Cotton disease survey yields mixed results

31 January, 2005

___Cotton growers have had some relief from seedling disease, but other soil borne diseases are on the rise compared with previous seasons, according to NSW Department of Primary Industries research scientist, Dr David Nehl._

Interviewed on the weekly CSD Web on Wednesday video, Dr Nehl said low temperatures had induced a significant number of cold shock nights (below 11°C) during October and November at Myall Vale.

“Combined with consistent rainfall, those cold, wet conditions were favorable for soil borne pathogens of cotton.

“In terms of seedling disease, that was probably a bit of a brighter note than in the last few years. A lot of growers that got their crops in with the warm weather at the end of September actually got good stands. We had growers at Hillston saying it was their best stand establishment ever.

“With the cooler conditions we did see some areas where we had some post emergent damping off due to Rhizoctonia and Pythium, but not too much of that this year and certainly much less replanting this year, so for seedling disease it wasn’t such a bad year,” Dr Nehl said.

He summarised the mid-season disease situation as follows:

Black root rot: Cool, wet conditions were favorable for the black root rot fungus, and fields that had a high infestation have had severe stunting and are still struggling to catch up.

Verticillium wilt: We really haven’t seen much of that on our early season surveys. We’re certainly not seeing that as a big issue, as resistant varieties are widely grown.

Fusarium wilt: It’s been an ideal set of climatic conditions for the Fusarium wilt pathogen. Last season we got those cold temperatures in October/November but it was relatively dry and a lot of areas reported much less severity of Fusarium wilt during the season.

“This year in fields with severe infestations of Fusarium wilt the plants have been dying quite rapidly through late October and November, and this will continue in heavily infested fields through the season, so it’s shaping up as a worse season for Fusarium wilt unfortunately.

“We really want growers to be aware of this disease. It hasn’t spread everywhere but it is still on the move and we want people to exercise good farm hygiene and keep their eyes open, as the earlier you can discover a new infestation the better you can control it.

Alternaria leaf spot: We really didn’t see anything in our disease surveys. It’s probably not going to have a big impact on yield in most of the conventional or Bollgard® II varieties, it would only be in Pima crops where we have a big issue with Alternaria.

Bacterial blight: Only Pima cotton is susceptible and I haven’t heard any reports of bacterial blight in those crops, which are currently few in number.

“Probably the two biggest issues we’re talking about would be black root rot and Fusarium wilt. With black root rot, warm conditions will definitely favour growth in the plant

“In most cases if we get a long, warm period the plants will just pull away and even crops with severe infestations can recover. Cotton can compensate well after severe black root rot if the season is warm.

“For Fusarium wilt I think we can expect an ongoing progression of disease through the season. Even though warm conditions favour plant growth, they will also coincide with wilting, defoliation and death of plants that were infected earlier in the season.

“Even if there are no external symptoms, the vascular (woody) tissue of infected plants is obstructed, effectively blocking the plant’s ‘plumbing’.

“Even with irrigation, those plants that are affected internally just can’t get water to their leaves and will start to defoliate, so Fusarium wilt will continue to progress even with warmer conditions,” he said.


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