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Pathogen Biology

Aphanomyces euteiches is classified as an Oomycete (a fungal-like protist), in the kingdom Chromista (Stramenopiles or Straminipila). Aphanomyces is in the order Saprolegniales, and is the order’s only genus to contain species pathogenic to plants. Aphanomyces cochlioides, a pathogen of sugar beet, and A. raphani, a pathogen of radish, are also economically significant pathogens within the genus. Like all Oomycetes, A. euteiches has coenocytic (no septa/cross walls) hyphae (Figure 10), cellulose in its cell wall (true fungi have chitin), and produces motile spores (zoospores). Nuclei are diploid (2N) in vegetative hyphae and all spore stages. It is a soil-borne pathogen that spends a part of its lifecycle in the soil and part of its lifecycle in its host.

Figure 10

The sexual spore of A. euteiches, the oospore (Figure 11, 12), is a spherical structure with a double cell wall that remains dormant in the soil for several years in the absence of a host. Oospores are formed following meiosis. A haploid (1N) female gametic nucleus formed in the oogonium is fertilized by a second haploid nucleus (male gametic nucleus) that has migrated from an antheridium (male gametangium). Once fertilization occurs, a single oospore will develop within the oogonium. Aphanomyces euteiches is characterized as homothallic (same body) meaning both the oogonium and antheridium arise from the same hypha and are compatible (self-fertile). As a result, separate mating types are not required for sexual reproduction.

Figure 11

Figure 12

In the soil, oospores are stimulated to germinate by chemical signals exuded by host roots. Oospores form a germ tube that can either proliferate as hyphae or function as sporangia. Sporangia, which are indistinguishable from vegetative hyphae, arise from the germinating oospore and grow to about 8 to 10 times the diameter of the oospore. Nuclei (2N) migrate through the sporangium and develop a cell wall to form a primary spore. Spherical primary spores aggregate at the apex of the sporangium and will release zoospores that emerge through a pore in the cell wall of the primary spore. Zoospores are motile, kidney-bean shaped spores possessing one, long “tinsel” flagellum and one, short “whiplash” flagellum, both attached laterally. Both flagella assist the zoospore in migrating through water-filled soil pores towards host roots. Once a zoospore has reached the rhizoplane (root surface) it loses both flagella, encysts, and germinates by formation of a germ tube. Hyphae, derived from the germ tube, can directly penetrate epidermal host cells and colonize throughout root and subterranean stem tissues. Within host tissues, hyphae differentiate into antheridia and oogonia when host tissues begin to decline and oospores are formed.

The host range of A. euteiches includes several legume species. Within populations of A. euteiches, isolates express a diversity of pathogenic types some of which are host specific. The classification of isolates of A. euteiches for host specificity is based on the ability of the pathogen to progress into hypocotyls or epicotyls and initiate symptom development.

Currently, the forma specialis (f. sp.) concept (a taxonomic classification based on host range) has been applied to isolates pathogenic to pea, A. euteiches f. sp. pisi, and isolates pathogenic to bean, A. euteiches f. sp. phaseoli. This classification system is based on observations that A. euteiches isolates obtained from pea are non-pathogenic or weakly pathogenic on bean and alfalfa, and isolates from bean are non-pathogenic or weakly pathogenic on pea and alfalfa (Figure 13). However, in vitro studies of isolates of A. euteiches from pea and bean indicate that they are sexually compatible and capable of exchanging genes for virulence. The forma specialis concept has not been applied to isolates of A. euteiches pathogenic to alfalfa or clover species although isolates from clover are weakly pathogenic on alfalfa, and vice versa (Table 1; Figures 14, 15).

Figure 13

Figure 14

Figure 15

Table 1. Symptoms of Aphanomyces root rot (ARR) observed on root and stem tissue of various legume hosts by isolates of Aphanomyces euteiches (Ae).

	Pea		Bean		Alfalfa
Origin of Ae isolate	Root	Epicotyl	Root	Hypocotyl	Root	Hypocotyl
Pea	+^a	+	+	-^b	+	+
Beam	-	-	+	+	+	-
Alfalfa Race 1	+	+	+	-	+	+
Alfalfa Race 2	-	-	+	-	+	+

^a (+)-Isolation of Ae and symptoms of ARR observed in root or stem tissue.
^b (-)-Isolation of Ae and symptoms of ARR were not observed in root or stem tissue.

As mentioned above, many isolates of A. euteiches are capable of infecting roots of several legume species. However, progression of the pathogen into the hypocotyl or epicotyl and the degree of pathogenicity vary by subpopulation of A. euteiches and the host species from which an isolate of A. euteiches was isolated. This suggests the host range of A. euteiches, as determined through controlled inoculations in laboratories and in greenhouses, may not represent the actual host range of A. euteiches in a natural environment.

Within a host species, cultivars may react differently to isolates of A. euteiches, suggesting the existence of races. The existence of physiological races has been reported for pea, but is most evident for isolates within the alfalfa population of A. euteiches. Currently, alfalfa isolates are categorized as either race 1 or race 2 based on differential responses of specific alfalfa populations to isolates pathogenic to alfalfa.

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