An outbreak of the foliar disease leaf rust is threatening the North Coast’s bumper soybean crop due to be harvested in coming weeks, according to NSW Department of Primary Industries (DPI).

With 13,000 hectares of soybeans planted between Macksville and Tweed Heads and a near-record 36,000-tonne crop waiting to finish off, growers are being advised to monitor their crops for rust and treat if necessary and possible.

NSW DPI Grafton-based research agronomist Natalie Moore said the current cool, rainy weather was favourable for rust growth, which was a concern with harvest so close.

“Rust can develop rapidly during showery weather, causing leaf drop and significant yield losses,” Dr Moore said.

“The rust forms as grey-brown raised pustules on the underside of leaves, usually starting lower down the plant and working its way up.

“If left unchecked and with favourable conditions, the rust can prematurely defoliate the soybean plants and significantly reduce seed size and tonnage.

“Growers need to protect the green leaf on the plant now, especially in the late-planted crops where the pods have not yet finished filling. The green leaf plays a critical role in making sure the pods fill to achieve yield potential.”

Dr Moore said growers were looking towards a good harvest after last year’s crop was hit hard by flooding rains, especially in the Richmond Valley and then leaf rust.

“Some crops were set back this season due to the hot, dry conditions in January and late planting – so there is a fair bit riding on this harvest,” she said.

Dr Moore said growers should assess the stage of their crop – and determine whether they have enough green leaf to finish the pods.

“If crops have five weeks to run and the current wet conditions continue, growers need to protect the green leaf with a rust treatment as soon as they can get back on their paddocks,” she said.

“There is a new permit available for Folicur 430 SC (tebuconazole) for use against leaf rust in soybean, so growers should discuss treatment options with their NSW DPI district agronomist or their rural supplier,” she said.