Evaluation of a stem inoculation technique for assessing resistance to Phytophthora cinnamomi in Leucospermum cultivars
An important aspect of integrated disease management of Phytophthora root rot, the most serious root disease of Proteaceae in South Africa, is the use of resistant rootstocks. Most commercial cultivars are propagated from stem cuttings that have been rooted in a rooting bed. Inherent resistance properties should, therefore, be present in the stems. The stem inoculation technique for evaluation of resistance properties of various genotypes is desirable because it is very efficient in terms of space, time, labour and cost. Ten isolates of P. cinnamomi Rands representing the A1 and A2 mating types and a range of aggressive abilities were selected for stem inoculations. The effects on lesion length of depth of inoculation and position of measuring the lesion were assessed. The stem inoculation technique was standardised by using superficial inoculations and measuring the lesion just below the bark after 6 days incubation at 22°C. Lesion development on five Leucospermum cultivars was compared after winter and summer inoculations were carried out over three consecutive years. Results showed that it is important to standardise the stem inoculation technique to reduce variation as a consequence of method. The performance of some cultivars was very consistent over all test periods, but others demonstrated variability in lesion length. Assessment of resistance on results of a single test is thus not recommended.