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Pathogen Biology

Phytophthora nicotianae is a fungus-like organism in the Kingdom Straminipila (sometimes written as Stramenopila), phylum Oomycota, class Oomycetes. All stages of the organism are diploid; whereas, most true fungi are haploid. The organism can grow vegetatively at temperatures between 5°C and 37°C, with optimal growth occurring between 26°C and 32°C. Extended exposure to temperatures above 40°C is lethal to the organism.

Hyphae

Hyphae are hyaline (colorless, transparent), aseptate or coenocytic, and typically irregular in width (3-11µm) with few to numerous hyphal swellings (Figure 22). With age, hyphae acquire pseudosepta and colonies become light yellow. There is considerable morphological variation in colony type among isolates of P. nicotianae (Figure 23). The growth habit or colony morphology of an individual isolate may vary when grown on different culture media (Figure 24).

Figure 22	Figure 23
Figure 24

Asexual reproduction

Sporangia are ovoid, pear-shaped, or spherical, and have very conspicuous papillae (Figure 25). Sizes of sporangia vary (18-70 x 14-39 µm) with isolate and the growth medium. Germination is either direct by production of hyphae or indirect by the production of five to 30 zoospores (Figure 26).

The zoospore is typically kidney-shaped, with a ventral groove from which two flagella emerge (Figure 27a). The posterior flagellum is whip-like and the anterior flagellum, a tinsel type, is shorter with hairs along its length. After settling down on a plat surface, zoospores encyst (produce a cell wall), and a single germ tube emerges from the spore (Figure 27b,c).

Chlamydospores are asexual, thick-walled spores produced at the tips (terminal) or in the middle (intercalary) of hyphae, and range from 13 to 60 µm in diameter, with walls approximately 1.5 µm thick (Figure 28). Chlamydospores serve as the primary survival propagule and as the primary inoculum that initiates epidemics. Survival for 4 to 6 years in soil has been reported.

Figure 25	Figure 26
Figure 27	Figure 28

Sexual reproduction

Phytophthora nicotianae is heterothallic, requiring two mating types (A1 and A2) for the production of oospores. Some single-strain cultures may develop oospores with age. Oospores are thick-walled, between 13 and 35 µm in diameter, almost filling the spherical oogonium cavity (between 15 and 64 µm in diameter). The antheridium is amphigynous (the oogonium grows through the antheridium at mating), spherical and remains permanently attached to the oogonium (Figure 29). It functions to contribute a nucleus during fertilization of the oogonium. Since many fields contain only one mating type and the oospores rarely germinate in culture, they are not thought to serve as a primary survival propagule or initiate infections that lead to epidemic development.

Figure 29

Nomenclature

Currently, the most frequently used name for the black shank pathogen is Phytophthora nicotianae. Phytophthora parasitica var. nicotianae and Phytophthora nicotianae var. nicotianae have also been used, but their use is declining. See the Appendix below and Erwin and Ribeiro (1996) for a more detailed description of the taxonomy of this organism.

Host Range

The species P. nicotianae (synonym P. parasitica) has a very wide host range. However, there is much evidence of host specialization within isolates obtained from various hosts. Isolates from tobacco are generally considered to be important pathogens only on tobacco. Isolates of P. nicotianae from other hosts also have shown some level of host specialization as well. However, current understanding of the level of specialization in the species has not resulted in development of forma specialis, which is a sub-specific level of taxonomy based on host specialization used with some other plant pathogens. See Erwin and Ribeiro (1996) for a comprehensive list of hosts for this species.

Other Phytophthora spp.

In addition to P. nicotianae, other species of Phytophthora also can cause root rot of tobacco. Although black shank has not been reported from Brazil, the primary causal agent of the yellow stunt disease has been identified as P. glovera, a previously unreported Phytophthora spp. Other unidentified species of Phytophthora spp. have been isolated from diseased tobacco from Kentucky and North Carolina, but their contribution to disease losses has not yet been determined.

APPENDIX: Additional Information on Pathogen Nomenclature

J. van Breda de Haan first described the pathogen as Phytophthora nicotianae in 1896 from Indonesia, but failed to submit a Latin description of the species. Additionally, his original drawings of the organism were inaccurate due to the presence of a contaminant, probably a Pythium species. Dastur also described the species from India in 1913, but gave it the name P. parasitica. In 1928, Ashby proposed that the name of all strains of this pathogen from tobacco be merged under the properly described name P. parasitica. Tucker in 1931 chose the name P. parasitica var. nicotianae to designate isolates based on pathogenicity, but pathogenicity should not have been used as a basis for variety because variety status is based on differences in morphology. Since isolates from tobacco display host specific pathogenicity, the use of forma specialis (f. sp.) would have been more appropriate, but again, studies have not been conducted to confirm the validity of this subspecific classification for tobacco isolates compared to isolates from other hosts. In fact, some isolates from tobacco will infect other plants.

It is generally agreed that P. parasitica is the more appropriate name, but the International Code of Botanical Nomenclature restricts its use. In 1963, Waterhouse replaced the name P. parasitica with P. nicotianae because it had priority despite errors in the original description. She further separated the species into P. nicotianae var. parasitica and P. nicotianae var. nicotianae based on morphological differences. Since 1963, multiple studies have shown that the morphology of these two varieties overlaps greatly and is not a valid basis for separating the species into varieties. Use of Phytophthora nicotianae is slowly gaining acceptance in the United States, including by tobacco pathologists; most pathologists in other countries have used this nomenclature for a number of years. We have used the name P. nicotianae in this disease lesson as this is the most widely recognized name for this organism at the current time.

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