Ensure irrigation strategy success – Potato Review

“Once you are clear that you have the capacity, you can decide how you prefer to build your irrigation strategy, choosing between model-based irrigation, which uses predictions of weather and soil moisture, and/or the use of tensiometers and in-crop sensors.

“We need to get better at monitoring and measuring what is going on in the soil and the critical times for irrigation.”

Timing of measurement is key, and Andrew suggests digging across the ridge down to 50-60cm and looking at the rooting zone and to assess the soil moisture levels. However, he warns not to leave it until too late as then any application of water would have a limited, if any effect.

Moving on to talk about irrigation tools, he points out that booms, rain-guns and drip systems all have advantages and disadvantages.

“Trickle irrigation makes the most efficient use of water, as it can be applied accurately with sensors. It works better than the other methods in hot, dry seasons as you do not lose so much to evaporation.”

As water is applied little and often, room can be left for rainfall when it is forecast, he adds.

However, costs of set-up are considerable, and it is an ongoing process that needs careful management, and drip irrigation is not suitable for rectifying a problem as not enough water can be applied quickly.

Rain-guns are more of a ‘fire-brigade’ approach which can be useful, says Andrew. However, because of the speed of application, 25ml can flatten the haulm, and it can lead to run-off.

“If soils are smeared, infiltration is compromised and the water does not reach down to the root zone, the water is lost.

“If it is very dry, the ‘fire-engine’ approach can sometimes lead to too much water going on, particularly if it is followed by heavy rain, as too much water can lead to growth cracks.”

These can occur when the skin is unable to stand the internal pressure created by very wet irrigation regimes which leads to over-hydrated tubers.

Water rates can be adjusted when using booms, which facilitates a more uniform application of water than when rain-guns are used, however, they can still create run-off if the field is on a slope.

“When choosing your field for the crop, it is a good idea to think at the same time about your preferred irrigation method, any effects of topography and whether you have nearby water storage areas, boreholes and rivers.

“Using a boom can also be more difficult if there are trees and poles to contend with.”

Andrew notes that using any sort of irrigation will raise the carbon footprint of the crop because of the energy used to pump the water. For every 1l of fuel, there is a 2.51 carbon penalty. “However, on the plus side, potatoes are good at soaking up carbon in the atmosphere, particularly as they are still green in July and August when other crops are senescing.”

Following weather forecasts remains important, particularly when summer storms threaten in July and August, because they are likely to bring heavy rain. “This is when we see the benefit of a well-structured soil which is able to use water more efficiently. In our trials, we have noted that soils with a reasonable level of organic matter (OM) are able to hold on to water so there is a bigger reservoir in the rooting zone.

“Nonetheless, if heavy rain is forecast it is better to hold off with the irrigation.”

Knowing the soils in the field being used for growing potatoes remains key, and while most growers know the soils on their own farms well, and the areas which tend to become saturated, sometimes rented land can bring surprises.

Soil type and texture determine water holding capacity because sand, silt, and clay particles have different pore sizes. Soils with higher amounts of clay have more small pores than sandy soils, helping them to retain more water.

“If you are planting in a sandy soil, it is worth considering growing in beds made for two or three rows with no furrow between them, as this can help water stay in the all-important root zone.

“Gentle rainfall is the best for our industry. On one hand if the soil in the ridges has capped, quite often gentle rain can help break it, but also it can help to reduce greening as it fills up cracks which have appeared in the ridges.”

Soil type, irrigation and varietal susceptibility all play key roles in reducing the incidence of common scab (Streptomyces scabiei), says Andrew.

The standard scab control regime is to irrigate for 28 days from tuber initiation (TI). This is the time when the crop is susceptible because new tissue is being formed by new cells, and is reduced once the tuber reaches the size of a golf ball, he explains. Tuber initiation takes place 15-40 days after emergence but in some varieties, depending on weather conditions, it can occur in two bursts.

He recommends that the soil in the ridges should be moist during this time, and so irrigation needs to start in good time. In addition, if irrigation is stopped too early, it can create an even better environment for the pathogen to proliferate, resulting in higher scab levels.

“If emergence is uneven, it is best to keep the crop moist until all the tubers have had a chance to grow out of the susceptible phase, which is when the tuber has stopped producing new cells.”

