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 mL was effective to
induce resistance. These doses were not phytotoxic to green bean.
BTH at 25-70 µg mL¹ 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 rustconducive 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.
Legume and cotton seeds soaked in BTH at 25 - 50 µg mL 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 ẞ-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 good marker of enhanced resistance at these growth stages and that it could be
causally related to the resistance. Measurements of ẞ-1,3-glucanase activity at late
stages of crop growth and many weeks after BTH application, however, did not
relate to the then observed lesser disease in faba bean and pea. This indicated that the
activity was not a useful indicator of expressed resistance at these late stages in crop
development, although a higher activity some weeks earlier may have contributed
along with other induced changes to the decreased growth of the pathogen.
Overall, the studies showed that BTH at the concentrations used did not adversely
affect legume or cotton growth and did not directly affect fungal pathogens but that it
changed legumes systemically so that they became more resistant to fungal
pathogens for many weeks under field conditions. Part of the systemic change in
younger plants was a heightened activity of at least one PR-protein, which may have
contributed to their enhanced resistance, and this evidence was discussed in relation
to general understanding of the process of induced systemic resistance in plants.
BTH application to legume crops at early stages of their growth gave useful findings
of durable and stable resistance under field conditions and the effect of resistance
often being measurable at the end of the seasons, although being then sometimes
diminished when weather conditions favoured particular pathogens. Seed treatment
with BTH enhanced the resistance against foliar or root pathogens in faba bean even
in full grown plants and the effect of SAR was not diminished. This implies that an
early induction is possible by seed treatment with BTH and it could be an effective
tool to induce resistance against other plant pathogens.