Disease Cycle and Epidemiology In the absence of lettuce, the pathogen survives in infested debris and as microsclerotia in the soil. Upon germination, microsclerotia produce hyphae and/or conidiophores capable of spore production. Free moisture from dew, rain, or overhead irrigation is required for conidia production, germination, and infection. The fungus may penetrate directly through the leaf epidermis or enter through stomatal openings. Under favorable temperatures, periods of leaf wetness as short as 4 h may be suitable for infection, although longer durations are much more conducive. Splashing water is the most important method of dissemination, but conidia may also be spread by mechanical means when the foliage is wet. Anthracnose is favored by cool, wet conditions because of its water requirement. Maximum disease development occurs at 20°C. The host range for the pathogen is confined largely to species of the genus Lactuca, although endive has also been reported as a host. The possibility of infected seed serving as an inoculum source for lettuce anthracnose has generally been discounted. Host plant resistance has been reported, along with at least five pathogenic variants of the pathogen.
Management Methods for managing anthracnose include crop rotation, plowing to bury debris and microsclerotia, and controlling collateral hosts, such as wild lettuce. These measures serve to reduce the level of primary inoculum in the field. Cultural practices should be designed to minimize leaf wetness and to avoid physical contact with the plant when the canopy is wet. Where conditions are frequently favorable and disease pressure is severe, fungicide applications may be economically justified. Strobilurin fungicides and phosphorous acid compounds were shown to be most efficacious in an Australian trial. Breeding for anthracnose resistance has not generally been a priority. However, genetic resistance has been identified among commercially available cultivars and may offer management potential. Selected References Brandes, E. W. 1918. Anthracnose of lettuce caused by Marssonina panattoniana. J. Agric. Res. 13:261-280. Couch, H. B., and Grogan, R. G. 1955. Etiology of lettuce anthracnose and host range of the pathogen. Phytopathology 45:375-380. Galea, V. J., and Price, T. V. 1988. Survival of the lettuce anthracnose fungus (Microdochium panattonianum) in Victoria. Plant Pathol. 37:54-63. Galea, V. J., Price, T. V., and Sutton, B. C. 1986. Taxonomy and biology of the lettuce anthracnose fungus. Trans. Br. Mycol. Soc. 86:619-628. Ochoa, O., Delp, B., and Michelmore, R. W. 1987. Resistance in Lactucae spp. to Microdochium panattonianum (lettuce anthracnose). Euphytica 36:609-614. Patterson, C. L., and Grogan, R. G. 1991. Role of microsclerotia as primary inoculum of Microdochium panattonianum, incitant of lettuce anthracnose. Plant Dis. 75:134-138. Patterson, C. L., Grogan, R. G., and Campbell, R. N. 1986. Economically important diseases of lettuce. Plant Dis. 70:982-987. Rogers, G., and Kimpton, T. 2012. Developing a Strategy to Control Anthracnose in Lettuce. Rep. no. VG10123. Horticulture Australia Limited, Sydney, New South Wales. Wicks, T. J., Hall, B., and Pezzaniti, P. 1994. Fungicidal control of anthracnose (Microdochium panattonianum) on lettuce. Aust. J. Exp. Agric. 34:277-283.
tributed on lettuce. When it occurs, plants can be significantly stunted because of extensive damage to small feeder roots.
Symptoms Affected lettuce seedlings can be stunted and delayed in development but otherwise appear healthy. If black root rot is severe early in the crop cycle, infected lettuce plants will remain small, the field will appear uneven (Fig. 11), and plants will not reach marketable size (Fig. 12). Stunting is usually most evident from the rosette stage through plant maturity. In advanced cases, the lower leaves of the plant may turn yellow or even brown. Root symptoms of black root rot include the formation of numerous small (4–8 mm long), dark-brown to black bands and lesions on small feeder and larger secondary roots (Fig. 13). As the disease progresses, the lesions coalesce and cause significant damage to the root system. The lesions do not penetrate into the root vascular tissue. Black root rot affects both crisphead and leaf/romaine cultivars.
Causal Organism Black root rot is caused by the soilborne fungus Thielaviopsis basicola (syn. Chalara elegans). This pathogen is widespread throughout the world and has a broad host range. On most agar media, colonies of T. basicola are gray with a velvety texture. Endoconidia are hyaline, thin walled, single celled, and cylindrical, and they measure 3–6 × 7–17 µm. Conidia form within slender phialides and are then pushed out from the tips of the conidiophores in fragile chains that readily break apart. Because these spores are catenulate, the ends of the endoconidia are truncated. T. basicola also forms aleurioconidia that are dark brown, thick walled, single celled, and subrectangular. Aleurioconidia are 5–8 × 10–16 µm and form in stacks or chains of five to eight or more spores; these stacks tend to remain intact until the spores reach maturity, at which point they break apart. Differences in isolates of T. basicola may exist. When lettuce isolates were inoculated onto lettuce and various other crops known to be hosts, T. basicola was very aggressive and caused significant damage to lettuce and bean; however, inoculation did not result in disease on cotton or other crops. Thus, T. basicola isolates from lettuce may have a host preference for lettuce and a few other plants.
Disease Cycle and Epidemiology Aleurioconidia enable T. basicola to survive in dry soil for extended periods (more than 1 year). Under moist conditions, however, survival is reduced to 7 months or less. Optimum temperatures for infection and disease development range
(Prepared by R. N. Raid)
Black Root Rot Black root rot of lettuce has been reported in Australia and the United States (California) but otherwise is not widely dis26
Fig. 11. Uneven growth of romaine lettuce plants, caused by black root rot (Thielaviopsis basicola). (Courtesy S. T. Koike—© APS)