- The fungus Rhizoctonia solani (hereafter referred to as “Rhizoctonia“) causes stem and stolon canker and the sclerotia on tubers form black scurf.
- Stem canker can kill developing stems and stolons, delaying crop growth.
- Rhizoctonia infection can also result in:
- delayed formation of tubers and uneven tuber development
- misshapen tubers
- “elephant hide” blemishes on tubers
- Inoculum may be seed borne or soil borne.
- On seed tubers, inoculum may be present as sclerotia (black scurf) or as hyphae in eyes. As stems begin to grow, hyphae spread initially over the surface. Hyphae can spread from stolons to the surface of developing tubers but do not penetrate the tuber. Sclerotia (black scurf) develop as the haulm senesces. More …
- Levels of soil borne inoculum may be low if rotations of more than four years are used. The highest disease risk results from a combination of seed and soil borne inoculum.
- Yield of individual plants may be reduced if more than 5 – 15 % of the seed surface is covered with black scurf but crop yield may be reduced only if patches of infected plants occur. However infected plants within the crop can result in variability in tuber quality. More …
- Nematode infestation of roots can increase Rhizoctonia infection. More …
Risk of black scurf
- There can be a risk of black scurf even at low levels of inoculum on seed.
- Sclerotia formation is triggered by senescence of the potato plant and the risk of black scurf formation is increased by a fast rate or long duration of senescence.
- The greater the amount of mycelial growth (seen as stem and stolon canker) the greater is the risk of black scurf formation. The risk may therefore be reduced by rapid emergence and a short growing season.
- Rhizoctonia in soil can be measured. However experiments have shown that tests provide a poor indicator of risk, as inoculum can increase rapidly when a susceptible crop is grown.
- Infestation by potato cyst nematodes can increase the risk of Rhizoctonia infection . More …
Control of Rhizoctonia
- Plants become resistant to infection after emergence, so rapid emergence reduces the risk of stem canker, though some experiments have shown greater infection of stems from chitted compared with unchitted seed.
- Rhizoctonia may be controlled effectively by application of seed treatments containing flutolanil (“Rhino”) or fludioxonil (“Maxim 100FS”) or by in-furrow treatments of azoxystrobin (“Amistar”) or fluxapyroxad (“Allstar”).
- Seed treatments should be applied prior to or at planting according to manufacturer’s instructions.
- Rhizoctonia inoculum may be present on the seed even in the absence of visible sclerotia and seed treatment of any crop for a pre-pack market may therefore be beneficial.
- Seed treatment of a processing crop may also be beneficial, particularly if:
- uneven tuber development could affect fry quality or
- more than 5 – 15 % of the seed surface is covered with black scurf or
- there is some black scurf on seed and a history of Rhizoctonia in the field or
- the next potato crop to be planted in that field is intended for a pre-pack market.
- Use of appropriate cover / biofumigation crops can reduce the risk of infection by Rhizoctonia. More … (brassica section)
See the manufacturers’ websites for details of agrochemical use.
Stem canker and black scurf notes
Nematodes 2021 (Includes effects of biofumigant crops on Rhizoctonia solani).
Cover crops in the potato rotation 2020 (Includes influence on black scurf infection)
Transmission of seed borne diseases 2018 (Spread of infection from seed to growing plants)
Rhizoctonia 2014 (Influence on yield and on quality in addition to black scurf; biology; effects of time of harvest, curing and storage conditions on black scurf; rotation and survival of sclerotia in soil; anastomosis groups; effects of seed and soil borne inoculum; trials with seed treatments including penflufen and fludioxonil)
Tuber blemishing diseases 2012 (Includes conditions in which there is a risk of Rhizoctonia infection)
Blemishing disease updates 2011 (A soil test can show the presence of Rhizoctonia but cannot quantify the risk of infection)
PCN and Rhizoctonia interactions 2011 (Infestation with PCN increases the risk of Rhizoctonia infection)
Nutrition and quality (Includes enhancement of black scurf control with a zinc product)
Rhizoctonia detection and control 2005 (Trials with fungicides applied to seed and soil; introduction to flutolanil; potential tests for Rhizoctonia in soil)
Amistar incorporation 2004 (Method of application of azoxystrobin to soil for suppression of Rhizoctonia infection)
Rhizoctonia 2001 (Cultivar resistance; disease severity after storage)
Tuber blemishing diseases 1999 (Includes stem canker and black scurf: yield and quality effects; seed a soil borne inoculum; risks of infection)
Tuber blemishing diseases 1998 (Includes black scurf: relationships between levels of infection at different growth stages; separating haulm and tubers before harvest reduces black scurf)
Rhizoctonia 1997 (Life cycle; biology including effects of stolon pruning)