The influence of climatically different seasons on the reproductive strategy of Venturia inaequalis and the RIMpro disease forecasting model

A previous research project (A-15-USP-PM20) suggested that conidia, in addition to ascospores, of the apple scab (fusi) pathogen Venturia inaequalis, may be an important source of primary inoculum during dry and warm autumns and winters. RIMpro is a disease forecasting model that can assist in the management of fusi primary infections if predictions are accurate. The objectives of the study are to (i) use a population genetics approach to investigate the role of conidia as primary inoculum source, (ii) develop a pre-bud break ascospore assessment method and (iii) evaluate RIMpro.

For the population genetics study, fusi lesions were collected from trap- and orchard trees in three seasons using published simple sequence repeat (SSR) markers.

Pre-bud break evaluation of ascospore production was conducted by overwintering fusi leaves in orchards from the 2018/19 to 2020/21 seasons. At set times, leaves were moved to the laboratory to accelerate ascospore formation and maturation followed by counting the number of ascospores released.

RIMpro was evaluated in three seasons using spore samplers and trap trees. Fusi leaves were overwintered in autumn to determine the biofix.

Results from the population genetic analysis of lesions from trap trees and orchard trees were inconclusive due to the low success of genotyping lesions with the SSR markers.

Pre-bud break analysis conducted in three seasons were unsuccessful since the positive control did not yield ascospores.

Considering all three seasons, evaluation of RIMpro showed that the model is accurate in some years and orchards but not in others. Furthermore, the prediction of peak ascospore releases were sometimes accurate but not the prediction of the most severe infection events. The biofix could not be determined in any of the three investigated seasons, since no ascospores were found in the samples.

The population genetic approach for assessing the importance of conidia and ascospores in the epidemiology of apple scab was unsuccessful due to SSR genotyping from the lesions not being successful. The isolation of the pathogen from lesions followed by SSR genotyping from the culture DNA may have been a more successful approach.

The absence of ascospores in biofix samples and the low ascospore quantities detected on rotorod spore samplers suggest that the primary inoculum contributed by ascospores was low in the 2018/19 to 2020/21 seasons. The detection of viable conidia on overwintering leaves attached to trees suggests that this may be a source of inoculum in the Grabouw region in addition to conidia surviving on the inner bud scales and pygmy fruit.

The accuracy of the RIMpro model could not be evaluated reliably due to the low ascospore numbers detected on spore samplers and relatively low levels of fusi infections occurring on trap trees.

A laboratory protocol for assessing the potential ascospore dosage just before pre-bud break could not be developed successfully. The protocol would have been useful for assessing whether early fall and winter conditions were conducive for ascospore production, i.e. severity of epidemics in the new season.

Climate change may have resulted in a change in the source of primary inoculum for apple scab epidemics. Ascospore may no longer be the main source of inoculum, conidia overwintering on trees may also contribute to the primary inoculum. This requires further investigation since if this is the case, spray programs will have to be initiated earlier and using RIMpro as a fusi forecasting system for primary inoculum may not be relevant