The Gram‐negative bacterium Erwinia amylovora, causal agent of fire blight disease in pome fruit trees, encodes a type three secretion system (T3SS) that translocates effector proteins into plant cells that collectively function to suppress host defences and enable pathogenesis. Until now, there has only been limited knowledge about the interaction of effector proteins and host resistance presented in several wild Malus species. This study tested disease responses in several Malus wild species with a set of effector deletion mutant strains and several highly virulent E. amylovora strains, which are assumed to influence the host resistance response of fire blight‐resistant Malus species. The findings confirm earlier studies that deletion of the T3SS abolished virulence of the pathogen. Furthermore, a new gene‐for‐gene relationship was established between the effector protein Eop1 and the fire blight resistant ornamental apple cultivar Evereste and the wild species Malus floribunda 821. The results presented here provide new insights into the host–pathogen interactions between Malus sp. and E. amylovora.

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Pythium isolates from diseased and dead bait plants of maize and cress grown in compost or various soils (maize fields, parkland under deciduous trees, grassland) were characterised and tested for pathogenicity to maize (Zea mays L.). In pot tests performed under controlled conditions, pathogenicity of the isolates to maize was apparent by reduction of root and shoot growth, whereas damping-off of maize seedlings was less frequent. Contrarily, pea seedlings were killed by pathogenic Pythium isolates. Pythium isolates from diseased maize seedlings and pathogenic strains from other gramineous plants (P. phragmitis, P. aff.phragmitis, P. catenulatum) were not necessarily more virulent to maize compared to isolates originating from dicotyledonous plants (cress). The most virulent isolates originated from compost and caused a reduction of maize shoot growth of up to 60%. Phylogenetic analysis revealed that they were very closely related to P. ultimum var. ultimum and P. arrhenomanes, respectively. Isolates originating from maize fields, grassland and parkland under deciduous trees, a reference culture of P. arrhenomanes and strains of P. phragmitis, P. aff. phragmitis and P. catenulatum with known pathogenicity on reed were non-pathogenic on maize. Isolates from compost, and from maize fields generally had a higher temperature optimum for mycelial growth (30 °C) and a faster growth rate (1.5–2.0 mm h−1) compared to the isolates from parkland under deciduous trees and grassland soil (20–25 °C, ~1.0 mm h−1), respectively. This study indicates a potential impact of pathogenic Pythium on maize plants even in the absence of visible symptoms.

The presence of foliar pathogens often leads to yield losses in winter wheat (Triticum aestivum L.), the most important crop in Germany. In this study the efficacy of different host resistance levels of eight wheat cultivars and three fungicide strategies on fungal disease control was studied in terms of yield and net return in field trials at five sites over three crop years. Fungicide treatments included a situation-related strategy in which cultivars were treated individually based on disease control thresholds, a practice-related strategy in which all cultivars were treated after disease thresholds had been exceeded in one cultivar, and an untreated control. Disease severity and incidence differed between cultivars and were reduced by fungicide treatments compared to the untreated control. On average over all locations and years, the Fungicide Treatment Frequency Index (TFI) of all cultivars treated with the situation-related strategy was significantly lower than those treated with the practice-related strategy, except the highly susceptible cultivar JB Asano. A reduction of the TFI by up to 82% was possible in the situation-related strategy. Despite slightly increased yields in the practice-related strategy compared to the situation-related strategy, these could not compensate for the higher fungicide costs in most cases and led to lower net returns. The cultivars with multi-disease resistance showed clear advantages. Their potential benefits are not only demonstrated by the attainment of high yields, but also in fungicide savings without reducing net returns, provided that their disease resistance characteristics are taken into account.

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