Long preharvest deficit irrigation as a tool to reduce purple spot incidence in ‘Algerie’ loquat

Abstract

• Purple spot is a physiological disorder of loquat that strongly depreciates crop value. • Season and block effects on purple spot incidence were confirmed. • Early and midday oriented fruit were more affected. • Long preharvest deficit irrigation attenuated purple spot incidence. • Calcium and nitrogen fruit content did not differ between spotted and healthy fruit. Loquat is a subtropical pome fruit of high economic value, since it is the earliest spring fruit reaching the markets. Different agricultural techniques may advance loquat fruit ripening and increase therefore its crop value. Unfortunately, a technique of great interest in this regard such as preflowering deficit irrigation (DI) has been found to increase the incidence of a specific loquat physiopathy known as purple spot. In order to clarify the environmental and cultural factors involved in purple spot appearance, we have compared its incidence in three consecutive years, determining the physiological stage and the size of the fruit at which the physiopathy appears, the amount of damaged fruit growing in different sections of the canopy, and compared fruit size and mineral composition of pulp and skin of fruit in damaged and undamaged fruit. We also compared purple spot incidence among five irrigation treatments (full irrigation, preflowering DI, and three different combinations of preflowering plus preharvest DI) in order to select DI treatments that reduce purple spot incidence while maintaining the benefits of harvest advancement. Our results show that purple spot was first detected at color break, when fruit had almost reached its final diameter. Late and earlier purple spot appearance was very rare. Damaged fruits were only slightly larger than healthy fruit. Significant variations in the incidence of purple spot were found among experimental blocks and years, with a low incidence in 2008/09 and 2009/10, and much larger in 2010/11. In all years, early maturing fruit and fruit growing in the canopy sections receiving more sunlight (South oriented) were more damaged by purple spot. In addition, DI treatments advancing fruit ripening provoked a higher incidence of purple spot. In spite of this response, the irrigation treatments in which water stress was also imposed at preharvest reduced the damage suggesting that altering pulp/skin water status and fruit growth rate might reduce purple spot. Finally, potassium, magnesium and especially copper were in much higher concentration in damaged fruit, especially in the skin. Nitrogen and calcium seem not to play any role in this physiopathy despite early assumptions and commonly applied preharvest calcium treatments.