The Effect of Weather and Location of Fruit within the Tree on the Incidence and Severity of Citrus Black Spot on Fruit

Katherine E Hendricks,Mary C Christman,Pamela D Roberts

doi:10.1038/s41598-020-58188-z

Abstract

Citrus black spot (CBS) caused by the fungus Phyllosticta citricarpa occurs in tropical and sub-tropical citrus production regions and affects all varieties of citrus. In Florida, the disease cycle is unique, having only the asexual spore. This work examines incidence and severity of CBS (hard spot symptoms) on fruit in two citrus groves during 2013–2014, 2014–2015 (Grove III) and 2015–2016 (Grove II) citrus seasons. Disease incidence and severity on fruit were analyzed based on citrus season, side of tree evaluated, height within the canopy, tree health, and tree age. Results indicate an increase in CBS incidence in Grove III between 2013–2014 and 2014–2015 seasons, with more infected or symptomatic fruit on the road side of the canopy and a higher incidence above 2 meters. Tree health status affected incidence but not severity and tree age had a significant effect on severity of CBS in Grove II. Analysis of weather data conducive for infection, between 2010 and 2017, indicated an average of 172 days per year (range: 104–261 days) when the temperature (15–35 °C) and relative humidity (RH ≥ 90% for 8 consecutive hours) were conducive for infection of fruit and an average of 98 days per year (range: 72–123 days) when the fruit were susceptible to infection.

Highlights

Citrus black spot caused by the fungus Phyllosticta citricarpa was first discovered on sweet orange in southwest Florida in April 20101
Since the buildup of inoculum in the grove is an important factor to consider, we looked at the weather conditions conducive for conidial infection across the entire year, defining IN6 to IN10 as weather conditions conducive to infection of citrus tissue other than the fruit
Overall there were more days throughout the years when conditions were conducive for the infection of citrus tissue defined as days with temperatures between 15 and 30 °C and relative humidity (RH) ≥ 90% in all years examined, especially in 2013 where 261 days out of the year were conducive to tissue infection based on this definition (Table 1)

Summary

Introduction

Citrus black spot caused by the fungus Phyllosticta citricarpa was first discovered on sweet orange in southwest Florida in April 20101. Florida’s citrus affected by this pathogen are placed under quarantine by the United States Department of Agriculture, restricting interstate movement of citrus materials. Reduced viability compared to the ascospore, recent work suggest that water and wind may disperse conidia over longer distances than previously reported[16]. Both the sexual and asexual spores of CBS are present on citrus[17] with the exception of Florida. The situation in Florida gives a unique opportunity to study the incidence and severity of CBS hard spot lesions in a system lacking ascospores. This work examines the incidence and severity of CBS on fruit spatially distributed within the tree canopy over a 3-year period (2013/14 to 2015/16) in one grove with 100% tree incidence of CBS at the beginning of the study and for a single citrus season (2015/16) in a second grove with 26.6% tree incidence

Methods

Results

Conclusion