Downy mildew

Downy mildew
Pathogen Responsible

Bremia lactucae

Description and Sympotoms

Downy mildew causes light green to yellow angular spots on the upper surfaces of leaves. White fluffy growth of the fungus develops on the lower sides of these spots. With time these lesions turn brown and dry up. Older leaves are attacked first. Severely infected leaves may die. On rare occasions the pathogen can become systemic, causing dark discoloration of stem tissue.

Genetics of Resistance

The known Dm genes are located in at least five clusters in the lettuce genome (Hulbert & Michelmore, 1985; Farrara et al., 1987; Bonnier et al., 1994).  The major cluster contains over nine genetically separable Dm specificities, as well as resistance to root aphid.  Another large cluster contains several Dm genes, resistance to the root-infecting downy mildew Plasmopara lactucae-radicis, and the hypersensitive reaction to Turnip Mosaic Virus (Witsenboer et al., 1995).  In addition to mapping the known resistance genes, we have conducted germplasm screens for resistance to downy mildew, corky root, and anthracnose (Brown et al., 2004; Farrara & Michelmore, 1987; Michelmore, 1994 & unpublished).  For Bremia, we have identified over 100 new genotypes that were resistant to all isolates of B. lactucae tested; these represented the most virulent isolates from our worldwide collection.

References
  • Farrara, B., Michelmore, R.W.  (1987).  Identification of new sources of resistance in Lactuca spp.  HortScience 22:647-649. 
  • Farrara, B., Illott, T.W., Michelmore, R.W.  (1987). Genetic analysis of factors for resistance to downy mildew (Bremia lactucae) in lettuce (Lactuca sativa).  Plant Pathol. 36:499-514. 
  • Maisonneuve, B., Bellec, Y., Anderson, P., Michelmore, R.W. (1994).  Rapid mapping of two genes for resistance to downy mi ldew from Lactuca serriola to existing clusters of resistance genes.  Theor. Appl. Genet. 89:96-104. 
  • Michelmore, R.W., Paran, I., Kesseli, R.V. (1991).  Identification of markers linked to disease resistance genes by bulked segregant analysis: a rapid method to detect markers in specific regions by using segregating populations.  Proc. Natl. Acad. Sci. (USA) 88:9828-9832. 
  • Truco, M.J., Antonise, R., Lavelle, D., Ochoa, O., Kozik, A., Witsenboer, H., Fort, S., Jeuken, M.J.W., Kesseli, R. V., Lindhout, P., Michelmore, R.W., Peleman, J. (2007).  A high-density, integrated genetic linkage map of lettuce (Lactuca spp.). Theor. Appl. Genet. 115:735-746.