Evaluation of phytophthora root rot resistance in avocado

Abstract

Avocado trees (Persea americana) affected by Phytophthora root rot caused by Phytophthora cinnamomi produce copious amounts of small, poor quality fruit and without intervention, affected trees eventually die. Current management practices involve an integrated approach to disease management, including planting on well-draining soil with a high total cation exchange capacity, application of organic mulch, carefully monitored organic and inorganic nutrition, phosphonate applications and the use of tolerant rootstock varieties. Selection of rootstocks in breeding programs is a time consuming process involving extensive glasshouse and field screening but little is understood about the mechanisms underlying tolerance in elite selections. The overall aim of the research described in this thesis was to identify phenotypic differences across a range of avocado rootstock varieties with varying degrees of resistance/ susceptibility that may be associated with known field susceptibility. The first hypothesis tested in this thesis is that field resistant rootstock varieties have greater root regenerative ability than susceptible varieties. This hypothesis was tested using a glasshouse study of inherent root growth of seedlings as well as an inoculation study to investigate root system health and wilting in response to infection by P. cinnamomi. To investigate whether inherent root growth correlates with known field susceptibility of seedling rootstock varieties, a 2-pot root regeneration system was successfully developed and varietal differences in root: shoot ratio across nine seedling avocado varieties assessed. Seedling ‘Hass’ produced the greatest amount of feeder roots relative to the surface area of mature leaves, although overall differences between seedling varieties were not significant. The main finding of the glasshouse root growth studies was that there were no strong indications that inherent root growth and root system necrosis correlated with known field susceptibility. The second hypothesis tested was whether increased surface feeder root growth of mature trees and other tree physiology attributes such as stored non-structural starch, yield, tree health and growth measurements such as tree height and canopy volume have any association with known field susceptibility. At one experimental site, feeder root growth of mature trees measured over two seasons was not significantly different between rootstocks ‘Dusa’ and ‘SHSR-04’ and no significant differences or associations were found in stored starch, tree health, yield or tree growth parameters. At the second experimental site, there were also no significant differences detected in feeder root growth of rootstock varieties. Significant differences in tree physiology were detected among rootstocks with ‘Velvick’ having the strongest determinate vegetative growth measured and the highest level of stored non-structural starch, although it wasn’t significantly higher than field-susceptible ‘Reed’. Overall, results of field studies at both sites indicated that there was no evidence that increased feeder root growth or tree physiology attributes were associated with known field resistance. To investigate whether the anti-oomycete compound Mefenoxam had a localised effect on P. cinnamomi in the soil that manifested as increased surface feeder root growth, it was applied to half of the root window sites. Mefenoxam application had site-specific effects. The third area of investigation involved testing if there were differences in disease lesion length and extension of P. cinnamomi hyphae within tip-inoculated feeder roots of the susceptible ‘Reed’ and resistant ‘Velvick’ varieties. The response of ‘Velvick’ was more rapid than ‘Reed’ as lesion lengths of inoculated ‘Velvick’ roots were significantly longer than ‘Reed’ at 72 and 96 hpi. Margins between necrotic and healthy tissue were significantly more likely to be distinct in inoculated ‘Reed’ roots than ‘Velvick’. Results of the PCR assay showed that P. cinnamomi was detected further ahead of the lesion front in ‘Reed’ than ‘Velvick’ but the difference was not significant. Results indicated that significant differences in host response were observed between tip-inoculated ‘Velvick’ and ‘Reed’ roots, but differences in pathogen extension through host tissue were not significant. The main findings of this thesis were that seedling ‘Hass’ had the most root growth relative to the surface area of mature leaves. In both glasshouse studies using seedlings and in the field, root regenerative ability does not appear to be a significant mechanism of tolerance/ resistance to Phytophthora root rot. Rootstock differences in physiology parameters of mature trees were not associated with known field resistance. Also, in an inoculation study, significant physiological differences were observed between tip-inoculated ‘Reed’ and ‘Velvick’ root tissue in terms of disease lesion length and margin with healthy tissue. Results of the inoculation study indicate that restriction of P. cinnamomi growth through root tissue does not appear to be an effective resistance response for the field-resistant ‘Velvick’.