Seed tuber-borne inoculum of Rhizoctonia significantly contributes to Rhizoctonia disease epidemics on potato and pathogen population genetic changes Norman Muzhinji and Prof Jacquie van der Waals (University of Pretoria) The fungus Rhizoctonia solani causes a reduction in the marketable yield of potato tubers in South Africa and other potato producing countries. Rhizoctonia solani is a complex species made up of 13 anastomosis groups (AG). Anastomosis group 3, the potato infecting subgroup (AG 3-PT), is the dominant AG associated with potato diseases worldwide and it infects potato at any stage of the crop development. The underground parts of the potato crop, especially the stems, stolons and roots, are infected early in the season. Stem infection is characterized by dark brown Page 22 CHIPS Maart/April 2017
necrotic lesions commonly referred to as stem canker (Fig 1A), which result in delayed emergence, death of sprouts, and reduced plant vigour. Stolon infection results in stolon pruning with a consequent reduction in the number of tubers per plant. At the end of the growing season black sclerotia form on the surface of progeny tubers, resulting in the appearance of black scurf (Fig 1B) and elephant hide (Fig 1C). If not controlled in the field, Rhizoctonia infection on potato can cause marketable yield losses of up to 70%. A B Sources of inoculum In nature, R. solani AG 3-PT exists primarily as vegetative mycelium and sclerotia in the soil and on plant debris. Soil-borne inoculum of R. solani includes mycelia and sclerotia already inhabiting the soil where the potato is planted. Rhizoctonia is mainly introduced into non-infested soil through seed-borne inoculum. Infected seed tubers are the mechanism of long-distance dispersal and dissemination of R. solani among and within potato production areas. Long distance dispersal causes the establishment of new genotypes into new locations through introduction, reproduction, selection and survival of the introduced organism. Once introduced into the soil via infected seed tubers, the fungus may establish itself and become soil-borne. Rhizoctonia solani can then survive in the soil as sclerotia for extended periods, up to six years in the absence of a host. Inoculum of R. solani can be seed tuber- and/or soil-borne sclerotia or hyphae, however, it is not clear what the relative contribution of each source of inoculum to disease development on potato is. Understanding the contribution of each source of inoculum in causing potato diseases is an important first step in implementing effective disease management strategies for R. solani. In this article, the contribution of seedborne inoculum to potato development and the control measures available are emphasized. Addressing the knowledge gap C Figure 1 Rhizoctonia solani symptoms on potato (A) Stem canker, (B) Black scurf, (C) Elephant hide. A two-year study was conducted by the Potato Pathology Programme Research group at the University of Pretoria to evaluate the contribution of each source of inoculum using an integrative experimental approach combining field trials and molecular techniques. In general, seed tuber-borne inocula was found to result in significantly higher levels of disease on stems, roots and stolons, and black scurf on progeny tubers than soil-borne inoculum (Tables 1-3). The results from our research provided plausible evidence about the predominant importance of seed tuber-borne inoculum of R. solani in causing diseases on potatoes supporting earlier observations in other potato producing countries. Therefore, efforts to reduce primary inoculum should focus on reducing pathogen population on seed tubers. Based on our experiments, the introduction of tuber-borne inoculum with genotype different from soil-borne inoculum causes changes in population genetic structure of Rhizoctonia within a field over and across growing seasons. CHIPS March/April 2017 Page 23
Table 1 The effects of seed tuber and/or soil-borne inocula of Rhizoctonia solani on the number of haulms and percentage emergence Number of haulms Percentage emergence Control 2.8 a* 3.2 a 77.3 a 90.0 a Soil 2.2 b 2.7 b 59.9 b 55.2 b Soil and tuber 1.8 c 2.5 c 56.9 b 52.2 b Tuber 2.0 c 2.5 c 57.4 b 55.2 b *Means compared with Fisher's protected least significant difference (LSD) (P = 0.05). *Values in columns followed by the same letter are not significantly different according to Duncan s Multiple range test (P = 0.05). Table 2 The effect of seed tuber- and/or soil-borne inocula of Rhizoctonia solani on potato root infection and stolon canker disease index Root infection Stolon canker Control 0.0 c 0.0 c 0.0 d 0.0 d Soil 14.5 b 10.5 b 8.5 b 15.0 a Soil and tuber 17.5 a 12.5 a 10.2 a 12.0 b Tuber 13.0 b 6.0 c 5.0 c 8.0 c a Disease index for roots and stolons was calculated by the formula: DI= Σ [0(n0) + 0.2(n1) + 0.4(n2) + 0.6(n3)201 +0.8(n4) +1(n5)] x 100/ (Ntotal) where nx = number of roots or stolons in the x rating class and N = total number of roots or stolons. *Values in columns followed by the same letter are not significantly different according to Duncan s Multiple range test (P = 0.05). Table 3 The effect of seed tuber- and soil-borne inocula of R. solani on stem and black scurf disease index (DI) Stem canker a Black scurf b Control 0.0 d* 0.0 0.0 c 0.0 d Soil 22.5 c 14.5 c 13.4 b 5.8 c Soil and tuber 37.0 a 22.5 a 17.6 a 11.3 a Tuber 29.0 b 17.4 b 12.6 b 7.8 b a DI for stems was calculated by the formula: DI= Σ [0(n0) + 0.2(n1) + 0.4(n2) + 0.6(n3) +0.8(n4) +1(n5)] x 100/ (Ntotal) where nx = number of stems or stolons in the x rating class and N =total number of stems or stolons. b DI for black scurf was calculated by the formula: DI= Σ [0(n0) + 0.25(n1) + 0.5(n2) + 0.75(n3) +1(n4)] x 100/ (Ntotal), where nx = number of tubers in the x rating class and N = total number of tubers in each of the category. Rhizoctonia disease control complexity Rhizoctonia solani is very difficult to control once established in the field as there is no single practice that is completely effective against Rhizoctonia. Potato growers have to use holistic plant health strategies in the management of the Rhizoctonia disease complex. The holistic strategies that have been helpful in Page 24 CHIPS Maart/April 2017
