Severe corky crack symptoms on potato tubers (Photos:
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 rhizoscab and may appear similar to growth cracks (physiological disorder) or fissure scab (caused by Streptomyces species). 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, was recently conducted by the ARC-VIMP 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 (growth cracks) 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. More recently, the major Streptomyces species resulting in fissure scab were reported (Cruywagen et al., 2021). 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).
Both the pathogen groups 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. The work conducted at the ARC-VIMP showed that sometimes only Streptomyces species were recovered from symptomatic tubers, while sometimes only Rhizoctonia species were recovered and yet sometimes both. Both pathogens can induce crack type symptoms on its own, but the resulting symptoms have subtle differences. Symptoms with the typical corky appearance inside the crack (as illustrated by the photos above), always yielded Rhizoctonia either alone or in combination with Streptomyces (often saprophytic species), whereas the more star-shaped cracks always yielded pathogenic Streptomyces species. The cracks with a smooth inner surface are typical of growth cracks with no pathogens associated with it. Unfortunately, these symptom types remain difficult to diagnose through visible inspection alone and laboratory tests are required to confirm the causal agent. 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.
Contact: Dr Mariette Truter
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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