Reaction of certain Solanaceous and Asteraceous plant species to inoculation with Phytophthora infestans in Cameroon

Fontem D A, Olanya O M and Njualem B F. 2004. Journal of Phytopathology 152: 331–336. © 2004 Blackwell Publishing Ltd.

Experiments were conducted to detect potential hosts of Phytophthora infestans, causal agent of potato late blight among weeds occurring in Cameroon. Isolates of P. infestans isolated from garden huckleberry (Solanum scabrum), potato (S. tuberosum) and tomato (S. lycopersicon) were inoculated on detached leaves of 12 solanaceous and 14 asteraceous species collected from the potato agroecosystem in the western highlands of Cameroon. Isolates of P. infestans from huckleberry and potato infected the same host plants as well as gboma eggplant (S. macrocarpon) and two asteraceous weeds; Billy goatweed (Ageratum conyzoides) and Dichrocephala (Dichrocephala integrifolia). Inoculum from potato caused late blight symptoms on haemorrhage plant (Aspilia africana); while inoculum from tomato resulted in late blight on worowo (Solanecio biafrae). This is the first report of late blight infection on S. macrocarpon, A. conyzoides, Sol. biafrae and Asp. africana in Cameroon. The research results indicate that some asteraceous and solanaceous weeds may be alternative hosts of P. infestans in the potato agroecosystem.

Corresponding author: Modesto Olanya, E-mail: modesto(at)maine.edu

Phenotypic and genotypic diversity in Phytophthora infestans on potato in Great Britain, 1995-98

Day J P, Wattier R A M, Shaw D S and Shattock R. 2004. Plant Pathology 53: 303-315. © 2004 Blackwell Publishing Ltd.

A total of 2691 single-lesion isolates of Phytophthora infestans was established from samples of late-blight disease from 354 commercial and garden/allotment sites in Scotland, England and Wales over four growing seasons, 1995-98. The A2 mating type was rare (3·0% of isolates) and was detected at only 34 sites. In vitro tests of sensitivity to the phenylamide fungicide metalaxyl showed that 316 sites yielded isolates with some insensitivity (resistant and/or intermediate); these were more often commercial sites than garden/allotment sites. Over the four seasons, the frequency of isolates with intermediate fungicide sensitivity increased, while the frequency of resistant phenotypes decreased. Resistant isolates were always of A1 mating type. A subset of 1459 isolates from 326 sites was analysed for molecular diversity. Mitochondrial DNA (mtDNA) haplotype Ia predominated (91·0% of isolates); haplotype IIa was present at 54 sites and both haplotypes at 33 sites. The multilocus RFLP probe RG57 detected 30 fingerprints. Four fingerprints were particularly common (RF002, RF006, RF039 and RF040) and 10 were unique to a single site in a single year. The three commonest fingerprints (RF039> RF002>RF006) were of A1 mating type and the fourth (RF040) was A2. RF002 isolates were resistant to the phenylamide metalaxyl and were more common in Scotland than in England and Wales. Small sample sizes limited the usefulness of estimates of diversity. Although approximately half of all sites appeared to be colonized by RF039 genotypes, some sites (both commercial and garden/allotment) showed a higher diversity, having both common and unique genotypes. The genotypic diversity within isolates collected from commercial sites and those from garden/allotment sites were similar. The contributions of sexual reproduction and alternatives to sex to the generation of variation are discussed.

Corresponding author: Richard C. Shattock, Email: r.c.shattock(at)bangor.ac.uk

Aggressiveness and host specificity of Brazilian isolates of Phytophthora infestans

Suassuna, N D, Maffia L A and Mizubuti E. S. G. 2004. Plant Pathology 53: 405-413. © 2004 Blackwell Publishing Ltd.

The population of Phytophthora infestans in Brazil consists of two clonal lineages, US-1 associated with tomatoes and BR-1 associated with potatoes. To assess whether host specificity in these lineages resulted from differences in aggressiveness to potato and tomato, six aggressiveness-related epidemiological components - infection frequency (IF), incubation period (IP), latent period (LP), lesion area (LA), lesion expansion rate (LER) and sporulation at several lesion ages (SSLA) - were measured on detached leaflets of late blight-susceptible potato and tomato plants. Infection frequency of US-1 was similar on potato and tomato leaflets, but IF of BR-1 was somewhat reduced on tomato. Incubation period was longer on both hosts with US-1, although this apparent lineage affect was not significant. Overall there was no host effect on IP. On potato, BR-1 had a shorter LP (110.3 h) and a larger LA (6.5 cm2) than US-1 (LP = 162.0 h; LA = 2.8 cm2). The highest LER resulted when isolates of BR-1 (0.121 cm2 h-1) and US-1 (0.053 cm2 h-1) were inoculated on potato and tomato leaflets, respectively. The highest values of the area under the sporulation capacity curve (AUSC) were obtained for isolates of US-1 inoculated on tomato leaflets (6146) and for isolates of BR-1 on potato leaflets (3775). In general, higher values of LA, LER, SSLA and AUSC, and shorter values of LP were measured when isolates of a clonal lineage were inoculated on their original host than with the opposite combinations. There is evidence that there are quantitative differences in aggressiveness components between isolates of US-1 and BR-1 clonal lineages that probably contribute to host specificity of P. infestans populations in Brazil.

Corresponding author: Eduardo Mizubuti, Email: mizubuti(at)ufv.br

Analysis of differentially expressed genes in a susceptible and moderately resistant potato cultivar upon Phytophthora infestans infection

Ros B, Thümmler F and Wenzel G. 2004. Molecular Plant Pathology 5:191-201. © 2004 Blackwell Publishing Ltd.

