1 and twelve as A2 mating types. The response of 76 isolates to metalaxyl and to propamocarb was tested. The majority (42) of the 76 isolates was classified as resistant to metalaxyl; 31 were partially sensitive and only three isolates were sensitive. The results with propamocarb were less discrete; 10 isolates were classified as resistant and three were clearly sensitive. AFLP fingerprinting was used to examine the genetic structure of the southern German P. infestans population collected in 1999 and indicated that the tested population can be sub-divided into a tomato group, a potato group and a mixed group containing isolates collected from both crops. The presence of Ia and IIa mitochondrial DNA haplotypes indicates that the German P. infestans isolates belong to the new pathogen population that has also been reported in neighbouring regions of Europe. The present study indicates that at the beginning of the season only a few genotypes were present, and the population became genetically more variable at the end of the growing season.

For organic potato producers, the two main challenges are disease and nutrient management. Both are limited by regulations that on the one hand prohibit the use of chemical fertilizers, especially nitrogen, and on the other hand prohibit most synthetic pesticides. Late blight caused by Phytophthora infestans is commonly thought to be the most yield-reducing factor. However, because there is no really effective fungicide available to control late blight, there are virtually no yield loss data available for organic farming conditions. In this paper, the state of the art of organic potato tuber growth under on-farm conditions with respect to disease and nutrient management is summarized by field trials and on-farm surveys on commercial organic crops carried out in the years 1995–1998. Soil nitrogen (N) levels, plant N uptake, disease development of P. infestans and potato yield were measured. Results indicated that N availability was most important in limiting yields in organic potato crops. From on-farm data, a model including disease development, growth duration of the crops until foliage decay and different parameters related to N status of the crop could explain 73% of the observed variation in yield. Only 25% of this variation in yield could be attributed to the influence of late blight. Differences in N availability explained 48%. In conclusion, several points emerged from the results. In organic farming, yields are mainly limited by nutrient availability in spring and early summer. The effects of late blight on yields may often be overestimated and cannot be deduced from results in conventional farming because of the strong interaction with nutrient status. Depending on N availability, tubers stop growing between mid-July (70–90 kg N ha−1 uptake), the end of July (110–140 kg N ha−1 uptake) and mid-August (140–180 kg N ha−1 uptake) due to N limitations. The higher the N status of a potato crop, the longer the growing period needed to achieve the attainable yield and the higher the probability that late blight stops further tuber growth and becomes the key tuber-yield-limiting factor. In the second part of this paper, the interactive effects of soil N availability and the impact of P. infestans on yield in the presence and absence of fungicides from 1996 to 1998 for mid-early main crops are reported. An empirical schematic model of disease impact depending on N availability was developed.

As a basis for a prognosis model for the protection of sweet cherries against the cherry leaf scorch, caused by Gnomonia erythrostoma, this research investigates the formation and propagation of the spores of the disease between 1998 and 2001. G. erythrostoma forms perithecia with ascospores as sexual fruiting bodies and pycnidia with conidia as asexual fruiting bodies. It was observed that the beginning of the spore ejection differed considerably between the years concerned; in 1998 the first ascospores, which were released from perithecia formed in leaves during the previous year, were detected at the end of April, in 1999 at the end of March and in 2000 at the middle of March. The duration of spore maturation depends on the temperatures between autumn and spring. The higher the temperatures the faster the maturation and the earlier the spore dispersal. The ejection of spores takes place in the spring, only under the condition that the dead and dried leaves from the year before are thoroughly wet. The perithecia-covered leaves remain on the tree throughout the winter until summer, which makes the disease very distinct. These conditions for ejection occur usually only after sufficient precipitation or after nights with extremely high humidity (≥ 90%). The end of the spore dispersal depends on the frequency of precipitation and the amount of the inoculum. Regular rainfall results in short spore dispersal, longer periods without precipitation lead to a longer spore propagation. Two conditions have to be met for the infection to occur: first, the leaves have to be developed completely or the fruit has to be visible; and second, the leaves have to be thoroughly wet for at least six hours for the spores to germinate and successfully infect the leaves or fruits. An infection with conidia can be excluded due to their lack of germination ability.