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Expert guide – Rhizoctonia solani in potatoes

Rhizoctonia solani – which causes yield-sapping stem and stolon canker and tuber skin blemishes such as black scurf – is now seen as one of the most important diseases for potato producers in fresh, seed and processing sectors.

Here, Harper Adams potato expert Dr Matthew Back offers an in-depth look at the epidemiology and control of this damaging pathogen to maximise output and produce a bright and clean skin finish.

What is R. solani?

It is a pathogen belonging to a large group of fungi known as the basidiomycetes, which also include cereal diseases such as the smuts, bunts and rusts.

It has an extremely large host range and causes economically important diseases in countless arable crops around the world, including cereals, oilseed rape, pulses, sugar beet and potatoes.

Is all R. solani the same?

The pathogen species is made up of a number of sub-groups, known as anastomosis groups (AG). The AG often dictates which host or hosts it favours and, in some cases, even the symptoms caused.

In the field, belonging to the same AG means hyphae – the branches of a growing fungus – from each organism are able to fuse together, share resources and help the pathogen thrive.

Some AGs are more damaging to potatoes than others. For example, AG 2-1, AG 3 and AG 5 are important for potatoes in the UK. AG 8 has been detected in soil, but not on daughter tubers.

There is variation in symptoms, depending on the AG group or groups present. AG 3 is the most common and causes typical stem and stolon canker and black scurf on tubers later in the season.

AG 2.1 causes mainly stem and stolon canker, with less black scurf. AG 8 only affects roots and might not be picked up by agronomists.

What is its lifecycle?

The pathogen is one of the great survivors amongst plant diseases. It is a saprophyte, so can rest on decaying organic matter, which allows it to survive on senesced crop and other debris through the winter.

In addition, sclerotia – resting structures in the form of a compact mass of mycelium – are formed at the end of the growing season and remain in the soil for a period of time until a suitable host is found. Additional sources of inoculum for following crops include infected seed, volunteer potatoes or groundkeepers and a number of weed hosts.

Research suggests survival in the soil decreases over an 18-month period, but with such a vast host range, combined with seed transmission and infected volunteers, its proliferation is very difficult to supress during a cropping rotation.

As a soil-borne disease, R. solani is affected by soil conditions. Symptoms tend to be more common and severe in cool, moist soils at temperatures of 16C-23C. When soil temperature is above 25C, stem canker reduces in severity. The disease can also be more severe on dry, light soils or where pH is neutral to acid (£ 7).

It is very common in all UK soils, although it does have a greater preference for soils where there are pores or cracks, as mycelium can grow and infect hosts more readily.

The most important part of the R. solani lifecycle is this asexual phase. It does have a sexual phase where basidiospores are produced, but its importance and impact in potato production is not well understood.

When and how does initial infection occur?

Whether R. solani is present in the soil or introduced on seed tubers with black scurf or less visible mycelium on the surface, initial infections take place shortly after planting.

The fungus is stimulated by carbohydrates produced by the plant as it grows. These same secretions guide hyphae to the surface of the shoots, stolons or stems, where they aggregate to form a structure known as an infection cushion.

This secretes toxins and a series of cell wall-degrading enzymes which soften the plant tissue. The combination of pressure from the cushion and the cell wall-degrading enzymes allow the fungus to invade, macerate and kill the surrounding tissue. As R. solani is a saprophyte, is survives on dead material and aims to kill as much of the plant as possible.

What are the symptoms?

There are two phases to the disease in potatoes. The first is the stem and stolon phase, which is typically seen when infection occurs soon after planting. Symptoms include stolon pruning, where lesions develop on tips of stolons within the soil, or lesions on stems known as stem canker.

This can cause uneven emergence and growth and as plants have fewer primary stolons, there is an increase in very small and very large tubers, so less marketable yield in the size brackets packers and processors are looking for. Optimal timing of inputs such as herbicides, fungicides and desiccants becomes more difficult.

A further consequence of stolon infection at tuber initiation is a hard, necrotic patch at the rose end of the tuber. As the tuber bulks up, this is stretched and creates an unsightly scab, which resembles an elephant’s hide.

In the worst cases, deep cracks can develop as soft tissue grows around the scarring, forming jigsaw-like necrotic pieces that can sometimes run down the length of the tuber.

The final evidence of R. solani infection can be seen towards the end of the season in the form of black scurf. It forms as black sclerotia on the surface of the tuber as the crop begins to senesce and can be scratched off easily with a fingernail.

Potatoes free of black scurf may still be infected with R. solani, because hyphae barely visible to the naked eye can be growing across the surface. These thin black lines can be picked up by using a x10 hand lens.