To gain deeper understanding of the host-pathogen interaction in the system potato-Phytophthora infestans, subtractive hybridization in combination with cDNA array hybridization was used. Leaflets of a moderately resistant and a susceptible potato cultivar were inoculated with P. infestans. The infection of the potato leaves was quantified by real-time quantitative PCR. Using infected and control tissue, two cDNA libraries highly enriched for P. infestans-induced genes were prepared. Within 531 clones randomly picked and sequenced from the libraries, 285 unigenes were found, from which 182 clones were selected for further analysis by cDNA array hybridization. Sixteen hours post inoculation genes were not induced significantly, whereas 72 h post inoculation induction of gene expression was clearly detectable. In both cultivars, 143 genes were induced moderately (> two-fold), and 35 of the selected genes appeared to be strongly induced (> seven-fold). Among these clones were mainly genes associated with stress and/or defense mechanisms. The strongest gene induction was found in 4-week-old susceptible plants. In the moderately resistant cultivar, transcripts of a number of genes accumulate with plant age; as a result, induction of gene expression upon infection was less pronounced. Down-regulation of three genes was observed in both cultivars, upon infection. Transcript levels of these three genes increased in uninfected plants within 4 weeks of growth. Other differences in defense responses of the two cultivars could be determined and their effects are discussed

Corresponding author: Barbara Ros, Email: barbara.ros(at)wzw.tum.de

The generation and quantification of resistance to dimethomorph in Phytophthora infestans

Stein J M and Kirk W W. 2004. Plant Disease 88:930–934.

The generation of dimethomorph resistance in Phytophthora infestans was attempted using ethidium bromide/UV light mutagenesis and repeated culturing on dimethomorph-amended medium. Ethidium bromide/UV mutagenesis created two isolates of P. infestans with resistance factors for dimethomorph >20, i.e., the ratio of the 50% effective concentration (EC(50)) of the mutant to that of the wild-type. With repeated culturing on dimethomorph-amended medium, the rate of growth (mm diameter/day) increased until the tenth subculture for most P. infestans isolates. Resistance factors generated from repeated culturing were <8 for all isolates. For most isolates, the generation of dimethomorph resistance resulted in reduced growth rates on nonamended medium, regardless of the level of resistance or induction treatment. Additionally, the frequency of infection of leaf disks and whole tubers was significantly reduced in >20% of the isolates repeatedly subcultured on dimethomorph-amended medium. Regardless of the induction treatment, reduced fitness was common for all P. infestans isolates, indicating a potential biological cost associated with dimethomorph resistance. Based on these results, the development of field resistance to dimethomorph in P. infestans is unlikely with the currently employed resistance management strategies.

Corresponding author: Willie Kirk, Email: kirkw(at)msu.edu

Survival of Phytophthora infestans in surface water

Porter L D and Johnson D A. 2004. Phytopathology 94:380-387.

Coverless petri dishes with water suspensions of sporangia and zoospores of Phytophthora infestans were embedded in sandy soil in eastern Washington in July and October 2001 and July 2002 to quantify longevity of spores in water under natural conditions. Effects of solar radiation intensity, presence of soil in petri dishes (15 g per dish), and a 2-h chill period on survival of isolates of clonal lineages US-8 and US-11 were investigated. Spores in water suspensions survived 0 to 16 days under nonshaded conditions and 2 to 20 days under shaded conditions. Mean spore survival significantly increased from 1.7 to 5.8 days when soil was added to the water. Maximum survival time of spores in water without soil exposed to direct sunlight was 2 to 3 days in July and 6 to 8 days in October. Mean duration of survival did not differ significantly between chilled and nonchilled sporangia, but significantly fewer chilled spores survived for extended periods than that of nonchilled spores. Spores of US-11 and US-8 isolates did not differ in mean duration of survival, but significantly greater numbers of sporangia of US-8 survived than did sporangia of US-11 in one of three trials.

Corresponding author: Lyndon Porter, Email: ldporter(at)uidaho.edu

Management of potato late blight in the Peruvian highlands: Evaluating the benefits of Farmer Field Schools and Farmer Participatory Research

Ortiz O, Garrett K, Heath J J, Orrego R and Nelson R J. 2004 Plant Disease 88: 565–571.

Farmer field school programs incorporating farmer participatory research (FPR-FFS) have the potential to provide important benefits to their participants and to other farmers who benefit from improved cultivars and management techniques. An FPR-FFS program in San Miguel, Cajamarca, Peru, has been in place since 1999 with an emphasis on management of potato late blight, caused by Phytophthora infestans, the most important problem facing Andean potato growers. Farmers’ knowledge of late blight was surveyed to determine useful components for the FPR-FFS curriculum. The benefits to participants of FPR-FFS programs were evaluated first by measuring knowledge of late blight management of participants and nonparticipants. Studies of the Peruvian FPR-FFS program indicate that participants are more knowledgeable and that their expertise further increases after an additional year of participation. The benefits to participants can be evaluated more directly by comparing the productivity of participants’ farms compared with nonparticipants’ farms. For the Peruvian FPR-FFS program, participants had significantly higher average levels of productivity. The benefits of FPR-FFS programs for the development of better cultivars and management techniques can be evaluated indirectly in terms of improved estimates of performance. Estimates of the overall performance of a particular cultivar or technique become more precise as data from more sites are included in calculating the estimate. A more direct evaluation of benefits from FPR-FFS input may be based on the ability of farmers in FPR-FFS programs to eliminate undesirable genotypes and to recommend desirable genotypes based on criteria in addition to those used by breeders.

Corresponding author: Karen Garrett, Email: kgarrett(at)ksu.edu

These abstracts were reprinted with the kind permission of Blackwell Publishing (www.blackwellpublishing.com) and the American Phytopathological Society (www.apsnet.org)