By David Jones, Broad Acre Agronomist

This week I’m looking at timing the ear spray fungicide for Fusarium control and dispelling some myths about what rates and products to use. There’s also some worrying news on Net Blotch in Barley on the horizon, an update on fungicide activity for next season, and first a look at avoiding disasters by spotting Potato Cyst Nematodes (PCN) early in potatoes…

Potato Cyst Nematode – know what you’re dealing with before you plant

Because of the devastating and persistent nature of PCN, it is vital to know what populations you are dealing with in your soil and what sensible steps should be taken to manage them. But just because you are not seeing visual patches of damage or detecting them in the soil, should you be relaxed?

In many fields we can assume that PCN are present where potatoes are widely grown, and indeed identified by looking for Cysts. When we want to look the potential damage that they would cause in order to more effectively manage it, we need to work out the population size which means looking for eggs rather than cysts.

US research shows that quite often 3-4 years of continuous potato crops can be grown in a field before the likelihood of detecting PCN. The problem is two-fold; the chance of finding eggs in a small sample of soil is low, and testing enough of the field to randomly encounter a patch of nematode eggs.

In severely infested fields, 20-30 eggs per gram of soil can be picked up provided enough cores are taken. 1,500mls of soil per hectare it what we should be testing give ourselves a fair chance of detecting eggs, made up of 20 individual sampling points per hectare and thoroughly mixed.

On more prone sites (non-irrigated, light soils) take more samples, particularly if a tolerant variety is being grown.

The graph above courtesy of UK’s Agriculture and Horticulture Development Board (AHDB) Potatoes, shows how long it can take to pick up eggs in an infested field – in a 3-year rotation for example it can take four crops before they are likely to be identified, reinforcing the point that in a close rotation, much more detailed sampling is required.

10 eggs per gram of soil is the target; above this level you will start to encounter problems. Tolerant varieties are fine, but remember that these still allow PCN to multiply, causing problems in successive crops. At 20-30 eggs/gram you really need to be using a resistant variety and using granular nematicides such as Mocap and Nemathorin in-furrow.

So when is the right time to test?

I always advise growers with known PCN problems to test immediately AFTER the crop is harvested. If you are sensible then you won’t return to that field for 6 years as an absolute minimum, but sample known patches in the field while they are fresh in your mind – then re-sample those areas to monitor the decline before you decide to replant in that field.

Knowing the species of PCN is crucial – Rostochiensis resistance is fairly easy to find in a marketable variety, but Pallida less so. Pallida also tends to have a much longer hatching period in my experience – meaning that longer rotations are less effective.

When deciding on control measures, also keep in mind the final population. Yield loss from 5-10 eggs/gram may be very low, but allowing populations to build up will be costly for those farms on a tight rotation. Using a nematicide in this situation can be money well spent even though there may not be an immediately yield benefit in that crop.

T3 timing in wheat

Fusarium is not only a yield robbing disease in wheat (and barley), but it also produces Mycotoxins on the grain that can be harmful to human health. Fungicides can effectively control it in most seasons, but appropriate doses have to be used.

Confused about when to apply the ear spray on your wheat? Do NOT go by days after your last spray!!! Apply it at early flowering to ensure it gets in before the disease. Useful picture (above) from the Southern States Co-operative.

The recent data from several years and 7 different sites across the UK and Ireland collected by the AHDB in high rainfall environments makes fascinating reading, and dispels the common myth that very high (often twice the legal rate) doses of a cheap fungicide (tebuconazole) are the most cost effective way to control the disease.

The researchers there are suggesting that 140-200g/ha of prothioconazole is needed to effectively control the disease (note that this in excess of the approved label rate here in Kenya).

Why not just apply a massive dose of a cheap product?

However, if you look at the light blue line below which shows the dose response to Proline (straight prothioconazole sold in Europe), and compare this to the pink line for Folicur you can see that even at 2x the legally approved rate of tebuconazole (500g a.i.) the researchers were not getting a response.

In fact, there wasn’t a response above 0.75 l/ha of tebuconazole 250, although remember that we often apply a full rate for Stem Rust control.

Also look at Caramba 90 which is straight metconazole – available here in mix with epoxiconazole. This is no more effective than Folicur and is more expensive on a rate for rate basis.

A few more comments on disease control…

Take away messages from the AHDB Agronomists’ Conference last month:

  • Chlorothalonil is working well in protectant situations against Septoria and is very low resistance risk. I would be adding it to more Septoria prone varieties like Robin or Gregory in wet seasons.
  • Epoxiconazole is still as good as Prothioconazole on Septoria – go to know as it is very well priced and adds variation to our fungicide programs to minimise fungicide resistance.
  • Strobilurins azoxystrobin and pyraclostrobin continue to provide the best persistence against Rust.
  • Tebuconazole noticeably behind epoxiconazole and prothioconazole on Septoria – no surprise there – but still very effective on Yellow Rust.
  • In a protectant situation (fungicides applied before the disease arrives) triazoles are not losing their efficacy. Curative activity is still declining however – the message as always is get in early before disease is established.
  • The researchers all voiced their concerns that wherever you look around the world, resistance to the SDHI fungicides (e.g. Skyway) is a very real threat. Use no more than two per crop.
  • Net Blotch in barley – this is a worry. In 2015 field performance from SDHI’s (Bixafen, isopyrazam, fluxapyroxad, benzovindiflupyr) were ALL superior to the triazoles. In 2017, monitoring across Germany, France, Ireland Denmark and the UK all suggested that their efficacy had declined and was now LESS than prothioconazole. They are still worth using, but we need to be very careful not to overuse them as an industry.

Look at the dark blue line in particular in the AHDB’s graph above – in 2015 it was more effective than prothioconazole against Net Blotch. Recent data from right across Europe suggests its efficacy has declined rapidly.

Our trials are examining some of the other actives that are showing less of a decline in performance against Net Blotch, so that we have alternatives, again to add variation to the programs and improve disease control.

Until next week!

About David


David Jones is the Broad Acre Specialist at Crop Nutrition Laboratory Services Ltd. (CROPNUTS). David has a keen interest in soils and no till farming systems where he has undertaken work looking into weed levels and changes in soil structure, and has extensive experience in field trials and in the development of precision farming techniques. In his spare time he enjoys playing rugby.