Steve Allen, Senior Research Scientist - CSD and CSIRO, outlines techniques used for ranking fusarium and verticillium wilt.

Steve what do we mean when we talk about ‘F.ranks’ and ‘V.ranks’?

Well ‘F.ranks’ and ‘V.ranks’ were basically designed in response to the needs of growers to have an understanding of the relative resistance or susceptibility of varieties to disease. In the case of ‘F.rank’ we’re talking about fusarium wilt and it is important if the disease is present the growers use the more resistant varieties.

The ‘F.rank’ system basically allows a comparison between varieties as to how resistant or susceptible they are and is very important in making the planting decision, similarly for the ‘V.ranking’ case of verticillium.

What is the data that needs to be collected to produce these rankings?

Well the ranking is based on the proportion of plants that survive through the season with minimum levels of disease, either no disease or just a very small amount of disease.

In terms of collecting data for a trial you need to first of all know how many plants are there at the beginning of the season and then at the end of the season, usually with a stem cut process you work out by looking inside the stem and seeing what proportion of those plants actually survive to the end of the season with no diseases at all. So we talk about percent survival; in other words proportion of plants that survive the whole season with little or no disease and that’s used to calculate the ‘F.rank’.

In each of the trials we have to have the standard and in the case of the ‘F.rank’ system we have the Sicot 189 as the standard and so we look at the percent survival in a new variety, compare that with the percent survival in the standard variety, the Sicot 189, divide one by the other, multiply by 100 and so you get an ‘F.rank’, which may be 150, which means that the new variety is 50% more plants will survive until the end of the season.

If a variety has an ‘F.rank’ of 200 it means twice as many plants will survive in this variety compared to the standard variety.

What type of trials are used to collect this ‘F.rank’ data?

There’s a range of different trials that can be used, the main prerequisites are that it’s going to be fully randomized, replicated, set up properly, there’s a minimum plot size of 10m, there’s a minimum disease pressure.

There can be no more than 70% survival; in other words you can’t use a trial where there’s a low level of disease pressure, you’ve got to have significant disease pressure. So at least 30% of the plants have got to be dying throughout the season before you can use the trial. They are the basis prerequisites of this. You also need to send off a sample to the QDPI to get it confirmed to make sure which race of the pathogen it is so that data’s kept as well.

So we’ll just talk about one of  the types of trials that is used and that’s what’s called the Box Trial. If you could just outline in a bit greater detail what’s involved in setting up a Box Trial?

Well a Box Trial is basically a way of using a commercial planter to do a research trial. If you’ve got an 8 row planter you put a different variety in each of the 8 boxes. You usually use 30m plots or 50m plots.

At that point you stop and you rotate the boxes so that you don’t get any one variety turning up twice in the same row. You plant another 30m, stop the tractor, rotate the boxes again and you do that at least 6 times, you need at least 6 replicates or 8 replicates is even better.

So when we talk about varieties of cotton, it is important for all new varieties to have an ‘F.rank’ or a ‘V.rank’?

It is very important to have an ‘F.rank’, I mean other wise it can be very dangerous for a grower to plant a new variety, which is very susceptible in the field where the disease is present.

It’s important in making the decision and I suppose it’s particularly rare the disease is present, you know the disease is present, it’s very important that you use varieties that have got the highest level of resistance. So every new variety that’s produced, you’ll find an ‘F.rank’ there for it or a ‘V.rank’ for it and you can know whether or not it’s the right variety in terms of disease resistance for that site.

And as far as the number of trials that a variety has data collected on it, what is the ideal thing?

Well that’s very important; if you look at the data we have on ‘F.ranks’ and ‘V.ranks’, you’ll see a number in brackets after the actual ranking, so for instance Sicala 45 has a ‘F.rank’ of 154 and in brackets afterwards you’ve got the number ‘8’.

That means there’s been 8 separate replicated trials and on the basis of those 8 trials the ranking is 154, in other words 54% more plants survive in this variety than in the standard.

So the number in brackets afterwards indicates gives you a measure of how much you can trust the number, so if you’ve got an ‘F.rank’ which has got 150 based on one trial you can’t trust that as much as one based on 5 or 10 trials and for varieties like Sicot 80 now and some of those more established varieties, Sicot 71 where 15 to 20 trials and so you can be quite sure that that ranking is reasonably accurate.

So what does the industry have in place to ensure the accuracy of ‘F.rank’ and ‘V.rank’ values that growers look at?

We’re moving towards a trial registration process. A protocol has been developed which both companies, which everyone is involved in setting the ‘F.ranks’ has agreed to.

We have independent people like Joe Kochman who run a trial as well and those numbers are fed into the system too and that’s like an independent third party trial, which is included. CSD prints out all of the data so that you can see where the trials were, the disease pressure  at each of the trial sites and look at the individual date of contributing to the ‘F.rank’ and I think that’s important.

Finally where can growers find ‘F.rank’ and ‘V.rank’ data for the varieties they are thinking of planting?

‘F.rank’ and ‘ V.rank’ data can be found on the CSD website (www.csd.net.au). It’s updated every year as we get a new set of trials contributing to the numbers and I urge all growers to consult the website and make sure that they’re using the best varieties in terms of resistance in those situations in those fields where the diseases exist.

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