Pseudocercospora fuligena (black leaf mould)

The initial symptoms of black leaf mould appear as minute, pale-yellow discolorations to light-green lesions, 1-20 mm diameter. Older lesions may have brown centres surrounded by zones of pale green to yellow (Subedi et al., 2015; Lookabaugh et al., 2018). Lesions on the lower leaf surface are initially covered with white mycelium that turns grey to black as the fungus sporulates (Yamada, 1951; Jain, 1955; Mohanty and Mohanty, 1955; Magda and Quebral, 1970; Sherf and MacNab, 1986; Hartman et al., 1991; Wang, 1996; Lookabaugh et al., 2018). Lesions have no definite margins on either the upper or lower leaf surfaces (Blazquez and Alfieri, 1974). Black, sooty fructification is produced on both surfaces. A large number of such areas appear on each leaf and may coalesce to form large patches (Jain, 1955). Infected leaves may become twisted, wilted, dried with age, brittle and generally remain hanging on the plant with a soot-covered appearance; but from time to time defoliation may also observed (Saroj et al., 2014; Lookabaugh et al., 2018). Infections also occur on petioles, stems and fruit peduncles (Mohanty and Mohanty, 1955; Sherf and MacNab, 1986; Wang, 1996) but there are no reports of symptoms occurring on fruit.

The black leaf mould pathogen is airborne and routine leaf removal may prevent spread to healthy plants. 

Resistant Varieties

Yamada (1951) found two resistant varieties, Dwarf Stone and Marglobe, among 35 varieties in the field.

Blazquez and Alfieri (1974) reported that the cultivars Walter and Florida MH-1 were equally susceptible to the disease, while Homestead was mildly susceptible and Floradel appeared to be resistant.

Inheritance of black leaf mould resistance was studied in four crosses involving two resistant Solanum accessions (PI 134417, S. habrochaites; PI 254655, S. lycopersicum) and four susceptible AVRDC tomato lines (CLN 657BC1F2-267-0-3-12-7, CL 143-0-10-3-0-1-10, CLN 698BC1F2-358-4-13, and CL 5915-93D4-1-0-3). The results indicate that resistance to black leaf mould may be conditioned by two recessive genes acting epistatically in both PI 134417 and PI 254655 (Wang et al., 1995a, b).

Among 46 Solanum accessions representing 10 species that were evaluated for reactions to this disease, accessions of S. lycopersicum were the most susceptible and accessions of S. cheesmaniae, S. chmielewskii, S. corneliomuelleri, S. habrochaites, S. pennellii, S. neorickii, S. peruvianum and S. pimpinellifolium had resistant reactions. The accessions of S. habrochaites were the most resistant (Wang, 1996).

Zahn et al. (2011) reported that three lines (the black leaf spot-resistant donor Solanum habrochaites, and the black leaf spot-susceptible recurrent S. lycopersicum parent) without visual symptoms and very low amounts of P. fuligena-specific rDNA shared an overlapping introgression on chromosome one that may be associated with black leaf mould resistance and the identification of an introgression conferring P. fuligena resistance represents a potential opportunity to fine map the relevant interval, which could allow marker-assisted selection for black leaf mould resistance breeding.

Black leaf mould occurs in many South-East Asian countries and it has been reported to be important in Japan (Yamada, 1951). It has not yet been shown to be economically damaging on host crops over a wide geographic region, possibly because its importance is not yet fully known (Hartman and Wang, 1992).

It was reported in 1974 that black leaf mould may become an economically important disease in Florida, USA, as an increasing number of growers were planting the susceptible cultivars Walter and Florida MH-1 (Blazquez and Alfieri, 1974).

Hartman et al. (1991) observed 54-86% leaf area infected on several varieties and advanced breeding lines in replicated yield trials at AVRDC (The World Vegetable Center). The authors suggested that this disease may be more economically important than has been previously reported.

Hartman and Wang (1992) reported that yield losses attributable to black leaf mould may be lower than for other foliar diseases because actual defoliation is delayed. Additionally, the disease seems to develop slowly, becoming severe only later in the season. Marketability is not affected because P. fuligena does not seem to cause direct damage to fruits.

Yield losses of 63, 46, 46 and 41% were observed in four commercial tomato cultivars (Tainan Selection No. 2, Known You 301, Taichung ASVEG No. 4 and Hualien ASVEG No. 5, respectively) (Wang, 1996). These results indicate that a high potential exists for serious economic damage to tomato production. Another two entries had 4-34% yield loss compared with the control plots. The yield reduction was due to a reduction in fruit weight and fruit number.

Saroj et al. (2014) observed black leaf spots on the leaves of ashwahandha (Withania somnifera) with 10-20% disease incidence in India during the monsoon of July 2011 and suggested that P. fuligena has the potential to reduce yield of W. somnifera.

Subedi et al. (2015) observed approximately 10 and 60% of tomato leaf area was diseased by P. fuligena in a home garden and a greenhouse, respectively, later in the season in Ohio, USA. 

Lookabaugh et al. (2018) reported that disease incidence caused by P. fuligena was moderately high with 25-35% of tomato plants affected in a commercial greenhouse in North Carolina, USA. They observed that fungicide applications and routine leaf removal prevented spread to tomato healthy plants.