Irrigation helps boost the populations of good microbes that compete for the same nutrients as the scab-causing Streptomyces, and resulting in fewer pathogens. However, he notes, extra irrigation does not give greater protection as there are no further increases in beneficial microbe numbers and may even lead to infection with powdery scab.

Irrigation, nevertheless, is one of the key strategies to reduce incidence of common scab, yet it is not the whole story. Correct planting depth for even emergence, avoiding over cultivation and compaction plus a good understanding of soil type are also critical.

“Soil health is also important as soils with good structure and organic matter (OM) with healthy numbers of microbes show lower incidence of common scab,” said Andrew.

He finds that the application of gypsum helps open up soil structure, and therefore can be useful in reducing common scab.

Although hydrogen ions do not cause scab soil pH may also be a factor, he says, noting that the effects of lowering soil pH by an application of ammonium sulphate in the ridge are inconsistent. This is down to the slow rate of the nitrogen breakdown which slightly increases acidity.

Sulphur pellets have been used to increase ridge acidity but owing to the speed at which the prills break down, the crop is already past the susceptible tuber initiation stage, says Andrew. “Another point about bringing down pH is that you reduce the plant’s ability to uptake nutrients, so you may be creating more problems than you solve, he said.

Manure can work as a useful soil conditioner, and may even do more than applying a biostimulant at planting. He gives the example that in Herefordshire many of the soils are low in calcium and high in magnesium, so applications of poultry manure which help correct not only calcium deficiency but also lower pH because of the nitrogen levels it contains.

“Scab control If the crops are under stress at this time, they can also be more susceptible, so anything you do to reduce this may be useful.

Powdery scab (Spongospora subterranea) isa fungal blemish disease of potatoes which often occurs when there are cool, wet growing conditions, or potato crops have received too excessive irrigation, says Andrew.

The powdery scab pathogen can sometimes carry and transmit Spraing (potato mop top virus or PMT’V).

He notes that crops grown in heavy soils can be more susceptible to the pathogen, particularly when there is poor drainage or standing water.

Infection can occur from diseased seed or it can be soil-borne, he adds noting that fields can carry inoculum if manure from stock fed with diseased tubers is applied, or if there is inefficient volunteer control after powdery-scab infested crops.

“If you are renting land for growing potatoes, you should look always carefully at the field history before taking it on, said Andrew.

Potato crops are most susceptible to the pathogen at tuber initiation. However it can take between four and eight weeks for symptoms to show.

“Part of the problem is that the ‘powder’ is made up of spore balls (cystosori) that are released into the soil can survive in the soil for up to ten years. These spores release motile (swimming) zoospores that penetrate root hairs and stolon epidermal cells, lenticels and tuber eyes. They can also enter via wounds on the developing tubers.”

Given the lack of effective treatment, particularly in ware crops, avoiding diseased seed is key, and he emphasises that careful checking of seed is crucial, particularly as tolerances are quite high.

“It can be confused with common scab so I prefer to use an eyeglass when doing his inspections as it can be difficult to distinguish between powdery and common scab. The shape of the lesion is slightly different.

The powdery scab pathogen produces circular pustules on the tubers which can act as ‘volcanos’ after they rupture.

“Another sign to look for is the size of the lesion – those made by powdery scab are often larger because they have merged together. Also, if you scratch the lesion with a fingernail, it leaves a greater depression on the surface than you get with common scab.

“If you suspect powdery scab, it is always a good idea to send a sample to a diagnostic laboratory for confirmation.”

Once in the soil, the zoospores can move through water across short distances to infect the new crop, and, if the mother tuber is diseased, they will move to the daughters.

“The zoospores infect the root hairs and stolons before the tubers develop, and can cause galls on the roots. The developing tuber produces corky tissue to halt the growth of the disease.

“We have found that some varieties are more susceptible to others, so it is always a good idea to check susceptibility of the variety you intend to grow alongside the field conditions and soil type to judge risks of growing in that particular field.”

Higher risk fields where seed potatoes are to be grown can be treated with an in-furrow treatment of fluazinam at 31/ha, but this then means that the fungicide cannot be used as part of a blight control programme.

“If you are growing a ware crops, it is more difficult. A zinc-based treatment (such as zinc oxide or zinc sulphate) applied to the soil could be useful, butat present there are no such treatments approved for powdery scab.”