Abstract
The effectiveness of a benzothiadiazole (BTH) was investigated as a chemical activator of systemic resistance against six foliar diseases of legumes under growth
room and field conditions and one root disease of legumes and cotton in a growth
room.
BTH was first compared with 2,6-dichloroisonicotinic acid (INA) and Colletotrichum lindemuthianum as foliar sprays to unifoliate leaves of green bean in a growth room as inducers of systemic resistance against C. lindemuthianum and Uromyces appendiculatus. Upper trifoliate leaves were challenged either with C. lindemuthianum or U. appendiculatus. In most experiments with green bean, BTH was more effective than INA or C. lindemuthianum. BTH induced marked resistance
when applied a few days to a few hours before challenge or even shortly after challenge. The protection was seen as a reduction in leaf area affected or as smaller and fewer lesions per plant. A BTH dose of 12.5 to 50 μg mL1 was effective to induce resistance. These doses were not phytotoxic to green bean.
BTH at 25-70 μg
mL ̈1
was then tested in field grown faba beans as one or two foliar sprays at early stages of crop growth or as a seed-soak. It caused significant protection against naturally occurring chocolate spot and rust diseases. The protection lasted for several weeks after application. However, with the advent of the aggressive stage of chocolate spot later in the conducive wet season of 1998, the protection against disease severity was largely lost but a greater retention of leaves was still apparent in BTH treatments. Again, in the continuously dry, warm and rust- conducive season of 1999, protection against rust was much weaker at the later
stages of crop growth. Therefore, there were limits to the protection offered by BTH under field situations other than normal.
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Legume and cotton seeds soaked in BTH at 25 - 50 μg mL1 and planted in soils infested with Thielaviopsis basicola in a glasshouse and growth room showed enhanced resistance to black root rot for 3 to 6 weeks. In some cases, chlamydospore formation was prevented on roots emerging from BTH-treated seeds. The possibility of use of BTH in minimising black root rot disease in cotton where legumes are used as an alternating crop was thus indicated. Evidence was also provided that, as in other cases, BTH had no direct antifungal activity.
One or two early applications of BTH at 25 to 50 μg mL1 to field crops of pea decreased the severity of downy mildew caused by Peronospora viciae through to the flowering stage. In a growth room study, INA performed better than BTH in protecting pea against Mycosphaerella pinodes, and pre-inoculation with M. pinodes did not give any systemic protection in pea against challenge inoculation with the same fungus.
BTH-induced resistance in field grown faba bean cv. Fiord was stable against rust even after interference through one or two later applications of urea to the soil. Similarly, BTH-induced resistance was durable in decreasing disease for 18 weeks after application to field grown faba bean cvs. Fiord and Icarus.
Enhanced PR-protein levels and activity of one PR-protein measured as B-1,3- glucanase occurred systemically after BTH-treatment of green bean and faba bean seedlings. B-1,3-glucanase activity was well correlated with systemic resistance as these seedlings developed in growth rooms. This indicated that the activity was a