## 4 Discussion
### Bunch Architecture and Botrytis cinerea Susceptibility  
Bunch architecture plays an important role in controlling bunch rot diseases, and in particular, _B. cinerea_. Several studies report a higher susceptibility of compact clusters to bunch rot [22; 23; 44]. Since berries in compact bunches are densely packed and cuticular membrane thickness decreases, while berry growth is ongoing [45], the bunch rot risk increased as _B. cinerea_ susceptibility was found to be negatively correlated with the impedance of berry cuticle and its waxes [46]. Therefore, a loose bunch architecture is one of the mayor preventive measures in viticulture to reduce _B. cinerea_ susceptibility and hence maintain grape quality. Several viticultural management strategies, including delayed winter pruning [11], late first shoot topping [47], bunch-zone defoliation [48; 49] or application of Gibberellic acid [50] are known to reduce bunch compactness and enhance resistance against bunch rot. In this study, SMPH indicates a lower rachis weight, lower single berry weight and less berries per bunch, resulting in a lower bunch weight compared to VSP, and thereby confirming previous studies of [PERSON] et al. [20]. Moreover, [PERSON] et al. [22] observed a lower bunch density index (i.e., less compact bunches) in SMPH compared to VSP. In this study, a higher ratio of rachis weight to bunch weight was correlated with a lower _B. cinerea_ susceptibility in SMPH training system, and thus, considered as an index for bunch compactness. Relying on this index differences in bunch compactness was not significant between VSP, SMPH and SMPH ST. Whereas, SMPH BT and SMPH GA resulted in less compact bunches. Thinning treatments modified the bunch architecture. The smallest berries were found in SMPH BT. During bunch thinning, larger berries were selectively removed from the bunches and a slower berry growth was observed during the onset of growth stage III (data not shown), leading to a substantial reduction of berry size at harvest. We assume that this might be due to a shock related reduction of growth processes caused by the mechanical forces during bunch thinning using a harvester. SMPH BT increased the rachis to bunch weight-ratio due to smaller and less berries per bunch, and thus, led to a looser bunch architecture. The increased berry size in SMPH GA and SMPH ST can be explained by the yield component compensation principle: SMPH GA had larger berries than SMPH due to reduced berry number per bunch at equal bunch numbers per meter of canopy, which is in accordance with results of [PERSON] et al. [50; 51] and [PERSON] and [PERSON] [34]. SMPH ST, with a significantly reduced bunch number (i.e., shoot), compared to SMPH, had a greater number and larger berries per bunch, confirming the results of [PERSON] et al. [52]. In their study, the magnitude of the berry size compensation effect (16%) was larger than in our study (10%), which might be explained by the fact that the vines in the present trial were not irrigated, or by the generally lower shoot number in their trial. SMPH also affected the kinetics of bunch rot infection. Bunch rot infection occurred earlier in VSP compared to SMPH, possibly due to a slower ripening and improved bunch architecture in SMPH as mentioned earlier. When harvest was conducted in VSP, bunch rot infection was significantly elevated, compared to SMPH. These differences disappeared or were even reversed, when _B. cinerea_ incidence and severity were compared at commercial harvest in 2017 and 2019 (no _B. cinerea_ infections occurred in 2018). This might be related to a longer period between veraison and harvest in all SMPH treatments compared to VSP, in which initial _B. cinerea_ infections had more time and GDD to spread. While, [PERSON] et al. [23] found a higher average humidity in SMPH canopy compared to VSP, we did not detect differences in relative humidity between the non-defoliated bunch zone of VSP and the canopy of SMPH during ripening from 15 August to the harvesting of VSP.