Downy mildew

About: Downy mildew is a research topic. Over the lifetime, 4739 publications have been published within this topic receiving 53418 citations.

Arabidopsis is susceptible to infection by a downy mildew fungus.

Abstract: A population of Arabidopsis thaliana growing locally in a suburb of Zurich called Weiningen was observed to be infected with downy mildew. Plants were collected and the progress of infection was investigated in artificial inoculations in the laboratory. The plants proved to be highly susceptible, and pronounced intercellular mycelial growth, haustoria formation, conidiophore production, and sporulation of the causal organism Peronospora parasitica were all observed. The formation of oogonia, antheridia, and oospores also occurred. In contrast, Arabidopsis strain RLD was resistant to infection and none of the above structures was formed. The fungus was localized very soon after penetration of RLD leaf cells, which responded with a typical hypersensitive reaction. The differential interaction of an isolate of P. parasitica with two strains of Arabidopsis opens up the possibility of cloning resistance determinants from a host that is very amenable to genetic and molecular analysis.

Disease-induced assemblage of a plant-beneficial bacterial consortium.

Abstract: Disease suppressive soils typically develop after a disease outbreak due to the subsequent assembly of protective microbiota in the rhizosphere. The role of the plant immune system in the assemblage of a protective rhizosphere microbiome is largely unknown. In this study, we demonstrate that Arabidopsis thaliana specifically promotes three bacterial species in the rhizosphere upon foliar defense activation by the downy mildew pathogen Hyaloperonospora arabidopsidis. The promoted bacteria were isolated and found to interact synergistically in biofilm formation in vitro. Although separately these bacteria did not affect the plant significantly, together they induced systemic resistance against downy mildew and promoted growth of the plant. Moreover, we show that the soil-mediated legacy of a primary population of downy mildew infected plants confers enhanced protection against this pathogen in a second population of plants growing in the same soil. Together our results indicate that plants can adjust their root microbiome upon pathogen infection and specifically recruit a group of disease resistance-inducing and growth-promoting beneficial microbes, therewith potentially maximizing the chance of survival of their offspring that will grow in the same soil.

Differential Recognition of Highly Divergent Downy Mildew Avirulence Gene Alleles by RPP1 Resistance Genes from Two Arabidopsis Lines

Abstract: The perception of downy mildew avirulence (Arabidopsis thaliana Recognized [ATR]) gene products by matching Arabidopsis thaliana resistance (Recognition of Peronospora parasitica [RPP]) gene products triggers localized cell death (a hypersensitive response) in the host plant, and this inhibits pathogen development. The oomycete pathogen, therefore, is under selection pressure to alter the form of these gene products to prevent detection. That the pathogen maintains these genes indicates that they play a positive role in pathogen survival. Despite significant progress in cloning plant RPP genes and characterizing essential plant components of resistance signaling pathways, little progress has been made in identifying the oomycete molecules that trigger them. Concluding a map-based cloning effort, we have identified an avirulence gene, ATR1NdWsB, that is detected by RPP1 from the Arabidopsis accession Niederzenz in the cytoplasm of host plant cells. We report the cloning of six highly divergent alleles of ATR1NdWsB from eight downy mildew isolates and demonstrate that the ATR1NdWsB alleles are differentially recognized by RPP1 genes from two Arabidopsis accessions (Niederzenz and Wassilewskija). RPP1-Nd recognizes a single allele of ATR1NdWsB; RPP1-WsB also detects this allele plus three additional alleles with divergent sequences. The Emco5 isolate expresses an allele of ATR1NdWsB that is recognized by RPP1-WsB, but the isolate evades detection in planta. Although the Cala2 isolate is recognized by RPP1-WsA, the ATR1NdWsB allele from Cala2 is not, demonstrating that RPP1-WsA detects a novel ATR gene product. Cloning of ATR1NdWsB has highlighted the presence of a highly conserved novel amino acid motif in avirulence proteins from three different oomycetes. The presence of the motif in additional secreted proteins from plant pathogenic oomycetes and its similarity to a host-targeting signal from malaria parasites suggest a conserved role in pathogenicity.

Systemic Fungicides and the Control of Oomycetes

Abstract: Diseases caused by oomycetous fungi of the order Peronosporales present major problems worldwide. Important foliar diseases include late blight on potatoes, blue mold on tobacco, grape downy mildew, plus a wide range of other foliar blights and downy mildews on cereals, fruits, vegetables, and ornamentals. Soilborne Phytophthora and Pythium spp. are equally wide­ spread and are the cause of major losses on crops as diverse as soybeans (148) and avocados (33, 51). In addition, Phytophthora and Pythium spp. are responsible for many preand postharvest problems on fruits and vegetables, including late blight of potato tubers (10), brown rot of citrus (35-37, 94), and black pod of cocoa (125, 126). Less than a decade ago our means of adequately controlling many of these diseases was extremely limited. Control of potato late blight and downy mildews involved repeated applications of protectant fungicides, starting well in advance of the general appearance of the disease. The control of many soilborne pathogens was even more limited. Fumigation with methyl bromide gas, or soil drenches with nematicides, offered only limited control for some high-value annual crops. Soilborne pathogens of field and perennial tree crops were not controlled by chemical methods, because of either cost or lack of efficacy of available chemicals. However, the discovery of systemic fungicides with good activity against Oomycetes in the early 1970s continues to revolutionize our concepts of