Early blight caused by Alternaria solani is a highly destructive disease of potatoes. Control of early blight mainly relies on the use of preventive fungicide treatments. Because of their high efficacy, azoxystrobin and other quinone outside inhibitors (QoIs) are commonly used to manage early blight. However, loss of sensitivity to QoIs has previously been reported for A. solani in the United States. Two hundred and three A. solani field isolates collected from 81 locations in Germany between 2005 and 2011 were screened for the presence of the F129L mutation in the cytochrome b gene; of these, 74 contained the F129L mutation. Sequence analysis revealed the occurrence of two structurally different cytb genes, which differed in the presence (genotype I) or absence (genotype II) of an intron, with genotype I being the most prevalent (63% of isolates). The F129L mutation was detected only in genotype II isolates, where it occurred in 97%. Sensitivity to azoxystrobin and pyraclostrobin was determined in conidial germination assays. All isolates possessing the F129L mutation had reduced sensitivity to azoxystrobin and, to a lesser extent, to pyraclostrobin. Early blight disease severity on plants treated with azoxystrobin was significantly higher for A. solani isolates with reduced fungicide sensitivity in the conidial germination assay compared with sensitive isolates. Data suggest an accumulation of F129L isolates in the German A. solani population over the years 2009-2011. It is assumed that the application of QoIs has selected for the occurrence of F129L mutations, which may contribute to loss of fungicide efficacy.

Epidemics of early blight caused by Alternaria spp. can cause significant economic damage to potato production if not adequately controlled. In order to improve control of Alternaria spp. in potato, studies were conducted to identify the optimal fungicide strategy and, if possible, to reduce the number of fungicide applications per growing season. Therefore, a disease-threshold-based framework was tested to define the optimal timing of fungicide application. The initiation and subsequent applications of fungicides were based on increases in disease incidence or severity. Adequate disease control was achieved by a three-time application with azoxystrobin, given that the applications were carried out at pivotal times in the epidemic. Targeted applications of fungicides reduced the number of sprayings required to protect starch yield. Results indicate that early blight can be effectively managed by using fungicide application thresholds based on disease progress.

Potato early blight and brown spot are important fungal diseases responsible for premature defoliation and yield loss of potato. Pathogens considered to be involved in leaf necrosis are Alternaria solani and A. alternata, respectively. Both diseases are commonly characterized by the visualization of leaf lesions. Current detection and identification methods for Alternaria species rely primarily on cultural and morphological characteristics, the assessment of which is time-consuming and not always suitable. Sensitive, reliable methods for estimating infection severity are therefore desirable. In this study, an Alternaria-specific real-time PCR assay was developed using primers based on internal transcribed spacers (ITS) 1 and 2. The assays facilitated species detection and clearly discriminated between A. solani and A. alternata. The use of realtime PCR allowed quantitative estimation of fungal biomass in plant tissues. Detection sensitivities were in the range of >100 fg. Real-time PCR applications used to accurately assess the extent of colonization by Alternaria spp. during disease development are reported here for the first time. Additionally, Alternaria genomic DNA levels were verified not only in potato leaves showing different levels of disease progress, but also in symptomless leaves. This assay provides a useful tool to quantify pathogen levels during initial latent stages of infection and will thus help in the early detection and quantification of Alternaria spp..

Field-grown potatoes showing scab infections were sampled in two successive years and analysed for prevailing Streptomyces strains. In 2008 and 2009, 293 Streptomyces isolates were collected in Germany and analysed for morphology, pathogenicity and strain type. Isolates varied in mycelium colour, sporulation and pigmentation. Based on their morphology, no clear differentiation of species was possible. At the genetic level, sampled isolates, as well as a number of type strains from culture collections, were characterized by PCR using 16S rRNA-specific primers and PCR-RFLP of the 16S-23S internal transcribed spacer (ITS) region with Hpy99I. Using this fingerprinting approach, Streptomyces species could be differentiated genotypically. The data from this study show that diversity among scab-causing species in Germany is much higher than previously thought. Isolates belonged to various Streptomyces spp. previously associated with common scab. This is apparently the first report of pathogenic strains of S. europaeiscabiei, S. stelliscabiei, S. acidiscabiei, S. turgidiscabiei and S. bottropensis within Germany. Streptomyces europaeiscabiei was the predominant species found. Other scab-causing species were identified, but their local distribution was uneven. For most of the isolates, the presence of the txtAB gene was demonstrated, indicating pathogenicity. This analysis is one of the first reports to examine the distribution of common scab-causing species in Germany.