Investigating the Cause of leaf blotch and fruit spot of apple in San Joaquin County California

Mohamed Nouri1, Karina Elfar 2, and Akif Eskalen2

1University of California Cooperative Extension San Joaquin County, Stockton, CA 95206, U.S.A.

2University of California, Davis, Department of Plant Pathology, Davis, CA 95616, USA.

Background

Symptoms of leaf blotch and fruit spot were recently observed (late summer 2020) in cv. Pink Lady apples in one orchard in Linden area of San Joaquin County, especially on leaves (Fig. 1). Microscopic examinations, molecular analyses and pathogenicity tests confirmed the causal agent as Alternaria alternata, A. arborescens, and A. tenuissima. Alternaria leaf blotch and fruit spot are significant threats to apple production in many parts of the world, and can be serious mid-late season fungal diseases affecting apple leaves and fruit respectively, in high spring/summer rainfall production areas.

In California, dry summer reduce the chance of Alternaria infection on apple orchards, but the late dormant season rainfalls and also overhead irrigation may increase the risk of infection. This pathogen requires warm temperatures and leaf wetness to infect apple trees – optimum condition could be created by the overhead water-based orchard cooling systems used on very hot days.

Symptoms:

Symptoms were mainly observed on leaves and were characterized by the presence of circular brown necrotic lesions, which enlarged in zonate circular or crescent-shaped rings, often with a dark brown margins (Fig. 2A & B). These symptoms called leaf blotch, followed by a yellowing of the affected leaves (chlorosis) and eventually defoliation (Fig. 2B & C). Older lesions often develop a black ring of spores (Fig. 2D). On fruits, round, dark-colored, dry, corky lesions were occasionally observed (Fig. 3).

Field observations indicate that symptoms initially appear during the summer and continue to express until the early fall. Typically, the leaves are infected in mid- to -late June or July, with the symptoms occurring several weeks later.

Comments on the Disease:

At present, it is unclear how wide-spread the disease is in California apple orchards, but surveys are being conducted to assess the prevalence of this disease in orchards. It is advisable to be vigilant throughout the growing season and monitor apple trees for the presence of the disease. Orchards should be scouted for signs of the disease in July through October. Due to the conducive/dry weather for mites, apple trees can contain a high population of mites without dropping leaves. Alternaria leaf blotch severity may be affected by severe mite infestation. Mites should be maintained at or below the established IPM thresholds. Ongoing research lead by Dr. Akif Eskalen from UC Davis is focused on further understanding the biology of these Alternaria pathogens as well as factors influencing disease expression in order to develop management strategies against this emerging disease.

Acknowledgments

We thank Michael Devencenzi, local pest control advisor, for bringing the disease symptoms to the attention of the authors.

Fig. 1. Alternaria leaf infections of cv. Pink Lady apples

Fig. 2. Alternaria leaf blotch symptoms of cv. Pink Lady apples

Fig. 3. Alternaria fruit spot symptoms on the surface of fruit of cv. Pink Lady