IAPAR-Instituto Agronômico do Paraná, CP 481, Londrina, PR, Brazil

Background and objectives
Severe epidemic outbreak of a leaf blight of cotton caused by Stemphylium solani occurred in the State of Paraná, Brazil, during 1994-96, causing up to 100% yield losses in some commercial fields of cultivar Paraná 3 of Gossypium hirsutum [1]. S. solani was thought to produce a phytotoxin. Studies were conducted to verify the relationship between the reaction of the pathogen and its toxin on different cotton cultivars as well as on some unrelated host species.

Materials and methods
Toxin-containing culture filtrates of 20 single-spore isolates of S. solani, obtained from different cultivars and locations in the State of Paraná, were used with or without autoclaving for two bioassay procedures in different dilutions. A partially purified toxin preparation of one of the isolates was also tested. Leaves and roots of 3-10-day-old cotton seedlings were infiltrated/suspended in the culture filtrates and 4 days later were evaluated for their reaction, considering leaf necrosis, increase in root length and seedling death. For correlative studies between a cultivar's resistance/toxin insensitivity, seven single-spore isolates of S. solani were arbitrarily selected and were used for inoculation/infiltration either alone or in mixtures. Young leaves of three adult plants of each one of the 42 cotton cultivars, were tested under controlled environmental conditions in two experiments in a randomized complete block design. Fifteen cultivars were common to both experiments. Plant species in 12 host/non-host genera were also tested. At 7 days after inoculation/infiltration, leaves were detached from the plants and the leaf area infected by the pathogen (LAI) and the leaf area necrotized by the phytotoxin (LAN) were assessed using a portable area meter. All data were subjected to statistical analysis.

Results and conclusions
The amount of toxin production was isolate dependent. Toxin preparation of five isolates killed 40-60% of the cotton seedlings when incubated for four days at 1:10 dilution. At 1:100, dilution toxin preparation of most of the isolates affected the development of the root system but failed to kill any seedlings. A partially purified toxin preparation was effective and killed 100% of the seedlings at 1:10 dilution in the root bioassay. One of the most striking features of the toxin activity was the inhibition of secondary roots. The phytotoxin was a stable element and was not degraded by autoclaving. Control plants infiltrated either with the liquid culture medium or with distilled water did not show any mortality. A high correlation coefficient between the LAI and the LAN of the 38 cotton cultivars was observed when one of the aggressive isolates No. 1168 and its toxin were tested (r=0.86). Cultivars CNPA-PRECOCE 2, CNPA TB-46, CNPA 87/61, PR 94-82, PR 94-204, PR 94-215 and PR 94-238 demonstrated resistance to the pathogen as well as insensitivity to its toxin. Cultivars showing intermediate reaction to the pathogen also showed intermediate reaction to its toxin. Similarly, the highly susceptible cultivars Paraná 3 and PR 93-129 were also highly sensitive, confirming the cultivar-specificity of the toxin.

A high correlation between LAI and LAN was also observed in a second experiment where a mixture of fungal isolates and their toxin preparations were tested on leaves of 19 cotton cultivars (r=0.88). Hosts susceptible to S. solani such as Crotalaria spectabilis, Lupinus angustifolius, Lycopersicon esculentum, Mucuna preta, Phaseolus vulgaris, Solanum tuberosum and Vicia faba were also sensitive to the phytotoxin, whereas non-susceptible hosts such as Avena sativa,Coffea arabica, Oryza sativa, Pisum sativum, Triticum aestivum and Zea mays were insensitive to the phytotoxin. The host-selective ability of the phytotoxin shows future perspectives in breeding work aimed at selection for resistance to Stemphylium leaf blight disease. This is the first report of a host and cultivar-specific phytotoxin produced by S. solani.

1. Mehta YR, 1998. Plant Disease 82, 1-4.