Growing kiwifruit under cover: challenges and opportunities

Since 2008 Psa ( Pseudomonas syringae pv. actinidiae) has become a threat for kiwifruit plantation in many Countries. A lot of research and trials have been carried out looking for chemicals able to stop the infection, acting directly on the agent of the disease. This study is targeted on the “environment” corner of the disease triangle, with the aim of creating an overall environment unfavorable to Psa development, considering the effect of change not only on the disease, but also on growing parameters. The objective is achievable growing kiwifruit under green house in different structures. Zespri in conjunction with the University of Bologna built up three years of growing experience under covers, so the present work wants to underline the outcomes of it. It has been proved that growing kiwifruit under cover reduces significantly the symptoms of Psa. This is likely linked to lower bacteria’s population, both epiphytic and endophytic. Beside the effect on bacteria’s population, effect on fruit quality was observed. Earlier fruit set was recorded under the cover due to the warmer spring temperature. It also resulted in bigger fruit size at harvest with less blemish. In fact, plastic covers protect the crop from adverse climate events, such as wind and hail. Generally, under cover kiwifruit needs a limited number of chemical treatments, with benefit on soil compaction (fewer sprays and in better soil condition – no wet soil). The controlled climate condition allowed to operate with no weather constrains: this was particularly helpful at pollination time, but also in a Psa safer environment for summer pruning, harvest and so forth, where wounds could get infected. In the other hand, growing kiwifruit under plastic cover showed some constrains. Primary, building a structure that can resist to climate events, like wind storms or hail and snow precipitation is generally highly expensive. In addition, the plastic covers last few years (5-6 years), or at least the light permeability decreases dramatically, requiring to replace it. This implies frequent substitution of the plastic sheet, with extra charge of cost to add to the balance. Data have been collected about change in amount and quality of the light. Although in Italy it is not a big issue during sunny days, it may become a problem when it is cloudy, causing a decrease of the photosynthesis: this can impact dry matter accumulation, resulting in lower fruit quality. Care must be paid to irrigation and nutrition management, since salinity can easily build up in the root area, due to the quality of irrigation water and/or the fertilizer application. In fact, the complete absence of precipitation avoids regular wash off/dilution of the salinity, accumulating in narrow layer compressed by the irrigation volume applied. In conclusion, this paper as result of ongoing monitoring, highlights all the side effects of plastic covers, both positive and negative, on kiwifruit crop