Dr.Stephen Allen, CSD Plant Pathologist, discusses the incidence of late season diseases in the 04/05 crop.

Stephen, the pathology teams have recently completed some of the late season disease surveys. Could you give us an update of what’s been found?

The NSW DPI have finished the NSW part of the surveys and ourselves along with Linda Swan and Joe Kochman from the QDPI have done the Queensland part and results are now in and I guess over the next few little while we will be putting them together. Generally, the season has been kind to growers.

Certainly in terms of the easy ones first the alternaria and boll rot generally very low incidence, except for maybe the Byee region where they seem to have the luxury in terms of water and rainfall and there were some rank crops.

Particularly the late season surveys, they had two issues of verticillium wilt and fusarium wilt and with verticillium wilt again in the Namoi and the Gwydir there has been quite a few people quite surprised and somewhat shocked at the level of verticillium in some of the crops late in the season but if you look at the weather that we have had for this last season, the cool conditions in November, the cool conditions at the end of December - there were even cold shock days in December the cool conditions at the beginning of January and then a week or so of cool overcast weather and again a couple of cold shocks in early February.

Under those conditions even the most resistant varieties we have got become susceptible to verticillium. They say that there is only about 5 degrees difference between resistance and susceptibility in our varieties. So considering that the conditions that we have had and the weather conditions and those cool spells its not surprising to see some high levels of verticillium and certainly in some fields we have seen up to 80/90% of plants affected with verticilium in varieties that we regard as being fairly resistant.

What sort of yield impact do you think that that could possibly have?

It all depends when the infection occurs. Generally if the infection occurs late in the season, or early in the season it does not have an impact but the problem is when you get the cool spell and the verticillium coming into the crop in the middle of the season when is doesn’t have time to compensate, that’s when we can see some yield impact.

What’s been happening on the Fusarium front?

Fusarium is an interesting one. Certainly the surveys show that the disease is still common and we have got two new records for new farms in NSW where the disease has been confirmed.

That’s the ones that we know about but our concern is that maybe some people are finding it and not reporting it, however, the disease wasn’t as obvious this year but nevertheless the incidence is still significant and I have seen areas of fields with 70 or 90% of plants affected and some of those fields with a fairly high incidence are still performing quite well. I mean people are using the more resistant varieties, the disease isn’t severe but certainly it’s there and it’s not going away and it’s still something to be concerned about.

Would you like to make a comment about how some of the new higher F-ranked varieties have performed up in high infection areas like the Downs. I am sure you have been monitoring some crops there?

It’s quite remarkable to see a high F-ranked variety next to a moderate F-ranked variety in the same field. From one hundred yards away you can see which one’s got the higher resistance.

I think we can only emphasise the importance of F-rank and V-rank in selecting variety. Certainly in areas where you have got the fusarium it is important to stick with the higher F-rank varieties. There was one other interesting aspect of our surveys which came through. We do transects across affected fields.

The transects that we did up in the Moura/Theodore the Northern end of the cotton growing areas, after even 4, 5, 6, 7 years there is still very low incidence of the disease. When we compare that with transects done over crops in the MacIntyre and after 4, 5, 6, 7 years we are finding 18, 20, 30, 40% of plants affected. So it could be that in the very warm areas the disease isn’t going to be as significant as we perhaps thought at one stage.

Because we certainly thought that the hotter areas were more at risk in the early days of the disease?

That’s right but some of the laboratory work and the glasshouse work has shown that if it gets much over 23 degrees you don’t get visible symptoms occurring and perhaps our say in these warmer areas the disease isn’t going to be as obvious. Certainly not spreading near as fast as it is in more Southern regions.

And Stephen, Alternaria, has it caused a problem anywhere this season?

As I mentioned earlier, the alternaria was virtually present in every crop but only at very low levels. Except for that Byee area where as I say there was a lot of rain and the crops were up to 2 metres tall in some places and under those very rank conditions boll rots, Alternaria - the crops were being defoliated the spots were moving up the plant and it looked like a real problem.

Further Information: Dr Stephen Allen