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Apple fruit spot disease has caused serious economic losses for years in China since the widespread application of fruit bagging in production. Although the three genera Trichothecium , Alternaria , and Acremonium have been reported to be the causal agents, studies on the disease etiology and pathogen biology are still sparse. Here, we report characterization of eight fungal isolates from lesions on 126 symptomatic fruit samples collected in Shaanxi Province, China. Pathogenicity of the isolates was assessed. DNA sequences were obtained at four loci, including D1/D2 domains of the large-subunit nrRNA gene, internal transcribed spacer regions 1 and 2, 5.8S nrDAN gene, a fragment of the actin gene, and a fragment of the β-tubulin. Based on phylogenetic analysis and morphological features, three new species were found: Acremonium mali , Sarocladium liquanensis , and Sarocladium mali . In addition, we made the first report of Sarocladium terricola as a plant pathogen. Temperature and moisture significantly affected in vitro conidial germination of five Acremonium -like species, and their impact on infection of apple fruit was tested using Acremonium sclerotigenum . Conidia of five species germinated from 15 to 35°C in free water; four of the species had optimum temperature around 25°C, whereas conidia of S. terricola had an optimum temperature of 30°C. Conidial germination rate increased as relative humidity (RH) increased. The five isolates had relatively high conidial germination rates at RH > 97%, with a significant decline at 95% RH. Incidence of infection also increased in proportion to RH. In free water, conidial germination was relatively unaffected by temperature.

Fruit bagging is a widely used orchard practice in China. Trichothecium black spot (TBS) is a disease highly associated with the fruit bagging. In this study, we characterized in vitro factors affecting the causal agent, Trichothecium roseum, and TBS development and infection histology on field-bagged apple fruit in situ. Under in vitro conditions, conidial germination required exogenous nutrients, and the germination rate was significantly promoted by high humidity, a condition mimicking the bag microenvironment. Germ tubes penetrated fruit via natural openings including stomata, lenticels, and surface cracks. To determine the chronology of infection by T. roseum, 'Fuji' fruit were inoculated in the field at different developmental stages. The earliest infection occurred 60 days after full bloom (dafb), and disease incidence increased as fruit maturity advanced. At harvest time (165 dafb), lesions on more recently inoculated fruit (105 dafb, 150 dafb) were larger than lesions from fruit inoculated on earlier dates. Histological observation showed that infection of younger fruit elicited stronger host lignification responses restricting lesion development. Taken together, our results support the hypothesis that high humidity in sealed bags and increased susceptibility associated with advancing fruit maturity are key factors promoting T. roseum infection and TBS symptom development on bagged apple fruit.

Alternaria spp. are pathogens of several diseases that pose significant threats to apple production. Several putative Alternaria sp. isolates were obtained from lesions of a disease commonly referred to as black dot on apple fruit in Shaanxi Province, China. Pathogenicity tests using mycelial plugs and conidial suspensions indicated that this isolate could cause leaf blotch, as well as moldy core and black dot on fruit. On the basis of sequence analysis of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), RNA polymerase second largest subunit, and translation elongation factor 1-α, an isolate clustered with the Alternaria sect. Ulocladioides. By combining GAPDH, major allergen Alta1, mating type protein 1-2-1, and the AGA1 gene sequence analysis and morphological description, the isolates were identified as a new species named Alternaria malicola. Our finding expands the documented diversity of apple pathogens within the genus Alternaria and clarifies the taxonomy of the pathogen assemblage that may be associated with three apple diseases.

Sooty blotch and flyspeck (SBFS) is a complex of about 80 fungal species that blemish the surface of apple fruit in humid regions worldwide. The dark colonies become visible in mid- to late summer, reducing the value of fresh fruit. Although many SBFS species can co-occur in the same orchard and even on the same apple, little is known about temporal patterns of these species, including the timing of colony appearance. To test the hypothesis that colonies of SBFS species appear on apples at characteristic times during the growing season, 50 apples were monitored weekly at three Iowa orchards in 2006 and six orchards in 2007 and 2008. However, a mean of 24.3 apples per orchard was assessed at harvest because of apple drop throughout the season. Colonies were marked with colored pens as they appeared. After harvest and after storage of apples at 2 °C for 3 months, SBFS colonies on each fruit were counted and classified by morphology, and a representative subset of colonies was excised from the fruit and preserved on dried peels for species identification using rDNA. Seventeen species were identified. Stomiopeltis spp. RS1 and RS2 appeared on apples 10 to 14 days before other SBFS taxa. Dissoconium aciculare was generally the last species to appear on apple fruit, and it continued to appear during postharvest storage. The most prevalent taxa in Iowa orchards were also the most abundant. Diversity of SBFS fungi in an orchard was positively correlated with cumulative hours of surface wetness hours due to rainfall or dew, which is believed to favor growth of SBFS fungi. Species-specific information about temporal patterns of appearance on apple fruit may lead to improved SBFS management strategies.