Rhizoctonia management include; the use of seed tubers that tested free of Rhizoctonia or treating seed tubers with fungicides or biocontrol agents as well as accurate and timely diagnosis of disease and crop rotation. However, fungicide treatments of seed tubers do not provide complete control if the soil is infested with soil-borne inoculum. There is also contrasting evidence regarding the use of crop rotation for controlling R. solani in potato fields because of the wide host range of the species complex. On top of that, a minimum of five-year rotations are recommended with non-food crops, when in fact there is not much land available to growers, making it difficult to still keep the farm profitable under such long rotations. Practical solutions for Rhizoctonia solani management by potato growers? The only practical Rhizoctonia solani disease control measures available to the potato grower are to avoid planting in Rhizoctonia infested fields and planting certified potato seed tubers. When seed tubers are contaminated, growers should treat seed tubers with fungicides. Research at the University of Pretoria has shown the effectiveness of fludioxonil, tolclofos-methyl and pencycuron in seed tuber treatment (Table 4-6). The value of this approach is to eliminate seed tuber borne inoculum and minimize the introduction of different genotypes of R. solani into the same field. Some of the steps that can be taken to reduce Rhizoctonia disease complex on potato include: The ideal would be stringent seed certification programs on a regional scale to limit long distance pathogen movement. Seed tubers shared among regions should be properly screened and infected seed tubers removed to prevent pathogen movement among the affected regions. Potato seed tuber certification systems should be able to detect even latent infections by qpcr, although this would prohibitively increase the costs of certification. This approach allows for proactive management that targets the seed tubers as the principle source of inoculum. proportion of daughter tubers with black scurf. Identification of R. solani AGs present in a soil and/or on seed tubers should be the first step in implementation of holistic management strategies, followed by fungicide selection. Our research has shown that various AGs that differ in host range and fungicide sensitivity are associated with potato diseases in South Africa. Risk assessment can be used as a criterion to decide on management measures to implement against specific Rhizoctonia anastomosis groups. Quantitative PCR assays, available at UP, have recently been developed with potential for use as a risk assessment and prediction tools for different Rhizoctonia AGs. Crop rotation using crops that are not host to AGs present in the field. In potato production, cereals have been observed to be good rotation crops since the AGs infecting cereals and potato crops differ. Long term strategy needed for disease management Currently there are no potato varieties that have been shown to have complete resistance to Rhizoctonia in South Africa or elsewhere but variations in tolerance have been reported in other studies, underscoring the need to develop potato cultivars that are resistant to Rhizoctonia. The successful integration of resistant cultivars and other strategies like determining the risk potential of the seed tuber or soil-borne inoculum using detection tools like qpcr, could significantly contribute to the mission of developing effective, environmentally friendly, durable, and sustainable management strategies for protection of potatoes against Rhizoctonia diseases. C Where soil-borne inoculum is present, in-furrow fungicides, especially fludioxonil and tolclofosmethyl, have shown to provide effective control of stem and stolon infection, as well as reduce the CHIPS March/April 2017 Page 25
Table 4. Stem canker disease index of plants after different in-furrow fungicides treatment of artificially inoculated soil by different Rhizoctonia AGs under field conditions. Azoxystrobin 29 b* 16.5 c 15 c 15 c 17.5 b 17 b - Tolclofos-methyl - - - - - - - Pencycuron 10 c 10 bc 17 c 11 c 18 b 13 c - Prodione 10 c - 8 b 11 c 12 b 10 c - Fludioxonil - - - - - 12 c - Benomyl 19 bc 10 bc 10 bc 12 c - 17.5 b - B. subtilis 20 bc 10 bc 12 bc 10 c 15 b 13 c - Control 53 a 32 a 35 a 46 a 28 a 55 a - *Means in a column followed by the same letter are not significantly different according to Duncan s Multiple Range Test (P 0.05) Table 5. Black scurf disease index of plants after different in-furrow fungicides treatment of artificially inoculated soil by different Rhizoctonia AGs under field conditions. Azoxystrobin 28 c* - 10 b - - 18 ab - Tolclofos-methyl - - - - - - - Pencycuron 10 bc - 7.5 b - - 11 c - Prodione 15 bc - 10 ab - - 10 bc - Fludioxonil - - - - - 10 c - Benomyl 15 bc - - b - - 13.8 bc - B. subtilis 20 bc - - b - - 18 c - Control 63 a - 25 a - - 44 a - *Means in a column followed by the same letter are not significantly different according to Duncan s Multiple Range Test (P 0.05). Table 6. Stem canker disease index of plants showing the effect of different fungicide potato seed treatments on different Rhizoctonia AGs under field conditions. Azoxystrobin 21 b* 14 b 17 b 8 b 26 b 12 b 4 b Tolclofos-methyl - - 10 c - - - - Pencycuron 6 e - 15 bc 9 b 14 c - - Prodione 12 d 8.9c - 5 c - - 6 b Fludioxonil 4.8 ef - - - - 10 c - Benomyl 10 d 10 c - 7.8 b 8 d - - B. subtilis 16 c 14 c 14 b 5 c 12 c 12 b 4.8 b Control 63 a 42 a 57 a 33 a 42 a 51 a 8 a *Means in a column followed by the same letter are not significantly different according to Duncan s Multiple Range Test (P 0.05). Page 26 CHIPS Maart/April 2017