Pea Seed Testing for Ascochyta

An increasing number of growers are having pea seed tested for Ascochyta and are asking how to proceed once they have received the results from the lab. There is big potential to save costs if you can get away with not treating pea seed, on top of benefits for the environment, time saved not dressing and better crop establishment from reduced handling and breakage of seed.

The question is, at what level of infection can you get away with not treating the seed, are there any potential risks, and if the level of infection if high what should you do?     

Firstly, Ascochyta is a complex disease caused by any one of three different pathogens. As the disease develops on the crop it causes purple-brown round lesions on the leaves and browning of the stems in severe outbreaks, which can lead to very serious yield loss – occasionally total crop loss in the very worst cases.

These pathogens can survive on crop residues, in the soil and on the seed, as well as transported by wind and rain splash. So using clean seed alone will by no means eliminate the risk of the disease developing.

Depending on which lab you have sent your pea seed to for testing, the result will give a % Ascochyta pisi and % Mycosphaerella pinodes figure. These two pathogens are the main sources of seed-borne infection.

Canadian guidelines suggest to apply a seed treatment if there is more than 10% infection on the seed and most research suggests that where levels are around 15% or above this can increase the overall level of disease later on in the crop. Inoculum on the seed however is not the main determinant of Ascochyta in the later stages of a pea crop; this is much more dependent on the weather conditions and the environment being suitable for the disease. Peas with more than 25-30% infection should not be used for seed if at all possible.

The one caveat to this is that we are talking about research done in controlled conditions, not in the field. Any farmer or agronomist with experience of what high levels of inoculum can do when spores are blown in from a neighbouring field will attest that the presence of a inoculum source matters absolutely as much as weather conditions.

What we do know from research is that the most marked effect of high Ascochyta levels on the seed is a reduction in germination % and establishment. This is what our expectation should be for seed dressing.

There is almost no varietal resistance (we had some great trials in high pressure conditions in 2019 and 2020 to confirm this), and I believe that fungicides have minimal effect if any on this disease. This is not a statement that I make lightly; but to ignore the results from several years of our trials would be folly.

Despite some very bold claims made by manufacturers and chemical salespeople, we have never ever been able to see these yield responses in trials or in the field, with a range of chemicals.

Remember that Bacterial Blights are NOT controlled by seed dressings, and if a crop was heavily infected the grain should not be retained for seed.

	Ksh per hectare
Seed	18,000
Seed treatment	6,350
Inoculant	3,000
Fertiliser & nutrition	1,450
Fungicides	7,900
Insecticides	3,550
Herbicides	12,700

Typical variable costs for growing peas

Aside from the significant cost saving of not having to apply a seed treatment, there are other benefits such as less chemical in the environment, reduced handling (potential breakage and germination reduction) and time savings from not having to apply a seed dressing.

The occasional dilemma is that seed treatments not only control Ascochyta, but also provide some degree of Fusarium and Pythium control, in the case of fludioxonil and thiram. Cymoxanil and mancozeb is approved for Downy Mildew control in some countries too.

If you experience losses from these diseases regularly, then proceed with caution before using untreated seed. The question also arises of what implication untreated seed has for insect pests, as pea seed dressings typically contain an insecticide for Cutworm and Thrips. Even if we do not apply a fungicide, is an insecticide still required?

The beauty of large seeded crops such as peas is that we (generally!) have enough time to identify a potential pest issue and react with an appropriate foliar insecticide. Not ideal, but if it is only required on occasions and avoids the routine use of one insecticide on the seed, this is a net win.

We know that peas are particularly adept at recovering from millipede damage for example, so some discretion needs to be exercised before applying any insecticide ‘routinely’.

In theory the insecticides are likely to be around 6 weeks of protection against aphids, but is this effective in practice? I would struggle to believe that it makes a notably difference in the field.

In conclusion, there are big savings to be had from testing seed if seed dressings can be avoided, but proceed with caution, consider all relevant factors and remember that peas are a high risk crop.

Barley Topdressing – Getting it as Right as Possible

As planting season gets underway in major barley growing areas of the country such as Mau Narok and then Laikipia and Timau in September, the crop will very soon have emerged and be prompting discussions around topdressing.

