Passalora personata

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Dothideomycetes
Subclass	Dothideomycetidae
Order	Mycosphaerellales
Family	Mycosphaerellaceae
Genus	Nothopassalora

Scientific Name

Nothopassalora personata

Scientific Name Synonyms

Passalora personata

Cercosporidium personatum

Phaeoisariopsis personata

Cercospora personata

Mycosphaerella berkeleyi

Common Name

Author: Barbara Shew and Garrett Ridge, North Carolina State University

Scientific Name Synonym

Mycosphaerella berkeleyi W.A. Jenkins [Teleomorph]

Pathogen

Passalora personata is an asexual fungus in the phylum Ascomycota. The fungus produces dense brown to black stroma and conidiophores (10 to 100 × 3 to 6.5 µm) borne in dense fascicles are pale to olivaceous brown with conspicuous conidial scars. Conidia (20 to 70 × 4 to 9 µm) are medium olivaceous, cylindrical, obclavate, usually straight to slightly curved, and rounded at the apex. Most conidia have three to four septa (range 1 to 8) and have a conspicuous hylum. The teleomorph for this species, Mycosphaerella berkeleyi, is rarely observed.

Hosts, Signs, and Symptoms

Passalora personata causes late leaf spot of cultivated peanut (Arachis hypogaea L.). It also has been reported in South America as causing a leaf spot on rhizoma peanut (A. glabrata Benth.), but other sources describe this species as immune. Late leaf spot (caused by P. personata) and early leaf spot (caused by C. arachidicola) are among the most serious and yield-limiting diseases of cultivated peanut. Both early and late leaf spot can be present whenever peanut is grown and mixed infections are very common. However, one species often predominates in a given location or season.

Mature sporulating lesions of late leaf spot may be apparent 14 to 24 days after infection has occurred. Although sporulation can be amphigenous, most sporulation occurs on the lower leaf (abaxial) surface and tufts of conidia, sometimes arranged in concentric circles, often are visible to the unaided eye (Figure 1).

Leaf spots are first visible on susceptible cultivars about 12 to 18 days after infection as small dark or chlorotic flecks (Figure 2). As lesions develop, they become darker and enlarge, producing a dense stroma. Lesions are circular, 1 to 10 mm in diameter, dark brown to black, occasionally coalescent, and sometimes have a well-defined chlorotic halo (Figure 3). Round to lenticular lesions may develop on petioles and stems. Eventually, infected leaflets and leaves turn yellow and defoliate, resulting in yield loss (Figure 4). Disease develops first on lower leaflets and defoliation proceeds from the lower to upper canopy. Defoliation reduces yield by reducing healthy leaf area, and is believed to weaken stems and pegs, causing pods to shed during digging and harvest. Yield loss is directly correlated with defoliation, especially once defoliation exceeds 30 percent. In severe epidemics, complete defoliation and plant death can occur, reducing yield by 50 percent or more.

Ecology

P. personata overwinters in lesions and stroma on leaves, stems, petioles, and pegs. Under favorable conditions, conidia form on old lesions and are the primary source of initial inoculum. Conidia are dispersed by wind, splashing water, and insects. The presence of abundant peanut residue and infected volunteer plants in fields where peanuts are grown in short rotations leads to early and rapid development of late leaf spot. Temperatures near 20°C and long periods (12 hours or more) of high relative humidity (> 93%) or leaf wetness are highly favorable for disease development. Little disease develops at temperatures greater than 28°C or less than 15°C, or if time periods for leaf wetness are not met. Under favorable conditions, several secondary cycles may occur per season, resulting in rapid increase in disease.

Geographic Distribution

Passalora personata causes late leaf spot wherever peanut is grown. Globally, this pathogen is ubiquitous in tropical and subtropical regions.

Management

Cultural

Because peanut is the only host of P. personata, rotation delays the onset of disease by reducing the amount of primary inoculum. Rotations of 2 to 3 years are sufficient to delay disease onset.

Several runner and virgina-type cultivars with moderate partial resistance to leaf spots are available in the United States and elsewhere. This resistance is not sufficient to eliminate the need for fungicide applications, but may reduce the number of sprays needed for disease control. Complete resistance to late leaf spot is not found in cultivated peanut.

Chemical

Multiple applications of fungicidal sprays usually are required to keep leaf spot below damaging levels when peanuts are cultivated in humid climates. Fungicides are applied on a regular schedule throughout the growing season, for a total of 4 to 7 applications per season, depending on the cultivar, location, and season. Disease advisories based on rainfall or on temperature and relative humidity can be used to allow fungicide application on an as-needed basis. Repeated use of selective fungicides with a single mode of action can result in selection of fungicide resistant strains and reduction or loss of efficacy. Fungicides with different modes of action can be applied in alternation, in tank mixes, or in pre-mixed products to deter the development and build-up of resistant strains. The use of a non-selective fungicide as the final spray of the season is strongly encouraged.

Diagnostic procedures

Symptoms of late leaf spot caused by P. personata are very similar to those of early leaf spot caused by Cercospora arachidicola. Early leaf spot lesions are typically light tan to reddish brown on the underside of leaflets, whereas late leaf spot lesions usually are brown to black (Figure 1). Early leaf spot usually has a prominent yellow halo, which may be lacking from late leaf spot lesions (Figure 5). These characteristics are not consistent for all peanut cultivars, however. Sporulation of late leaf spot typically occurs on the abaxial surfaces, whereas C. arachidicola sporulates on the adaxial leaf surface. Conidia of P. personata are produced abundantly and sporulation may be evident with the unaided eye, whereas sporulation by C. arachidicola usually can be visualized only with a hand lens or microscope. The most reliable method of differentiating between the two diseases is to identify conidia under the microscope. If conidia are not present, individual leaves or leaflets may be incubated in a moist chamber for 24-48 hours to induce sporulation.

P. personata is very difficult to isolate in pure culture and is extremely slow growing. Successful isolation on agar media usually requires transfer of individual conidia with a fine needle or single hair. Cultures must be examined under a microscope and individual germinating spores transferred after 24 to 48 hours of incubation. Visible colonies are very small and may take two weeks or more to develop. Media used to culture P. personata include water agar, V8 agar (preferred), and PDA.

Late leaf spot symptoms can be confused with symptoms caused by certain phytotoxic pesticides. Spots caused by pesticide injury are lighter in color in the center, less clearly defined on the abaxial leaf surface, and do not sporulate even after incubation. Typically, pesticide injury is relatively uniformly distributed over an area and severity is constant or reduced over time, whereas late leaf spot initially is found on lower leaves in widely scattered areas and becomes more severe and widespread as it progresses.

Additional References

• Mycobank. http://www.mycobank.org/Biolomics.aspx?Table=Mycobank&MycoBankNr_=335649>.

Acknowledgments

• National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-41530-30708 as part of "Diagnostic Image Series Development for Supporting IPM in the Southern Region" (USDA-NIFA-RIPM-003351)