Screening of apple rootstocks for resistance against the most prevalent Pratylenchus species affecting orchards in South Africa

Lesion nematodes (Pratylenchus spp.) are difficult to control once established. Screening newly imported, promising high potential apple rootstocks for resistance against prevalent species of root lesion nematodes occurring in SA will help to identify the most promising rootstocks for release to the industry. The aim of this project is to evaluate new rootstocks with potential value to the industry, including promising new Geneva rootstocks, for their comparative susceptibility, resistance or tolerance to the lesion nematodes prevalent in SA apple orchards. Initially, the screening of rootstocks was limited to pot trials, but an amendment on the project was requested to obtain additional data from apple rootstock trials that were planted to evaluate GENEVA® rootstocks.

Populations of lesion nematode species were obtained from the collections of ARC Infruitec- Nietvoorbij and cultured using the in vitro carrot disc method. Plants were inoculated with nematodes by burying infected carrot discs next to the trial plants. At 180 days after inoculation, final nematode populations (Pf) and numbers of nematodes per gram of root were assessed for each experiment. The reproductive potential of lesion nematodes in each rootstock was calculated using Oostenbrink’s reproduction factor (Rf = Pf/Pi), where Rf > 1 indicates susceptibility. In addition to this, the percentage of the nematodes g-1 roots of each rootstock in relation to the nematodes g-1 roots of the most susceptible rootstock was calculated and used as an additional parameter (%R) to classify the level of resistance, where %R < 10% indicates resistance.

None of the rootstocks tested could be confirmed as being resistant to lesion nematodes, but some seem to be more tolerant to infection by lesion nematodes. The rootstock that was the least susceptible to all 3 species was M9 T337, while G202 and G213 was also less susceptible to P. hippeastri and P. vulnus, but not to P. penetrans. G228 also showed reduced susceptibility in the field trials but we were unable to obtain tissue culture material of this rootstock from the suppliers during this project for use in pot trials.

The apple industry in South Africa is rife with lesion nematodes, from nurseries to orchards, and attention must be given to alleviate this problem. Starting with clean nursery material will contribute to healthier apple trees but will not totally eradicate the problem. A lot of progress has been made with evaluation of new apple rootstocks, and the result of this project will provide more information for producers to use when deciding on a suitable rootstock for their circumstances. Lesion nematodes are here to stay, and producers will be wise to select rootstocks which are less susceptible and/or tolerant to the lesion nematodes prevalent in SA apple orchards.

None of the rootstocks tested during this project are completely resistant to lesion nematodes, but results from this project will enable producers to select rootstocks which are less susceptible and/or tolerant to lesion nematodes.