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Severe corky crack symptoms on potato tubers (Photos: M. Truter)

The appearance of skin blemishes on potato tubers may result in the rejection or downgrading of seed consignments or processing potatoes, as well as rejection by consumers, which ultimately results in major economic losses to the potato industry. Besides the common typical blemishes such as silver scurf, black dot and powdery scab, there is a range of "atypical" blemishes, for which the nomenclature and pathology is less clear (or incorrect) and often a source of controversy and misunderstanding. One such blemish is corky cracks, also referred to as growth cracks or fissure scab. The confusion has arisen because of the different causes (abiotic and biotic) and nomenclature loosely associated with these symptoms. Research on tuber blemishes, funded by the Potatoes South Africa, is currently conducted by ARC-VOP in collaboration with the University of Pretoria.​

Tuber deformation and cracks are the results of physiological reactions of the tuber during its growing period. One of the main causes of tuber cracks and deformations is irregular water uptake during the growing period, e.g. with alternating dry and wet periods. Heat waves during the growing season and at the time of tuber bulking may lead to temporarily reduced or halted crop growth as well as reduced quality of the tubers. When temperatures drop again, resumed tuber growth leads to secondary growth symptoms and malformation, such as growth cracks and knobbiness. Another cause of growth cracking (usually unexpected) is the treatment of the foliage with an inappropriate pesticide causing phytotoxicity (herbicides), as well as over or poor nitrogen fertilisation. Some potato cultivars are more subject to such overall tuber deformation than others.

Such abiotic causes are often confused with biotic causes of corky cracks, which are similar in appearance. Compounding this problem is conflicting reports of the biotic causes of corky cracks. In 2012, Gouws and McLeod published a first report of an "unknown Streptomyces species" causing "deep longitudinal fissures (3 to 12 mm) containing scab-like lesions" on the surface of tubers. These lesions caused by Streptomyces species were named fissure scab. However, many authors have published scientific articles attributing corky cracks to infection by Rhizoctonia solani. The same was found to be true for corky cracks on potatoes in South Africa where R. solani AG 3-PT was shown to cause not only black scurf and stem canker, but also elephant hide and corky cracks on tubers (Muzhinji et al., 2014).

The pathogens R. solani and Streptomyces species can co-infect a tuber under conditions conducive to disease development. This therefore raises the question of whether corky cracks are indeed due to infection by only one pathogen, or if they can be as a result of co-infection by R. solani and Streptomyces species. Confirmation of the causal agents of these corky cracks, as well as knowledge of their biology and epidemiology, is paramount in formulating effective and practical management tactics and will enable the incorporation of correct nomenclature and pathogens of these cracks in potato seed certification schemes. The primary objectives of this project is therefore to confirm the causal agent(s) of corky cracks on potatoes in South Africa and to propose standardised nomenclature to describe corky cracks caused by the causal agents identified in this study, to avoid confusion in future.

Contact: Dr Mariette Truter

References

Muzhinji, N., Woodhall, J.W., Truter, M. & Van der Waals, J.E. 2014. Elephant hide and growth cracking on potato tubers caused by Rhizoctonia solani AG 3-PT in South Africa.  Plant Disease 98(4): 570. http://dx.doi.org/10.1094/PDIS-08-13-0815-PDN.

Gouws, R. & McCleod, A. 2012. Fissure scab, a new symptom associated with potato common scab caused by a Streptomyces sp. in South Africa. Plant Disease 96(8): 1223

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