How damaging is R. solani?

As a rule of thumb, yield loss is often cited as being about 30%. However, where poor size distribution and skin blemishes are rife, rejections or downgrading of marketable yield can make losses much worse.

Can you test for it?

Seed tuber infection not visible to the naked eye can be tested using real-time PCR assay extraction of the pathogen’s DNA and is considered a very reliable way of assessing the need for seed treatments.

More recently, a test for detection and quantification of soil inoculum has been developed in the UK and Australia using the same method and can help potato growers develop more robust management strategies.

Commercial testing services for presence of  diseases like R. solani on seed tubers are available at FERA and NIAB CUF in England and SASA in Scotland.

How is R. solani managed?

1. Source clean seed

Good seed health is important in minimising disease risk. Sourcing clean, certified seed is advised, and seed samples should be washed and inspected for signs of visible black scurf. Microscopic fungal growth that isn’t visible to the naked eye can also be present, so laboratory testing can help detect seed infection and help guide seed treatment requirements.

2. Know field history

Rhizoctonia is a sporadic disease and risk is difficult to assess, but field history can help. Consider the frequency of potato growing on a particular field and whether potato cyst nematode (PCN) is present. Where soils have been subject to intense potato production, risk from both R. solani and PCN is likely to be high. Soil testing can help quantify this risk.

PCN damage on potato roots leads to a greater loss of the carbohydrates that stimulate and guide R. solani hyphae to the surface of plant tissue, increasing infection risk. Where possible, minimise the impact of PCN on any potato crop with integrated control, including longer rotations, PCN-resistant varieties and nematicides. This should also help reduce the impact of R. solani.

3. Manage volunteers

Allowing potato volunteers to flourish through an arable rotation helps many soil-borne pests and pathogens to survive until the next crop is planted, including R. solani.

Ensure potato harvesters are set up to minimise tuber losses at harvest. An application of growth regulator/sprout suppressant maleic hydrazide can help minimise volunteer establishment. Any growing volunteers should be controlled with herbicides in all crops or stubbles through the year to reduce disease inoculum in the soil.

4. Variety choice

Varietal susceptibility to black scurf caused by R. solani varies and is linked to how long potatoes are in the ground, which is determined by speed of skin set. In high-risk situations and where growing for fresh markets, choose a less susceptible variety with rapid skin set.

Where this is not possible due to market requirements, carefully manage nitrogen applications and ensure rapid haulm destruction to speed up skin set. Also consider an early harvest to minimise build-up of black scurf on tubers.

5. Plant into warm, moist seed-beds

As slow crop emergence can increase the risk of R. solani infection, consider delaying planting where it could be a problem. If forced to plant in cold and wet conditions, place seed at a shallower planting depth to reduce the risk of compaction under ridges, prevent rooting issues and speed up emergence.

6. Consider chemical treatment

Where R. solani risk has been identified, chemical treatments may be required to minimise problems such as black scurf, particularly when growing for the packing market where skin blemish tolerance is low.

Whilst black scurf is not as important in processing crops, R. solani can cause deep necrosis on tubers in the form of netted scab and may affect tuber size distribution due to stolon cankers, so fungicide treatment may still be justified.

In addition, some processors now require a good skin finish for skin-on convenience products, which are becoming more popular with consumers.

The available fungicides negate stem and stolon canker and treatments are applied to seed tubers as a liquid at grading or a powder at planting. Accurate and even application of these seed treatments is essential for suppressing seed-borne sclerotia and mycelium of R. solani.

Liquid in-furrow fungicides at planting can also help reduce disease severity.

Matthew Back is a reader and researcher at Harper Adams University, specialising in potatoes. He supervises several PhD projects looking at the biology and management techniques of plant parasitic nematodes. He also speaks regularly to the potato industry and at academic conferences.

Choose RhiNo for flexible seed tuber protection

The availability of liquid and powder formulations of fungicide RhiNo (flutolanil) gives growers the flexibility of two different application methods at different timings for effective control of R. solani, says manufacturer Certis.

Liquid formulation RhiNo allows for the treatment of seed tubers using application equipment above a roller table or conveyor as seed is being graded, while powder product RhiNo DS is applied at planting via a specialist applicator fitted to the potato planter.

Both formulations are systemic and provide protection of stems and stolons during growth and guard against soil-borne infection, protecting total and marketable yield.

Flutolanil also has some curative effect on seed-borne R. solani and is active against all anastomosis groups. There is an added benefit of a reduction in silver scurf.