The infection biology of Bipolaris oryzae and the defence responses of its host rice were studied using the rice cv. MTL 189 inoculated with two isolates of B5 (highly compatible with MTL 189) and K2 (less compatible). In the former interaction, the hyphal progress was accompanied by tissue degradation and extensive sporulation after 8 days, whereas in the latter interaction, only very limited tissue degradation and sporulation was observed. Quantitative microscopy of the infection showed that the percentages of conidia and appressoria causing penetration and fluorescent epidermal cells (FEC) were lower for isolate K2 than for isolate B5 at 12 and 24 hours after inoculation (hai). Fluorescent papillae (FP) were only seen in the less compatible interaction and the percentage of conidia causing single FEC was highest in the less compatible interaction at 12 hai, but not at 24 hai. Qualitative examination of other defence responses showed that H₂O₂ started to accumulate at 4 hai in the less compatible interaction, whereas it appeared in the compatible interaction only at 12 hai. The level of H₂O₂ was generally higher in the less compatible than in the compatible interaction. Cross sections of leaves showed that H₂O₂ accumulated in the outer walls of epidermal cells. Likewise, accumulation of callose and polyphenolic substances was most pronounced in the less compatible interaction and occurred at the same places as H₂O₂. To our knowledge, this is the first report implicating H₂O₂ as an early defence response against the hemibiotrophic pathogen B. oryzae during early infection stages in rice. Understanding defence reactions may aid in resistance breeding.

Infection biology and defence responses of watermelon attacked by Stagonosporopsis cucurbitacearum (syn. Didymella bryoniae) were studied in two genotypes, accessions PI189225 (moderately resistant) and 232-0125/B (susceptible). On intact leaf surfaces, spores started to germinate 14 h after inoculation (hai) with one to three germ tubes, which subsequently developed and formed appressoria. Invasion of the host tissue started at 20 hai by direct penetration from appressoria or occasionally indirectly through stomata. In the susceptible accession, a significantly higher number of direct penetrations were observed than in the moderately resistant. After invasion, hyphal colonisation was restricted in the intercellular spaces in the moderately resistant accession, whereas they developed extensively, causing tissue decay, in the susceptible accession. Macroscopic symptoms were seen in leaves of the moderately resistant accession as small and dry lesions, whereas big, water-soaked lesions developed on the susceptible accession within 48 hai. Investigations of the defence responses of the two accessions showed accumulation of H₂O₂ at penetration sites beneath appressoria in the moderately resistant, but to a lesser extent in the susceptible accession. Such H₂O₂ accumulation correlated with a reduction in penetration frequency and a lower level of hyphal growth after infection in the moderately resistant accession. There was a rapid and early increase in total peroxidase as well as β-1,3-glucanase activity in the moderately resistant compared to the susceptible accession. These results indicate that fungal penetration and development in watermelon are inhibited by a consorted action of different responses including accumulation of H₂O₂, peroxidase and β-1,3-glucanase.

Imbibing watermelon seeds in 1 mM sodium tetraborate (Na₂B₄O₇) for 24 h systemically protected plants against foliar infection by Stagonosporopsis cucurbitacearum in detached leaves and under greenhouse conditions. The treatment resulted in both a reduction in the overall percentage of leaf infection as well as in the size of lesions. Studies of the mechanisms by which Na₂B₄O₇ protected watermelon showed that there was no direct effect on the S. cucurbitacearum mycelium growth in vitro. On the other hand, plants raised from seeds primed with Na₂B₄O₇ showed a higher frequency of fluorescent epidermal cells compared to the plants treated with water. This indicates that a higher number of cells expressed the hypersensitive response after Na₂B₄O₇ priming. In addition, there was an increase in peroxidase activity and an enhanced accumulation of a 45 kDa acidic peroxidase isoform during the early stages of infection in plants treated with Na₂B₄O₇ compared to plants treated with water and this was positively correlated to the reduction of leaf infection caused by the pathogen. These results indicate that Na₂B₄O₇ is able to induce systemic resistance in watermelon against S. cucurbitacearum by activating the hypersensitive reaction at penetration sites, increasing peroxidase activity and altering the peroxidase isozyme profile. Although each individual response may only have had a minor effect, their combined effects had a reducing effect on the disease.