Phytophthora Root Rot, caused by the oomycete Phytophthora cinnamomi (Rands), results in devastating decreases in avocado yields every year. Successful management of P. cinnamomi relies on many features, one of which is chemical control by phosphite. Despite its continuous use in South Africa for decades, there is yet to be a country-wide survey that assesses the current status of phosphite resistance in P. cinnamomi isolates. The European Union, which remains the greatest export market for South Africa, have enforced limits on phosphite residues on avocado fruits. Thus, simply increasing phosphite treatment concentrations would not be an option.
There are multiple components to consider when assessing the risk of chemical resistance occurring. The first of which is the mode of action of the chemical in question. Phosphite targets a diverse range of molecular targets within P. cinnamomi; it might be assumed that, because of this, resistance is unlikely. However, there are other aspects that need to be considered with equal measure, two of which we have addressed in this project: phosphite treatment history and pathogen genotypes.
Using an in vitro phosphite sensitivity assay, where the inhibition of mycelial growth was used as the measure, we assessed P. cinnamomi isolates from four South African avocado regions with varying phosphite application histories. Tzaneen (Limpopo) and Hazyview (Mpumalanga) have had extensive phosphite treatment use, Everdon (KwaZulu Natal) had only made use of phosphite applications two to three years before sampling, and George (Western Cape) had no phosphite treatment history at the time of sampling. We had found that previous exposure to the chemical did not correlate to the observed phosphite sensitivities. We had also assessed the differences between phosphite-sensitive and -tolerant isolates’ morphology following phosphite treatment. We found that hyphal stunting and distortions are found for all isolates; this was more pronounced for those that are sensitive. Suspected thick-walled chlamydospores were found for two of the eight phosphite sensitive isolates.
With the use of simple sequence repeat markers, we assessed the multilocus genotype groups in the P. cinnamomi isolates obtained. In our study, we had found 22 multilocus genotypes; none of these groups of isolates indicated major phosphite tolerance. Our study showed no evidence of a link between certain genotypes and
decreased phosphite sensitivity. It is suspected that the cost of fitness involved in becoming resistant to a broad range fungicide would effectively reduce fitness in terms of environment adaptation and competition. We believe that, should phosphite continue to be used in a multifaceted management regime, the risk of resistance would remain low.