If in doubt, check it out

Cotton growers are being urged to have the cause of any dying plants identified to allow better management of potentially damaging disorders.

Speaking on CSD’s Web on Wednesday broadcast, senior plant pathologist Dr Stephen Allen says cool, wet conditions in the middle of the season have given rise to several plant disorders particularly Fusarium, Verticillium and Sudden Wilts.

Dr Allen said Verticillium Wilt, a vascular wilt largely managed by resistant varieties, would still damage crops when the conditions were right.

“Verticillium wilt is favoured by cool conditions. The resistance we have is temperature sensitive – under normal seasonal conditions you don’t see it very much but give it cool conditions and even those varieties with V-ranks of over 110 will show symptoms fairly soon.”

“The real comparison is where you compare these varieties with varieties with V-ranks of 70 to 80 like we had a few years ago – there the disease symptoms are much more severe and much more common and the effect on yield is much more pronounced,” he said.

Dr Allen said the Australian strain of the Verticillium Wilt pathogen was relatively mild compared to defoliating strains of the disease that originated in the United States and have since spread around the world.

“The defoliating strains almost kill the plant, drop all the leaves, and cause drastic symptoms.”

“They’re now present in most parts of the world – in Europe, South Africa and the United States. We definitely don’t have and don’t want those strains – the mild strains that we have cause enough problems,” he said.

Dr Allen said Fusarium Wilt, similar to Verticillium, was favoured by cool conditions resulting in an increase in its prevalence since Christmas.

“The fungus that causes Fusarium gets in through the roots, it clogs up the vascular system and it’s favoured by cooler temperatures. If you have inoculated plants in a glass house and temperatures stay above 23°C you don’t get symptoms. You’ve got to have cool conditions for symptoms to develop.”

“Where you’ve had Fusarium, the inoculum from those infected plants is in the top 15 to 20cm of the soil so when you have a couple of weeks of wet weather you have shallow root development in that warm moist soil and that’s root development right where that inoculum is centred.”

“So where you’re bringing the inoculum and the plant together in cool conditions its prime for symptoms of Fusarium Wilt,” he said.

According to Dr Allen, there had also been recent reports of a less common disorder, Sudden Wilt.

“Sudden Wilt is also caused by a Fusarium but it’s more of a garden type Fusarium and you won’t see it in the same place twice. It’s not one of those things that survive in the soil for a long time in the same spot.”

“You’ll see all the plants in a part of a row or particular patches all wilt on the same day, sometimes dropping all leaves, and in many cases they come back and by the end of the season it’s hard to find where the spot was.”

“You can tell it apart from Verticillium and Fusarium because the brown discoloration within the stem is usually confined to the roots and lower stem only.”

“The problem is usually worse with root pruning and hot weather, particularly if you have the last cultivation followed by an irrigation followed by hot weather,” he said.

Dr Allen suggested growers with dying or suspicious looking plants should contact a plant pathologist or send samples to Dr Linda Smith at the Department of Employment, Economic Development & Innovation’s diagnostic laboratories at Indooroopilly.

“We always say, ‘if in doubt, check it out’ – the diagnostic service is free, it’s confidential, and why worry when there’s a fair chance you don’t need to worry,” Dr Allen said.

2 February 2010