Characterization and management of fungal pathogens associated with postharvest fruit rot of Guava (Psidium guajava L.)

Abstract

Guava (Psidium guajava L.) is one of the important commercial fruits that belongs to family Myrtaceae and is also known as “The Poor man’s Apple” or “Apple of Tropics”. It is native to tropical America and is now widespread in many tropical and subtropical countries. Being a delicious and nutritious fruit, guava is consumed directly or processed into several products such as jam, jelly, cheese, RTS beverages, etc. But, the incidence of postharvest diseases impairs the fruit quality, colour, taste and shelf life. Among the various postharvest diseases affecting the fruit, latent infections are presumed to be the origin of many rots that develop on maturing fruits. Hence this study was envisaged to document the postharvest fruit rot diseases caused by latent fungal infections, characterize the pathogens causing these diseases, and to exploit the antimicrobial compounds for developing management strategies for the major pathogen inciting the diseases. Purposive random sampling surveys were conducted in markets and homesteads of Thrissur (AEU 10), Palakkad (AEU 10 and AEU 23), and Ernakulam (AEU 9 and AEU 12) districts of Kerala. From the 19 different surveyed locations, 55 fruit samples were collected and brought to the laboratory. They were then surface-sterilized, and incubated at room temperature for disease development. A total of 36 fruit samples produced symptoms within three to twelve days of incubation including stylar end rot, soft rot, dry fruit rot, and brown to dark brown sunken lesions. The per cent disease incidence (PDI) of various diseases ranged from 16.6 to 75 per cent in the fruits sampled from various locations. The major pathogens isolated from diseased fruit samples were Lasiodiplodia and Colletotrichum. Lasiodiplodia sp. (13) was isolated from fruits exhibited stylar end rot, soft rot and dry rot. The PDS of Lasiodiplodia fruit rot varied between 13.3 to 50 per cent and the highest PDS (50 %) was recorded for fruits collected from Perumbavoor/ AEU 9, Chalakudy/ AEU 10, and Nellimolam/ AEU 12. Colletotrichum sp. (12) was isolated from brown to dark brown sunken lesions. The PDS of anthracnose ranged from 10 to 40 per cent with the highest PDS (40 %) was recorded from Kodakara/ AEU 10. Neofusicoccum sp. (2) was isolated from dry fruit rot developed near stylar end of the fruit, in the fruits sampled from Mannuthy/ AEU 10 and Kottappady/ AEU 12. Alternaria sp. (4) and Pestalotiopsis sp. (2) were isolated from brown lesions and rot developed on fruits sampled from Thrissur district. The lowest PDS (3.3 %) was recorded for fruit rot caused by Pestalotiopsis sp. Molecular characterization of six selected pathogen isolates was attempted by sequencing and analyzing the ITS or beta tubulin sequences. Based on this, the fungal pathogens associated with postharvest fruit rot were identified as Lasiodiplodia theobromae (TSG 2), Colletotrichum gloeosporioides (TKG 2), Alternaria alternata (TSG 1), Pseudopestalotiopsis camelliae sinensis (TMG 2), Neofusicoccum parvum (TMG 5), and Diaporthe phoenicicola (TPG 1). The in vitro efficacy of antimicrobial compounds viz., chitosan, hydrogen peroxide (H₂O₂), and methyl jasmonate (MeJA) was studied against selected pathogens L. theobromae (TSG 2), N. parvum (TMG 5), and C. gloeosporioides (TKG 2) using the poisoned food technique. The results showed that chitosan (2 %), H₂O₂ (2 %), and MeJA (0.1 %) consistently exhibited 100 % inhibition of all the selected pathogens. The standardization of antimicrobial treatments for managing fruit rot caused by L. theobromae (TSG 2) involved treatment of pathogen inoculated fruits with chitosan (2 %), H₂O₂ (2 %), and MeJA (0.1 %) for one min or two min duration. All the treatments reduced disease development. The treatment of fruits with chitosan (2 %) for two min was the most effective, showing slower lesion development, reduced physiological loss in weight (PLW) (2 %), and reduced respiration rates (73 ml CO2 kg-1 h-1). The subsequent effective antimicrobial compound was MeJA (0.1 %). MeJA treatment for two min, showed reduced lesion development, PLW (2.65 %) and respiration rates (101 ml CO2 kg-1 h-1). Hot water fruit dip treatment was also standardized. The treatment of fruits at 47 °C for 20 min emerged as the most promising treatment for managing fruit rot. It provided effective disease control, minimized PLW (2.38 %), and reduced respiration rate (95 ml CO2 kg-1 h-1) without causing significant browning. Evaluation of the efficacy of combined treatments of hot water and antimicrobial compounds was carried out in L. theobromae (TSG 2) inoculated fruits to select the best promising treatment. Hot water treatment at 47 °C for 20 min followed by treatment with chitosan (2 %) or MeJA (0.1 %) for 2 min completely inhibited pathogen growth, reduced PLW (2.36 % and 2.43 % respectively) and respiration rates (65 and 93 ml CO2 kg-1 h-1 respectively) of fruits, thus demonstrated their superiority to individual treatments.