The potential of combined rapid hot water treatment and yeast biocontrol for suppressing postharvest avocado anthracnose and stem-end rot diseases.

Abstract

Avocado (Persea americana Mill.) is a highly nutritious fruit, rich in vitamins, minerals, and antioxidants. In South Africa, the avocado industry experiences combined losses of about 50% due to anthracnose and stem-end rot. The fungi most commonly associated with these diseases are Colletotrichum gloeosporioides (Penz.) Penz. & Sacc and Lasiodiplodia theobromae (Pat.) Griffon & Maubl respectively. Acceptable control levels of these diseases have been achieved by postharvest treatments with prochloraz. However, a significant reduction of the maximum residue levels by the European Union has precluded the use of this fungicide from 2020. Therefore, this study aimed to develop an alternative treatment regime to control the primary postharvest diseases of avocado. The aim of the study was to optimize a rapid hot water treatment (RHWT) and to discover an effective yeast biological control followed by the integration of these two treatments. Temperatures tested for the RHWT ranged from 20 to 80°C, combined with exposure periods ranging from 10, to 180 seconds. These were applied to “Hass”, “Fuerte”, and “Pinkerton” fruit. Levels of disease occurrence were reduced when temperatures between 52°C and 58°C were combined with exposure times of 10 to 30 seconds, which also caused no heat damage of fruit. Overall, the best treatment was a temperature and time combination of 56°C for 10 seconds. More than 100 yeast isolates were isolated and screened against the two primary pathogens. Three yeasts performed exceptionally well, including a commercial yeast variety known as B13. The combination of RHWT and all four yeasts provided a level of control comparable with that provided by the fungicide prochloraz. Excellent and consistent control was achieved from the integration of yeast strain B13 and rapid hot water treatment of 56°C for 10 seconds.