Cotton Seed Distributors - Web on Wednesday

Dr David Nehl, Research Scientist - NSW Department of Primary Industry, comments on diseases this season.

David, we’ve had some pretty cool, wet conditions through most of the growing regions this season and it’s still not particularly warm. What impact does that have on the major diseases?

The conditions this season at the beginning were quite favorable for diseases particularly the soil borne ones which are the ones that are most serious in cotton in Australia. So over October I think a lot of areas would have had a number of cold shocks, we had about 18 nights below 13ºC here at Myall Vale at the Research Station in October and in November there were about another 12 nights below 13ºC. That was probably not too dissimilar from the previous season except this year that was combined with rainfall and consistent rainfall every week to 10 days to 14 days, so those cold, wet conditions are very favorable for soil borne pathogens of cotton.

In terms of seedling disease, that was probably a bit of a brighter note than 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, so we had growers at Hillston saying that 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. One field in particular where they’d rotated with a legume crop and they had a lot of residue in the soil we saw plants dying at the five leaf stage and that turned out to involve Pythium, but not too much of that this year and certainly much less replanting this year but for seedling disease it wasn’t such a bad year.

For black root rot those cool, wet conditions were very favorable for the black root rot fungus.

It generally starts infecting the plant about 10-12 days after sowing and generally it increases to a maximum level of infection about three weeks after sowing, sits there until about 5 weeks and as the season warms up the plants will pull away and grow out of that black root rot phase and slough off the dead tissue that’s on the tap roots. So the conditions were quite favorable for black root rot and fields that had a high infestation of the black root rot fungus have had severe stunting and are still struggling to catch up.

We’ve had quite a cool start through to November but also some cloudy conditions through the season so far.

In terms of verticillium wilt we really haven’t seen much of that on our early season surveys. There are a few reports of that happening now and the conditions will also have been favorable to Verticillium. Those problems are probably just going to be limited to field where growers have had a history of susceptible varieties that led to an increase in the pathogen in the soil and the past, so it’s not going to be as widespread as it was in the early 80s. We’re certainly not seeing that as a big issue

But for Fusarium wilt this season is looking like it’s been an ideal set of climatic conditions for the Fusarium pathogen. Last season we did get 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. Towards the end plants certainly did drop dead and defoliate but throughout the previous season it wasn’t too bad.

This year in fields with severe infestations of Fusarium Wilt the plants have been dying quite rapidly in through November and late October and that sort of thing 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’s not spread everywhere but it is still on the move and we want people to exercise their farm hygiene and keep their eyes open as the earlier you can discover this on a new infestation the better you can get on top of it.

Alternaria leaf spot, we really didn’t see anything in our disease surveys. In the few crops I’ve looked at since Christmas there’s certainly some alternaria starting on the leaves and the rainfall and humid, cloudy conditions would favor that pathogen but 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.

And that would be the same with Bacterial Blight?

Bacterial Blight in the Pima would be favored by the wet conditions. There’s probably only a few crops of Pima out in the Bourke regions but I haven’t heard yet of reports of Bacterial Blight in those crops. It would depend on a source of the bacterium coming through either in infested planting seed or staying in crop residues and I don’t think there’d be much in the way of crop residues due to the drought.

Given that we switch to more normal summer weather, as I said it’s still pretty cool here today but it’s expected to warm up. Any sort of thing that’s really going to happen in the way of those diseases?

Probably the two biggest issues we’re talking about would be Black Root Rot and Fusarium Wilt. With Black Root Rot the warm conditions would definitely favor growth in the plant and in most cases if we get a good, long, warm period the plants will just pull away and slough off that dead tissue and put on some root growth and grow through, so hopefully that will be sufficient for even those severe infestations to recover. Cotton really can compensate well after severe black root rot if the season is warm.

But for Fusarium Wilt I think we could expect that you will still see an ongoing progression of disease through the season. Even with the warm conditions that is going to favor defoliation or wilting and death because the plants have been infected earlier in the season, even if there are no external symptoms say in the beginning of December but the vascular tissue, the woody tissue is blocked up, effectively the plumbing of the plant is blocked up.

When you decide to irrigate you put the water on because the plants are transpiring and they need a drink and those plants that are affected internally just can’t get water to their leaves and then they will start to defoliate. The Fusarium Wilt will continue to progress even with the warmer conditions now.

You obviously are doing some work on VAM, particularly out in the Bourke area pre-season. Have you got any tips on how that has been a problem this year?

Everything is looking fairly positive on that note. We did a study pre-season back in March where we did some test plots and found that surprisingly there were quite adequate numbers or sufficient numbers of VAM fungi in the soil for cotton and linseed, we used linseed as a little test plot and that followed through to this season. We haven’t finished assessing the root samples from cotton, we have had a number of trials at Bourke that had up to 41 months of bare fallow at sowing in October and those crops actually looked fine, they are growing quite normally and as I said we haven’t assayed the root samples yet but we’re not expecting VAM to be a big issue and if you think about it the VAM fungi, a native fungi, they weren’t introduced to those soils and they’ve probably adapted over millions of years to survive droughts and floods and the drought seems to have not been a problem, at least for cotton anyway.

Contact: david.nehl@agric.nsw.gov.au

Further Information:  Robert Eveleigh, John Marshall, Craig McDonald or David Kelly