WA 38: understanding green spot origin, timeline, and development
Author: Stefano Musacchi
Published: 2022
Summary: Our preliminary research determined when the onset of green spot (GS) symptoms appears (last ten days of July) and identified a possible physiological reason (damage to lenticels). Fruit protected in bags during June did not develop green spot symptoms, while fruit bagged in July showed an increasing trend of damaged fruit. Bagging fruit late in the season (August) was ineffective in controlling GS symptoms. A difference in the appearance of the symptoms has been noticed with different rootstocks: in both years of the project, G41 showed a higher percentage of fruit affected by GS compared to NIC29. A significant increase in the average humidity level inside the bags was recorded in June-July-August in both years. This percentage increase of humidity in the bag ranged from 6.4% in June 2020 to 3.5% in August 2020 and from 7.8% in June 2021 to 2.3% in August 2021. A high and constant humidity level is a well-known condition that helps reduce lenticel damage. The temperature of fruit in the bag compared to the control was not significantly different in 2020, while in 2021, differences were significant in July due to the heatwave. The fact that bags can protect the fruit from GS onset focuses our attention on the role of lenticels in this disorder. The images taken under the microscope showed damage in the lenticels—further, the tissue surrounding them displayed the typical green discoloration of the peel. Peel and cortex on the sunken GS areas exhibit the presence of chemical compounds related to stress protection. An accumulation of glucose sorbitol, hyperin, and epicatechin in sunken green spot tissue and tissue surrounding sunken green spot have been identified. Also, compounds associated with photoprotection, barrier formation, nitrogen metabolism, and cell wall chemistry have been detected. In 2020 netting did not reduce the incidence of fruit showing GS symptoms, while in 2021, the net seems to reduce the percentage of fruit affected by GS only in combination with G41 rootstock. We know that bags are not a feasible option for growers due to the high cost and availability of labor. Bags are just a research tool to investigate the origin of green spot.
