NC State Extension Publications

Introduction

Glomerella leaf spot and fruit rot is a devastating fungal disease of apple that results in premature defoliation and diminished fruit quality or unmarketable fruit. Additionally the disease may cause decreased winter hardiness and reduced crop return. While Glomerella leaf spot and fruit rot must be managed annually in North Carolina, other apple growing regions in the eastern United States have reported occassional outbreaks during hot and humid growing seasons.

Pathogen

Glomerella leaf spot and fruit rot is caused by fungi in the Colletotrichum gloeosporioides species complex.

Colletotrichum spores

Conidia of Colletototrichum gloeosporioides.

Sara Villani, NC State University

Symptoms and Signs

Symptoms of the leaf spot stage of the disease begin as tiny purple flecks or specks on the front of the leaf. In the absence of fungicide intervention, the purple flecks rapidly expand into irregularly shaped, often concentric, lesions. Later in the infection process, leaves become chlorotic (yellow) and drop prematurely prior to harvest. Most commonly, early symptoms of Glomerella leaf spot will appear within the lower inner tree canopy, however, initial disease symtpoms have been observed on terminal leaves and high in the canopy. Glomerella fruit rot symptoms initially appear as small sunken lesions on either immature or mature fruit and are slightly larger than the size of a lenticel. The lesion may expand prior to or post-harvest and can evolve into a full rot. On yellow skinned cultivars at harvest, a red halo can be observed surrounding the rot lesion. Unlike more typical bitter rot symptoms, salmon colored sporulation is rarely to never observed with Glomerella fruit rot.

Glomerella leaf spot symptoms 1

Purple flecks and irregular lesions of Glomerella leaf spot.

Sara Villani, NC State University

Glomerella Chlorosis

Leaf chlorosis during advanced Glomerella leaf spot.

Sara Villani, NC State University

Glomerella Defoliation

Premature leaf drop resulting from Glomerella leaf spot.

Sara Villani, NC State University

Glomerella sunken lesions

Sunken lesions on immature 'Gala' fruit.

Sara Villani, NC State University

Glomerella fruit rot gala

Glomerella fruit rot on mature 'Gala' fruit.

Sara Villani, NC State University

Red halo Glomerella

Red halo symptom of Glomerella fruit rot on mature 'Golden Delicious.'

Sara Villani, NC State University

Environmental Conditions Favoring Disease

Hot and humid conditions favor infection by the Collectotrichum fungi causing Glomerella leaf spot and fruit and disease development. Infection by conidia occurrs between 59°F and 95°F, with an optimal temperature of 82°F. A minimum 2.76 hours of leaf wetness are required for infection to occur.

Sources of Inoculum

Research regarding the sources of inoculum for Glomerella leaf spot and fruit rot is currently being conducted. The main source of primary inoculum for the disease appears to be infected leaves overwintering on the orchard floor. Ascospores are released from perithecia sometime around the blom/petal fall growth stage. The development of perithecia on infected/fallen leaves has not been observed in North Carolina during the growing season. Other sources of Glomerella leaf spot and fruit rot inoculum that are currently being investigated include cankers, buds, mummified fruit, and other plant hosts.

Glomerella Perithecia

Perithecia developing on a leaf lesion, December 2016.

Sara Villani, NCSU

Management

Once the disease is observed in the orchard, Glomerella leaf spot and fruit rot can be difficult to control. Management, therefore should be focused on prevention and consist of a combination of cultural and chemical interventions.

Cultural

  • Pruning and removal of shoots inected with fire blight, dead wood, wood with cankers, and fruit mummies
  • Post-harvest application of urea, flail mowing, and/or leaf removal to reduce overwintering inoculum
  • Increase air-flow through the canopy to reduce trying time and decrease humidity within the canopy
  • Plant less susceptible/resistant cultivars. Cultivars with 'Golden Delicious' parents tend to be most susceptible to Glomerella leaf spot and fruit rot.

Fungicides

In North Carolina, fungicide programs to manage Glomerella leaf spot and fruit rot should begin at petal fall and continue on 7 to 14 day intervals through harvest. It is suggested that single-site fungicides be applied in tank mixture with a multi-site protectant for resistance management. The table below lists active ingredients/products that have demonstrated high levels of efficacy in field trials at NCSU. A more comprehensive list of registered fungicides for Glomerella leaf spot and fruit rot management may be found in the "Integrated Orchard Management Guide for Commercial Apples in the Southeast". Please note that fungicide labels are legal documents-always read and follow label directions.


Example Commerical Fungicides for Glomerella Leaf Spot and Fruit Rot
Active ingredient Example Formulated Product Pre-harvest inteveral (days) FRAC Code
Mancozeb Koverall 77 (3 lb/A rate only) M3
Captan Captan 80WDG 0 M4
Trifloxystrobin Flint 50WG 14 11
Trifloxystrobin + Fluopyram Luna Sensation 14 11 + 7
Pyraclostrobin + Fluxapyroxad Merivon 0 11 + 7
Pyraclostrobin + Boscalid Pristine WG 0 11 + 7
Potassium Phosphite Prophyt 0 33
Ziram Ziram 76DF 14 M3
Fluazinam Omega 500F 28 29

Author:

Extension Specialist (Apple and Ornamental Plant Pathology)
Entomology and Plant Pathology

Publication date: June 5, 2017

Recommendations for the use of agricultural chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by North Carolina Cooperative Extension nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your county Cooperative Extension agent.

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