In South Africa, potato (Solanum tuberosum) late blight epidemics from 1996 to 2007 were caused by Phytophthora infestans clonal lineage US-1 (McLeod et al. 2001; Pule et al. 2013). Similarly, surveys on tomatoes in the mid-1990s only identified the US-1 clonal lineage in South Africa (McLeod et al., 2001). On potatoes, populations from the Southern Cape and Western Cape regions consisted of persistent mefenoxam-resistant populations (McLeod et al. 2001; Pule et al. 2013). Limited mefenoxam (R-enantiomer of metalaxyl) screening in 2021 in the Western Cape showed that potato isolates were sensitive, which prompted our study. Potato late blight samples were collected in 13 potato fields in the 2021 to 2023 seasons in the Western Cape (n = 4), Free State (n = 7), Limpopo (n = 1) and Kwazulu-Natal (n = 1) Provinces, and one tomato sample in 2022 in the Limpopo Province. Fourteen samples, one per field, were simple sequence repeat (SSR) genotyped for 12 loci (Li et al. 2013) using as DNA template, FTA cards, or genomic DNA extracted from cultures. P. infestans isolations from lesions and DNA culture extractions were conducted as previously described (Pule et al. 2013). SSR genotyping revealed that all 14 P. infestans samples belonged to clonal lineage EU_23_A1 (EU23), which has a phenotype (A1 and metalaxyl sensitive) and SSR genotype matching the US-23 lineage (Saville et al., 2021). As expected, minor polymorphisms were detected among the samples at loci Pi02, G11, D13 and SSR4. Mefenoxam sensitivity testing of seven potato isolates from the Free State (n = 3) and Western Cape (n = 4), and one tomato isolate was conducted as previously described (Mcleod et al. 2001). All isolates were sensitive to mefenoxam since no infection and sporulation occurred at 3 µg/ml. This was expected since EU23 has been reported as mefenoxam sensitive in other countries (Kawchuk et al., 2011; McGrath et al., 2015). Replacement of the US-1 clonal lineage by EU23 suggests that the latter lineage is more aggressive or fit than US-1, but this must be verified especially on potatoes. On tomatoes, on the other hand, EU23 is known as a highly aggressive lineage (Kawchuk et al., 2011; McGrath et al., 2015; Saville et al., 2021). Therefore, population displacements may have first occurred on tomatoes from where the lineage spread to potatoes. In the Cape coastal potato production regions, population displacement may have been supported by the withdrawal of mefenoxam/metalaxyl from the region since 1996 because the EU23 lineage is mefenoxam sensitive, as opposed to the previously prevailing US-1 mefenoxam-resistant lineage. More severe potato late blight epidemics has not been observed in recent years in South Africa. However, tomato late blight has increased and is more prevalent in the Limpopo province. The source of the introduction of EU23 into South Africa is unknown. Only test-tube plants and/or greenhouse tubers may be imported into South Africa since 1997. Therefore, the illegal importation of planting material may have introduced the new genotype. Whether this could have occurred from neighbouring African countries is unknown since P. infestans genotyping has not been conducted in these countries. In Africa, EU23 has been reported in northern African countries (Tunisia, Algeria and Egypt) (Saville et al., 2021; El-Ganainy et al., 2023). Mefenoxam and metalaxyl applications will likely be effective again in the Western Cape, but more samples will have to be tested to confirm this. This will provide growers with a more cost-effective fungicide (metalaxyl) since alternative actives with comparable systemic and curative activity are more expensive.
Keywords: Causal Agent; Crop Type; Oomycetes; Pathogen diversity; Subject Areas; US-1 lineage; Vegetables; pathogen introduction.