Relations of Cercospora beticola with Host Plants and Fungal Antagonists
Cerco pora leaf pot (CLS) cau ed by Cercospora beticola Sacc., is sti ll considered to be the mo t important foliar di ease of ugar beel. The di ea e ha been reported wherever ugar beet i grO\\ n (Bieiholder and Weltzien 1972). Since the di ease wa fir t identified. management of CLS of ugar beet has been an ongoing mi ion of plant pathologists. Toda}. everal trategie are available and applied either s ingly or in combination to manage the di ea e. The e management trategie , which were ummarized by Windels et al. (1998), include cultural practice uch a deep tillage, rotation with non-host crops and identification and elimination of econdary weed host . Other include breeding and use of re i.tant cultivars and application of fungicide. The u e of re i tant sugar beet cultivars has long been an integral pan of CLS management; however, problems associated with selection of re i tant cuhivars again t CLS are wel l documented and were recently reviewed by Weiland and Koch (2004). According to the author , resistance to CLS in sugar beet has been de cribed as quantitatively in herited and rate limiting with respect to disease development (Smith and Gaskill 1970, Ro i et a l. 1999). Although resistant cultivars have proven effective in both North America and Europe, they nonetheless exhibit low heritability (Smith and Ruppel 1974), and cultivar bred for Cercospora resistance can still exhibit leaf pots if climatic condition favorable for the disease occur.
Differentiation and Detection of Sugar Beet Fungal Pathogens Using PCR Amplification of Actin Coding Sequences and the ITS Region of the rRNA Gene.
The DNA sequences of the actin genes of several fungi were compared and highly conserved regions in the coding sequence were identified. Deoxyoligonucleotide primers were synthesized based on conserved sequence blocks in the 5' and 3' ends of the open reading frame encoding the actin protein. In addition, primers (internal transcribed spacer [ITS] regions 1 and 4) based on conserved regions of the ribosomal RNA (rRNA) genes of fungi were synthesized. Use of the primers in the polymerase chain reaction (PCR) resulted in the amplification of DNA products from the genomes of sugar beet fungal pathogens of a size consistent with the amplification of the actin gene and rRNA gene sequences, respectively, in these fungi. With one primer pair (5FWDACT and MIDREVACT) directed to the actin gene, the major products amplified from the DNA of Aphanomyces cochlioides, Pythium ultimum, Cercospora beticola, Phoma betae, Fusarium oxysporum, and Rhizoctonia solani were of the sizes of 0.9, 0.9, 1.1, 1.1, 1.2 and 1.7 kilobase pairs (kbp), respectively, whereas no product was generated from the DNA of sugar beet (Beta vulgaris L.). Restriction endonuclease digestion of products amplified using 5FWDACT and MIDREVACT permitted the differentiation of A. cochlioides from A. euteiches. Use of ITS1 and ITS4 in PCR reactions employing the same template DNAs and reaction conditions yielded single products of 0.7, 0.8, 0.5, 0.5, 0.6, and 0.7 kbp, respectively, as well as a 0.7-kbp product from DNA of sugar beet. The data indicate that actin and rRNA gene sequences are appropriate targets for the development of PCR-based strategies for distinguishing sugar beet fungal pathogens at the genus level. The presence of A. cochlioides DNA in extracts of diseased sugar beet seedlings was detected using PCR with primers 5FWDACT and MIDRE-VACT.
Tissue specific localization of root infection by fungal pathogens: role of root border cells.
When roots of pea seedlings were inoculated uniformly with spores of Nectria haematocca or other pea pathogenic fungi, more than 90% developed lesions in the region of elongation within 3 days. More mature regions of most roots as well as the tip showed no visible signs of infection. Yet, microscopic observation revealed that 'mantles,' comprised of fungal hyphae intermeshed with populations of border cells, covered the tips of most roots. After physical detachment of the mantle, the underlying tip of most roots was found to be free of infection. Mantle-covered root tips did not respond to invasion of their border cells by activation of known defense genes unless there was invasion of the tip itself, as revealed by the presence of a lesion. Concomitant with the activation of defense genes was the induction of a cell-wall degrading enzyme whose expression is a marker for renewed production of border cells. Mantle formation did not occur in response to nonpathogens. The data are consistent with the hypothesis that border cells serve as a host-specific 'decoy' that protects root meristems by inhibiting fungal infection of the root tip.