Apple replant rootstock trials for determining a screening technique

The objective of this study was to quantify the variation in apple rootstock genotypes regarding resistance/tolerance to ARD

The performance of rootstocks from the Cornell Geneva range (CG3007, G778, CG4202) and standard industry rootstocks (M793, M7, MM109) were investigated under ARD conditions in field trials in field and pot trials. Pathogen load in soil in the immediate vicinity of plant roots was determined by qPCR for ARD-associated pathogens, namely the genera Cylindrocarpon and Phytophthora  and the species, Pythium irregulare, Pythium sylvaticum and Phytopythium vexans.

From the percentage increase in trunk and total shoot growth, as well as plant weight measurements with termination of the trial, it was clear that rootstock tolerance was site specific, with possibly the exception of MM109 that seemed to be most susceptible to ARD in both sites. However, this is in contrast to the previous round of field trials (2010-2012) which found MM109 to be the only rootstock that consistently showed tolerance to ARD conditions in the field.  All pathogens, except for P. irregulare, were present in varying concentrations at both sites (Vergelegen and Glen Fruin) and in both treatments (fumigated and unfumigated). For both sites there is a clear difference in the plant growth in fumigated versus unfumigated soil, however, it is only at Glen Fruin that a slight pathogen load effect is evident.

It was concluded that the pot trial results was not an accurate indication of results in the field. It seemed that pot size restricted growth in fumigated soil over a two year growth period, resulting in few differences between plant growth in fumigated and unfumigated soil. When taking into account rootstock growth over all four field trials, it becomes evident that there are site-specific factors contributing to rootstock performance. For example, in the 2010-2013 trials, MM109 emerged as the best-performing rootstock, however in the 2013-2015 trials it was the most susceptible. From the pot trials, we know that soil from the 2010-2013 field trials contained at least three of the ARD-associated pathogen groups as well as lesion nematodes, although unlike in 2013-15, the latter were not present in significant numbers.  From the 2013-2015 trial results, it appears that parasitic nematodes may have a site specific role as part of the ARD complex. Results from soil chemical and plant nutrient analyses could not be related to / did not affect plant growth. With regard to fungal and oomycete pathogens, although there was a difference in growth between rootstocks in fumigated and unfumigated soil at both sites, poor growth could not be definitively attributed to higher pathogen loads, given the differing results for Vergelegen and Glen Fruin.

Therefore, there appear to be a number of site-specific factors (some of which are as yet unknown) that need to be taken into account when selecting rootstocks for ARD management.