Field and postharvest biology of dendritic spot and stem end rot of mango

Abstract

Stem end rot is a well-established postharvest disease of mango caused by the fungi Neofusicoccum parvum (Pennycook & Samuels) Crous, Slippers and A.J.L. Phillips and Lasiodiplodia theobromae (Pat.) Griff & Maubl. Dendritic spot is a newly emerging disease of mango. The pathogenic fungi which cause anthracnose and stem end rot such as Colletotrichum gloeosporioides (Penz.) Penz & Sacc. in Penz and N. parvum respectively also cause dendritic spot of mango. However, a number of other fungi such as Fusicoccum sp., Alternaria sp. and Lasiodiplodia theobromae have also been isolated from dendritic spot symptoms from time to time. Very little work has been reported on dendritic spot as the symptoms of this disease are easily confused with other skin blemishes of mango such as tear stain. This study investigates the aetiology of dendritic spot. Fruit bagging experiments were carried out on Kensington Pride (KP) and R2E2 cultivars of mango to investigate the time of infection of dendritic spot and stem end rot. The fruit were bagged at four different growth stages. The results showed that fruit bagged at golf ball size which remained bagged until harvest had low incidence of dendritic spot and stem end rot. Higher incidence was observed in the fruit which were bagged close to harvest and the un-bagged control fruit. It was concluded from the results that pathogens invade the fruit at an early growth stage and fruit protection at this stage of growth can help in minimizing postharvest disease incidence. The effect of conventional and improved orchard practices on the management of anthracnose and stem end rot was also studied. The experiment was carried out in Pakistan. Three districts of Punjab province of Pakistan were selected for the study. The fruit of Chaunsa cultivar of mango were collected from growers’ conventional practice blocks and ASLP (Agriculture Sector Linkages Program) improved practice blocks. In the conventional practice blocks, there was irregular application of foliar and pesticides sprays, non-consistent use of fertilizers with a minimum use of cultural practices such as irrigation, pruning, weeding and soil nutrition analysis, use of recommended foliar sprays of fungicides for disease management at different fruit growth stages, pest management and nutrient management by annual application of NPK. Whereas, the improved practice blocks were maintained by cultural practices. A low incidence of anthracnose and stem end rot was observed in the fruit which was collected from the ASLP improved practice blocks as compared to the incidence observed in the fruit collected from the conventional practices blocks. Postharvest application of exogenous ethylene on mature fruit to induce uniform skin colour and ripening and hot fungicide dips to minimize postharvest disease incidence are common practices used by Australian mango growers. Early fruit ripening facilitated by exogenous ethylene is helpful in early fruit marketability, but it can result in higher disease incidence in case of prolonged fruit storage or higher fruit storage temperature. The effect of exogenous ethylene and hot fungicide dip with fludioxonil was investigated when both treatments were applied on the fruit alone and in combination with each other. The results showed that the application of exogenous ethylene alone caused higher incidence of dendritic spot and stem end rot when the fruit received frequent rainfall during the growing season. This higher incidence of both dendritic spot and stem end rot could also be because of accelerated fruit ripening and decline in fruit resistance. But, when low or no rainfall was received during the season, a low incidence of dendritic spot and stem end rot was observed when the exogenous ethylene was applied on the mature fruit. Fruit hot-dipped in the fungicide fludioxonil followed by the exogenous ethylene had zero to minimum incidence of both dendritic spot and stem end rot. Market surveys were carried out to determine the geographical distribution of the dendritic spot pathogens and to collect the dendritic spot and stem end rot pathogens. Disease pathogens were isolated from fruit from Northern Territory, North and Southeast Queensland. All isolates of Colletotrichum collected from anthracnose were identified morphologically and phylogenetically as C. gloeosporioides but the isolates which were collected from the dendritic spot symptoms were identified by Internal Transcribed Spacer (ITS) and Trans elongation Factor (TEF) 1- α as, C. gloeosporioides, C. siamense, and C. asianum. All the Neofusicoccum isolates collected from dendritic spot and stem end rot symptoms were identified as N. parvum and N. occulatum by using the ITS and TEF gene regions. One isolate of stem end rot was identified as Botryosphaeria dothidea. To determine the role of C. gloeosporioides, N. parvum, L. theobromae and Alternaria sp. in causing dendritic spot in mango, the isolates of these fungi were assessed for pathogenicity. This work identified C. siamense, C. asianum, N. parvum and N. occulatum as the most virulent pathogens causing dendritic spot.