Effects of heavy metal salts on susceptibility of rice seedlings to brown spot disease

Abstract

SUMMARYHeavy metal salts, known to induce phytoalexin production in different plant species, were used at dilute concentrations (10‐3 to 10‐5 M) in different treatments to control brown spot infection in rice seedlings caused by Drechslera oryzae. Foliage sprays, root‐dips for 24 h at the time of transplanting, or seed soaking for 24 h provided moderate to substantial protection to rice seedlings, at different stages after treatment, against artificial inoculation with the pathogen or natural infection under field conditions and mostly caused significant reductions in symptoms. Of the three methods, greatest and most durable protection was achieved with the wet seed treatment. The number of successful infections was appreciably reduced in most treatments and some also inhibited lesion expansion. Ferric chloride and nickel nitrate were the most effective and mercuric chloride the least effective of the chemicals tested. There was no fungitoxicity in leaf diffusates from 3‐ and 5‐wk‐old seedlings that had been treated with heavy metal salts and only mild toxicity in that from untreated, inoculated seedlings. However, treated seedlings inoculated at the 3‐and 5‐wk stages produced leaf diffusates with significantly greater toxicities which were in proportion to their abilities to resist attack.