Hot weather leads to charcoal rot warning
THE disease of charcoal rot has been building up in soybean crops in northern NSW due to the hot, dry conditions experienced during the past few summers.
Paddock selection is critical to minimise the impact of this disease and rotation with non-host crops is advised.
DPI research agronomist Natalie Moore had a message for growers: “It is timely to think about soybean paddock rotation, especially since charcoal rot has built up in recent summers.
“Back-to-back soybean planting is a high risk if the disease was severe in the previous season.”
Charcoal rot of soybean is caused by the soil-borne fungus Macrophomina phaseolina, a pathogen with a wide host range, including all summer crops but particularly soybean, sorghum, sunflower, mungbean, maize and cotton, many horticultural crops and trees, and also weeds.
In Australia, more than 80 host plants have been identified and the fungus has been found in all mainland states and territories.
The disease is the most common in soybeans in Australia, and can cause the premature death of entire crops.
Evidence suggests that infection occurs when the seedlings are growing actively and that the pathogen remains latent inside roots until the plants are stressed.
Physiological stresses associated with flowering combine with an external stress, most frequently high temperatures and/or low soil moisture, to stimulate the pathogen into activity. Stress from waterlogging followed by hot weather, and perhaps stem insect injury, can predispose plants to the rot.
During this stage, M. phaseolina produces a plant toxin which, together with a plugging of the water conducting tissue, causes rapid wilting and ultimately plant death. As infected plants die microsclerotes composed of tightly compacted fungal strands are produced in abundance in the roots and lower stems.
Charcoal rot symptoms appear on soybean plants usually at and after flowering, although hot, dry weather immediately after emergence can kill seedlings.
From flowering onwards, individual plants scattered across a paddock suddenly wilt, with leaves dying rapidly but remaining attached to the petioles. In furrow-irrigated crops, plants at the head ditch end often wilt first and plants growing in lighter soil tend to wilt first.
The stems of infected plants turn from green to a light yellow-tan colour, later becoming brown. On occasion, infected plants will display a dark brown lesion from ground level up the stem, ending in a gradation from dark brown to yellow into the green stem above the lesion.
A diagnostic symptom of charcoal rot in the early stages of development is an orange discolouration of the tissue just below the surface of the stem, commonly called “orange bark disorder”.
Charcoal rot-infected plants almost always die before maturity, and after death the stems usually turn an ashen grey colour with minute black specks on the surface. When the dead tap roots and basal stems are split this “pepper” symptom is also readily apparent.
The specks are the survival structures of the disease, forming a hard rind at the surface, enabling them to survive in the soil for years.
LOAD OF ROT: Early symptoms of charcoal rot in northern NSW.
Destruction caused by charcoal rot and drought stress at Inverell. PHOTOS: N MOORE, NSW DPI