Biotic and Abiotic Factors Causing Rind Blemishes in Citrus and Management Strategies to Improve the Cosmetic Quality of Fruits

Abstract

Citrus is the major tree fruit crop grown and traded worldwide. Citrus industry around the globe has been facing deterioration in fruit cosmetic quality and increased farmgate rejections, mainly due to high incidence of rind blemishes. Rind blemishes are caused by various biotic (diseases and insects) and abiotic (environmental, physical and physiological) factors at various stages of fruit development, however initial 8–12 weeks of fruit setting are the most critical. While the causes and intensity of blemishes varies with agroecological conditions and citrus species, and fruits position in canopy, the blemishes due to melanose, scab, canker, thrips, mites, scales, and wind are generally the most common. Being a complex issue involving multiple factors in the field (environment, pathogens, tree, fruit, cultural practices), its control has always been very challenging. R&D progress overtime showed a great deal of work done on the subject, however for commercial success, an integrated approach is essential to reduce rind blemishes and improve fruits cosmetic quality. The key interventions include maintaining tree vigor and hygiene with judicial pruning, selective and timely application of pesticides at critical stages, particularly during initial 12 weeks of fruit development. Keeping in view the increasing concerns regarding food safety, the application of horticultural mineral oils (HMOs) and effective biological tools need to be integrated. To some degree, harvest and postharvest supply chain operations may also contribute towards some fruit blemishes (oleocellosis, rind/stem end breakdown, chilling injury, etc.) and are to be managed appropriately. While various advance technologies i.e., near-infrared (NIR), ultraviolet (UV), ultraviolet fluorescence (UVF), laser backscattering imaging (LBI) and hyperspectral imaging (HI) have been tested /developed for blemish-based fruit sorting, their high cost is prohibitive in adaptation particularly in developing countries. Future research needs to focus on assessing the impact of climate change on dynamics of biotic factors, blemish free fruit production under small tree-framework system, development of new chemistry low residue pesticides, reducing cost of high-tech sorting machines and consumer education to have acceptance of blemished fruit (still with good internal quality) to some degree.