Too little Nitrogen and a lot of yield is sacrificed, too much and rejections for high grain Nitrogen and lodging can be equally costly. Then there is sulphur to consider too. Lots of factors go into making a sound decision, so we take a look at where to begin…

In most Nitrogen trials we do in Kenya, the yield improvement from nitrogen is very low relative to other countries. Our deep, high organic matter soils are constantly mineralising and releasing nitrogen which is available to the crop. Especially during a fallow, where mineralisation occurs and Nitrogen accumulates, as there is no crop taking it up.

When we test different amounts of Nitrogen in barley to try to identify the optimum dose rate, the yields increase, but only very slightly. In lower yielding years, on some sites there is in almost no yield increase at all. Check out the graphs below of two such trials conducted in Timau in recent years.

Nitrogen response trials in Timau. The rates of N are topdressing only and do not include the 29kg/ha of N in the seedbed.

The trials above were on a deep, fertile humic clay loam in Timau, Mount Kenya, looking at three rates of Nitrogen. Note that there was 29kg/ha of N applied in the seedbed fertiliser. The N rate then begins at 10kg/ha of N (so 39kg/ha of N in total) and goes up to 80 or 70kg/ha in either trial (109 and 99kg/ha total N).

Each coloured line represents the nitrogen response for one rate of sulphur. The blue for example has 0 N applied, the grey has 20kg/ha of sulphur. What we see is a gradual increase in yield at best, in fact the August 2021 trial barley showed any response when yields were lower – perhaps at 5 t/ha the crop had sufficient N available from the soil?

Remember that there is always noise in any trials data, but the general trend from these two and from other trials is a small yield improvement from topdressing. Sulphur appeared to make no difference.

We often take Deep Nitrogen measurements after harvest and find between 80-120 kg/ha of nitrogen in the top 70cm of soil; by the time the next crop is planted 5-6 months later it is often 160-220kg/ha.

Assuming the crop can use 50-60% of this nitrogen (a fairly typical figure for Nitrogen) this implies that the crop can access somewhere between 80 and 130kg/ha of Nitrogen.

I say that this implies what a crop can access, because farming is rarely predictable and mathematical. But a 5 ton/ha barley crop at 1.8% Nitrogen will remove 90kg of N in the grain, so it gives us a rough guide.

On the subject of Sulphur, as I have said many times before in Think Agronomy it is very rare to see a response to this nutrient in most crops in Kenya. Sulphur does get leached and it is relatively cheap to apply a small maintenance amount; I still apply 15-25kg/ha of sulphate (SO3) on most crops to make sure.

In conclusion, if you are in Timau or Mau Narok, or indeed on a Black Cotton soil in Laikipia and your yield expectation is only 3-4 t/ha you probably have enough Nitrogen in the soil and the planting fertiliser.

When you push yields to the levels that our soils are actually capable of, around 60kg/ha of total Nitrogen is probably required, especially if your grain Nitrogen levels at harvest at below 1.4%. For those regularly pushing above 7.5 t/ha, the total should be closer to 100kg/ha N.

Give canola priority at planting

Looking ahead to canola planting in September, do not delay. We so often wait for the perfect establishment by delaying planting until there is some sign of rain, but end up compromising the potential of the crop by making it flower and pod fill when the rain has finished.

The best crops I see are the most ‘untidy’ at the beginning, with a slightly split germination, but they end up winning because they make better use of the moisture later in the season. They are also less likely to be harvested in March which can be a disaster if the rains arrive early.

If we conserve the moisture we have now and get more in August, canola planting should take priority in September, starting with Jazz and finishing with Blazer.

And I may be going against the grain here, but if you are cover cropped during the fallow think very carefully about terminating the cover now to conserve what moisture you have. Having to plant in October will severely limit yields.

…and Companion Cropping to reduce Millipede damage in canola

Having tried some areas of canola sown with a companion crop of peas, phacelia and buckwheat last year, the millipede damage was definitely reduced and the establishment improved, as they appeared to be distracted by the other food sources available to them.

Whilst there might be some competition in the early stages this is far outweighed by the improved establishment and we can experiment with different types of companions and terminating them earlier or later.

In the photo below we used a mix of peas, buckwheat and phacelia. For me this makes so much sense. Avoid overthinking soil health, and focus instead on the building blocks of how to use fewer insecticides and fungicides where sensible. The benefits will follow.