Monitoring of Psa spread and epidemiological data
Due to the influence of different environmental conditions on Psa and bacterial canker, the role of agronomic practices commonly carried out by farmers (such as training systems, pruning, sanitation pruning, irrigation, fertilization) in the incidence and severity of the disease was explored. Moreover whether the presence of the bacterium in the plant could change fruit quality and storage life was investigated.
Influence of fertilization on disease development
Nutritional imbalances may cause stress in the plant and make it more susceptible to pathogens attack. Our findings showed that nitrogen fertilization promotes bacterial growth and disease development, possibly due to the emission of new succulent and more susceptible shoots. Furthermore, high nitrogen fertilization results in denser canopies which may provide favourable microclimatic conditions for Psa epiphytic growth. Finally, the stimulation of shoot growth will lead to the need of more pruning interventions, thus increasing the risk of infection through pruning wounds. The effect of nitrogen on Psa did not depend on its administration mode (root or spray), nor its form (ammonium and/or nitrate). In conclusion, we suggest a reduction in the use of nitrogen fertilizers.
In general, micronutrient deficiency, especially iron, promotes bacterial growth. Iron is a key nutrient for Pseudomonas spp. (Cornelis, 2010). However, iron deficiency also promotes the growth of the bacterium in planta, possibly due to the induction of early senescence, facilitating Psa growth by the inactivation of plant defences and the depletion of nutrient storages.
Influence of irrigation on disease development
We observed that water stress induced a more severe symptomatology. Our research showed a positive correlation between the disease incidence and the use of irrigation water pumped from artificial basins collecting rain water from the infected orchards, thus suggesting a possible role of irrigation water in the local spread of the disease.
Role of pruning in the disease cycle and curative pruning
Pruning cuts are possible entry points for Psa and, in the period when pruning is usually performed (late winter) they need more than a month to cicatrize. Therefore, minimising pruning, treatments to seal the pruning cuts, or the application of protective compounds to reduce Psa population, are recommended after pruning. However, pruning during vegetative dormancy are associated with the lowest disease risk.
The comparison of two common training systems (GDC and pergola) showed similarities in the microclimatic conditions inside the canopy, although the GDC training system allowed slightly higher temperatures and lower humidity compared to the pergola, possibly due to the increased illumination and air flow into the canopy. However, the more widespread Pergola system together with specific pruning interventions to be more suitable to decrease Psa infection risk.
Furthermore, the effectiveness of curative pruning of coppicing was tested. Within 4 months, the bacterium was able to colonize the whole plant both acropetally and basipetally. Therefore a prompt pruning of the infected limbs may prevent the systemic migration of the pathogen. Nonetheless, when symptoms (i.e. cankers) are evident on the cane, the infection has probably spread systemically. The branching nodes and the grafting point do not impede bacterial migration. Therefore, coppicing and re-grafting are not suitable to reconstitute a productive plant.
Role of pollen and pollinators in the spread of Psa
Data shows that pollen has a crucial role in vectoring Psa. Moreover, asymptomatic closed flowers may produce contaminated pollen, suggesting Psa adapted to the internal colonization of male flowers increasing the sexual transmission of the pathogen proving an evolutionary advantage to the bacterium. Therefore, great attention should be paid to commercial pollen production and use of assisted pollination in kiwifruit. New EU policy on pathogen free certification for pollen are envisaged.
Influence of bioregulators on disease development
The use of bioregulators is a widespread practice in A. deliciosa cultivation, while their use is regulated or forbidden in the major A. chinensis club varieties (Zespri and Jintao). Their application helps increase fruit quality parameters, although synthetic auxin imay increase the plant's susceptibility to Psa. In contrast, the synthetic cytokine (Forchlorfenuron), applied up to one month prior inoculation, may induce a moderate resistance.
Screening of the Actinidia genus to select resistant rootstoc
Different species were tested for their susceptibility to Psa in order to select possible candidates for new resistant rootstocks. The A. arguta genotypes were the most resistant, having the mildest symptomatology and the lowest Psa migration inside the tissues. However, further studies are necessary to evaluate A. arguta graft compatibility and effects on scion production.
Effect of Psa infection on fruit production, quality and storability
In order to verify whether fruit quality, as well as crop yield, was affected by the bacterial canker, fruits were separately harvested from healthy and symptomatic plants (specifically from symptomatic canes). The quality parameters and the storage of these fruits were monitored. In general, fruits from symptomatic plants were smaller and with shorter storability. Although not a typical climacteric fruit, kiwifruit produces ethylene in its post-harvest life. Kiwifruit produces ethylene in its post-harvest life (Koukounaras and Sfakiotakis, 2007), and a higher ethylene production was recorded by fruits from the symptomatic plants, in particular by those from the most infected parts, compared to fruits from asymptomatic plants. The higher ethylene production may induce a faster ripening of fruits from infected plants during post-harvest with consequent storage disorders.
University of Bologna