Powdery Mildews
Authors: Ridout Christopher James 
Published: September 2009

Powdery mildews are an important group of parasitic fungi causing disease in over 7600 species of host plants. They are obligate biotrophic parasites, obtaining their nutrients from the host through intracellular feeding structures known as haustoria. Many species of powdery mildews have a high degree of host specialization, infecting only one or a few closely related host genera. Some powdery mildews have well‐documented gene‐for‐gene (GFG) resistance interactions with their host plant, revealing information about the nature of host specificity. This article focuses on the interactions between powdery mildews and the host plant. Events in the establishment of haustoria and biotrophy are reviewed and their influence of infection on host metabolism described. GFG, durable

mlo

and nonhost‐resistance mechanisms are also described.

Key Concepts

Importance of powdery mildews
: Powdery mildews are extremely successful fungal parasites. Over 700 species have been described infecting over 7600 species of host plants, many of which are important crops species. Chemicals (fungicides) and resistance genes are used against them, but powdery mildews have the ability to rapidly adapt and defeat these control measures.

Obligate biotrophy
: All powdery mildews are obligate biotrophs, requiring a living host to grow and reproduce.

Haustoria
: The feeding structures produced by powdery mildews to obtain nutrients from epidermal cells of the host plants.

Host specificity
: Many powdery mildews have a high degree of host specialization, and can only grow on one or a few closely related plant genera. Infection can even be restricted to single genotypes of a specific host, as typified by gene‐for‐gene (GFG) resistance.

GFG resistance
: GFG resistance has been extensively investigated in the interaction between barley powdery mildew (

Blumeria graminis
f. sp.
hordei

) and the host plant, barley. In GFG resistance, an avirulence (
avr
) gene in the parasite matches a resistance (
R
) gene in the host causing a recognition event that triggers defence responses and prevents infection by the parasite. If either the
avr
gene or the
R
gene is lacking, the parasite is not recognized and can become established as a parasite. In barley, over 85 powdery mildew
R
genes have been described, and over 25 avr gene loci have been mapped in
Bgh.

mlo resistance
: A loss‐of‐function mutation in the barley MLO protein results in failure of
Bgh
to establish a successful infection.

mlo

resistance has been widely used in barley breeding, and has so far proved to be durable.

Nonhost resistance
: Plants are resistant to most invading pathogens, a feature described as nonhost resistance (NHR). Only those adapted microbes that are able to overcome this NHR are able to infect and become pathogens. The extreme specificity of some powdery mildew/host interactions has enabled investigation into the basis of NHR. A series of

pen

Arabidopsis

mutants with enhanced penetration by nonadapted
Bgh
have been defined. Genes coding for
pen1‐pen3
have been identified, enabling key proteins required for NHR to be identified.