Part I. Infectious Diseases Disease Caused by Algae Algae are ubiquitous in both aquatic and terrestrial environments. Some, such as blue-green algae, are classified as prokaryotes and are similar to bacteria. Others, such as green or brown algae, are eukaryotes and are similar to fungi and plants. Algae are an important component of soils and considered a primary component in stabilizing soil aggregates. While they are often associated with water and primarily spread by water movement, algae may also be spread by air currents. Selected References Bold, H. C., and Wynne, M. J. 1985. Introduction to the Algae: Structure and Reproduction, 2nd ed. Prentice-Hall, Inc., Englewood Cliffs, NJ. Metting, B. 1981. The systematics and ecology of soil algae. Bot. Rev. 47:195-312.
Algal Leaf Spot Algal leaf spot is commonly known as red rust because of the alga’s orange-colored reproductive structures. However, the name “red rust” is a misnomer because there is no rust fungus involved in this disease. Distribution of the disease is worldwide between latitudes 32°N and 32°S, and it occurs on many hosts outside the Palmae family. It is reported in association with various palm species from Australia, Brazil, China, the Congo region, Costa Rica, Honduras, India, Indonesia, Japan, Malaysia, Nicaragua, Nigeria, the United States (Florida, Hawaii, and Puerto Rico), and the various islands of the West Indies, but it is also likely to be present in many other countries. Not all algae associated with palms are parasitic. Some are epiphytic, resulting in no physical damage but considered aesthetically unacceptable.
Symptoms Algal infection first appears as yellow-green pinpoint spots, primarily on the upper leaf surface and on the rachis. The algal thalli expand to 1–3 mm in diameter and coalesce, resulting in algal patches greater than 1 cm in diameter. Tissue necrosis is observed beneath the algal thallus. Premature senescence of leaves and a reduction in vigor of young palms are the observed symptoms on susceptible palm hosts, but these effects have not been quantified.
Causal Organisms Cephaleuros virescens Kunze is a green alga in the family Chroolepidaceae (synonym = Trentepohliaceae) and the primary alga species causing plant disease. This species is subaerial in growth habit. The thallus consists of a disk with a dichotomously lobed, radiating margin and may be one cell to several cells
thick. Part of the C. virescens colony grows horizontally between the leaf cuticle and epidermal cell layers. Under low magnification, the thallus appears velvety because of the presence of sterile hairs or setae and fertile branches that terminate in one to eight pedicels, each bearing one globose zoosporangium (Plate 1). Reddish orange, quadriflagellate zoospores are released when water is present (Plate 2). At reproductive maturity, the thallus appears orange because of the formation of a hematochrome pigment. Less obvious gametangia form in the subcuticular portion of the thallus. These emerge through the host cuticle and bear a single exit pore. Gametes are motile and biflagellate. While C. virescens is the dominant pathogenic alga, other green alga species may also be involved in this disease. For example, in Malaysia, the related alga Trentepohlia sp. has been identified on palms with algal leaf spot. Cephaleuros spp. also form lichenus (symbiotic) relationships with various fungi. The lichenized forms of C. virescens are Strigula spp. (Plate 3).
Host Range and Epidemiology The pathogenic role of C. virescens has been widely documented on many crops. On palm species, however, the role of C. virescens as a pathogen or epiphyte has not always been clearly distinguished. Reports exist of the pathogenic role of C. virescens on Elaeis guineensis in the Congo region and Nigeria. Although this alga is reported on such palms as Arenga spp., Bactris gasipaes, Butia capitata, Caryota spp., Cocos nucifera, Phoenix dactylifera, Sabal palmetto, and Trachycarpus fortunei, this simply reflects the presence of the thallus, since pathogenicity has not necessarily been demonstrated. C. virescens infection is initiated by zoospore germination and direct mechanical penetration of the leaf cuticle. Zoospore release is correlated with the humid, rainy season. Wind may disseminate sporangia for some distance. Short-distance spread is attributed to water splash of sporangia or zoospores. Vegetative growth is slow and may span 8–9 months until reproductive maturity is reached. The thallus likely remains fertile for the life of the palm leaf or rachis. The incidence of algal leaf spot is higher on less vigorous, older tissue, particularly on palms in border rows or planted along roadsides. Suboptimal light, poor water drainage, inadequate air circulation, and other foliar diseases can predispose palms to algal leaf spot. Algal spots may exceed 20 per cm of leaf or leaflet length, and algal leaf spot is associated with up to 20% defoliation in plantation-grown E. guineensis.
Diagnostic Techniques The thallus is a reliable sign of C. virescens on palms. The role of the alga as a pathogen or epiphyte is determined by microscopic examination of freehand sections through the thallus on the host. Pathogenicity is indicated by the presence of subcuticular cells of the alga and the death of cells beneath the algal thallus. C. virescens can be cultured on modified Chu’s 5