Effect of calcium and boron nutrition on grey mould of capsicum (Capsicum annuum L.) and fruit quality.

Capsicum (Capsicum annuum L.) is mostly cultivated in humid and warm conditions, which increases disease development, particularly grey mould caused by Botrytis cinerea. Infection of capsicum fruit by B. cinerea often occurs preharvest but symptoms of grey mould are not usually visible until after harvest making the pathogen difficult to control. Appropriate fertilisation that ensures calcium (Ca) and boron (B) is sufficient in plant tissues, especially in fruit, has been suggested as an alternative to fungicides for disease management. This research studied the infection pathway of B. cinerea and the effect of Ca and B on grey mould development and quality of fruit in two capsicum cultivars (cv. Aries and cv. Papri Queen). Botrytis cinerea infected capsicum preharvest and flowers often died when inoculated at anthesis. The number of dead flowers increased when inoculum concentration increased. The extent of grey mould development on fruit inoculated preharvest was not affected by timing of inoculation [at anthesis, 3 days after anthesis (DAA) or 6 DAA], but was dependent on inoculum concentration and cultivar. When capsicum fruit were inoculated after harvest, grey mould developed most rapidly in red (R) fruit from cv. Aries and breaker red (BR) fruit from cv. Papri Queen. An inoculation of 10⁶ conidia mL¯¹ caused more disease on fruit than 10⁴ or 10⁵ conidia mL¯¹. Cv. Aries was more susceptible to B. cinerea than cv. Papri Queen regardless of whether inoculation occurred before or after harvest. The effect of both soil and foliar application of boron (B), at different concentrations, on grey mould development and fruit quality of capsicum was examined. Preharvest B application, from transplanting to harvest when fruit were mature and red, using 0.05 or 0.1 mM H₃BO₃ via soil amendment or 2.0 or 7.0 mM H₃BO₃ as a foliar spray increased B concentration in leaves and fruit of both cultivars. However, soil application was more effective than foliar application in increasing B concentration in plant tissues. Foliar application of B at low concentrations (0.025 or 0.075 mM H₃BO₃) did not increase B concentration in plant tissue. Increasing B concentration in leaf and fruit tissue reduced grey mould development on fruit inoculated with B. cinerea preharvest compared to the control, but did not affect grey mould development on red fruit inoculated with B. cinerea postharvest. Preharvest soil application of B increased shelf life of fruit, but did not affect quality of fruit including water content, firmness, total soluble solid content (TSSC) and titratable acidity (TA) at harvest or during storage. Symptoms of B toxicity were observed on leaves from plants that received high B concentration (0.1 mM H₃BO₃) in the soil, but no effect was observed on fruit. Preharvest application of calcium (Ca) via soil amendment [1.5, 4.0 or 8.0 mM Ca(NO₃)₂] or as a foliar spray [0.5 or 1.0 % w/v mM Ca(NO₃)₂] increased Ca concentration in leaves, but did not increase Ca concentration in fruit, regardless of cultivar. Soil Ca application appeared to increase Ca concentration in leaf tissue more effectively than the Ca foliar spray. Ca concentration in leaf tissue from cv. Aries was significantly higher than in leaf tissue from cv. Papri Queen when plants received the same amount of Ca, regardless of application method. Ca treatment did not affect quality of fruit at harvest or during storage. Preharvest application of Ca reduced grey mould development on fruit that had been inoculated with B. cinerea preharvest, but did not reduce grey mould in fruit inoculated postharvest. Symptoms of Ca deficiency were observed on plants that received no Ca or low Ca concentration [1.5 mM Ca(NO₃)₂] from transplant to fruiting. Dipping and vacuum infiltration with calcium chloride (CaCl₂.2H₂O) did not increase Ca concentration in flesh after treatment, but vacuum infiltration did increase Ca concentration in flesh after 10 days of cool storage (10°C). Ca treatment after harvest did reduce grey mould development on fruit, but did not affect the quality of fruit during storage. A directly inhibitory effect of Ca on fungal growth was responsible for reducing grey mould development on fruit. In conclusion, capsicum was most sensitive to infection by B. cinerea at anthesis and high inoculum concentrations caused a greater disease incidence in capsicum fruit, regardless of whether inoculation occurred preharvest or after harvest. Reducing inoculum concentration, especially during flowering, is therefore recommended to reduce losses in capsicum. Preharvest application of Ca or B may be used as an alternative method to reduce grey mould on capsicum fruit, but they had no effect on fruit quality. Postharvest application of Ca could also be recommended for cv. Aries fruit before or during storage for controlling grey mould on fruit. Findings in this research may therefore provide basic knowledge for management of B. cinerea in the capsicum industry.Thesis (Ph.D.) -- University of Adelaide, School of Agriculture, Food and Wine, 201