Lealy (1970) and Owino (2008) reported that root rot of vanilla caused by Fusarium oxysporum (syn. F. batatatis var. vanillae) was the most serious disease in vanilla plantations in Uganda. This and other species of Fusarium, including Fusarium solani have been reported in vanilla plantations in India (Balagopal et al., 1974a, 1974b; Philip, 1980; Anandaraj et al., 2005), Thailand (Ratanachurdchai and Soytong, 2008), Tonga (Stier, 1984), and China (Ruan et al., l998). The pathogen is observed in Reunion Island and Madagascar today, but was probably present as far back as 1871 and 1902, respectively (Bouriquet, 1954).
Symptoms and Disease Development
Symptoms of stem and root rot may appear at any growth stage of the vanilla plant: cuttings, young vines, and mature productive vines in the field. In addition to the stems and roots, the disease attacks other plant parts such as shoots and beans at any time of the year (Tombe et al., 1992a). In Indonesia, most cases of infection occur initially on the stems, followed by roots and shoots, and occasionally on beans and young leaves. Foliar infection is more commonly found on young leaves. During the rainy season infection on shoots is more prevalent than on other parts of the plant, although the damage is not as severe as on the stems.
Under adverse disease development conditions, symptoms appear as black spots on the stems with limited progress and obvious brown margins. On the other hand, under favorable conditions brown to dark brown lesions with less clearly defined margins enlarge and extend very rapidly and spread along the whole stem internode. A chlorotic zone is often observed between the lesion and healthy tissue (Figure 8.1a) (Soetono, 1962; Tombe, 1993a; Anandaraj et al., 2005). Consequently, the infected stem internode constricts, turns brown to dark brown and finally becomes dry and necrotic (Figure 8.1b). However, disease progress appears to be inhibited by nodes along the vine (Figure 8.1c). On the rotted and constricted parts yellowish-orangey-white spore masses are formed, consisting of fungal conidiophores and conidia. When the infected stem is longitudinally cut, necrosis is apparent, as indicated by a brown discoloration, developing from the inner to the peripheral tissue.
On roots the symptoms initially appear in the form of browning, followed by eventual death (Rachmadiono et al., 1982; Tombe, 1993a; Anandaraj et al., 2005). Aerial roots die promptly after entering the soil (Figure 8.1c), resulting in flaccidity and shriveling of the stem and consequently the vine droops. It turns dark brown and decays, and the rot is either soft and watery or somewhat dry, depending on the existing moisture conditions (Alconero, 1968). Fusarium rot of aerial roots is often diffi-cult to distinguish from anthracnose as both have very similar symptoms and the respective pathogens are often coisolated when present in the same farm. Tucker (1927) recorded in some instances that as much as the lower 3 m of a vine may rot away while the upper part remains green and continues its existence. It is, however, important to note that the pathogen can survive within green healthy internodes with no apparent internal or external symptoms.
FIGURE 8.1 Symptoms of Fusarium rot on vanilla vines: (a) dark brown lesion on stem internode with a chlorotic zone; (b) advanced necrotic stem internode and root; (c) numerous aerial roots are produced at the nodes but rot after reaching the soil.
Causal Organism
Tucker (1927) was the first to describe the pathogen of vanilla root rot by detailing the morphology of various spore types and identifying the pathogen as F. batatatis var. vanillae. This name has undergone multiple nomenclatural changes since then. This together with the association of the disease with various plant parts, resulted in the occasional confusion as to the true etiology of the disease. The species name given by Tucker (1927) was on the basis of morphological similarity to Fusarium batatis, the wilt pathogen of sweet potato, but the two pathogens from the respective hosts were not found to be cross-pathogenic, hence, a variety name was used for the vanilla strain. The host specialization of morphologically identical strains of Fusarium species led to the concept of forma specialis (Snyder and Hansen, 1940), whereby many Fusarium species previously named on the basis of host pathogenicity, despite morphological similarity to F. oxysporum Schlechtendahl, were renamed F. oxysporum with different forma specialis epithets according to the hosts. Fusarium oxysporum Schlecht. f. sp. batatis (Wollemw.) Snyder et Hansen was synonymized with F. batatis (Snyder and Hansen, 1940), but it was sometime later that F. batatatis var. vanillae (Tucker) was renamed F. oxysporum Schlecht. f. sp. vanillae (Tucker) Gordon (Gordon, 1965).
F. oxysporum f. sp. vanillae produces various asexual structures: the microconidia, macroconidia, and chlamydospores (Figure 8.2). Mycelia are immersed, sometimes running over the surface of lesions, hyaline, slender; sporodochia on decaying infected part of vanilla; microconidia in false heads and short conidiophores, abundant, single-celled, oval, 4–9 × 2–3.5 μm; macroconidia usually 3- septated, occasionally 1- to 2-septated, rarely 4- to 5-septated, abundant, hyaline, curved, pedicellate, 20–46 × 2.4–8 μm; chlamydospores present, singly or in pairs, thick-walled when old, brown, 6.5–10 μm (mean 7 μm). On potato dextrose agar (PDA) medium the growth of colony is rapid and the white aerial mycelia may become tinged with pale purple.
FIGURE 8.2 Morphology of the F. oxysporum conidia isolated from vanilla on carnation leaf agar: (a) Microconidia produced in short monophialides; (b) Macroconidia produced in aerial mycelium; (c) Chlamydospores formed singly or in short chains; (d) Macroconidia and microconidia.
The general morphological features of the Fusarium isolates from vanilla isolated in Indonesia, Reunion Island, the Comoros, and Central America today are all similar to those of F. oxysporum described by Messiaen and Cassini (1981). They also fit the key descriptions of F. oxysporum (Matuo, 1972). Morphologically, these isolates are also similar to those isolated from vanilla in India (Philip, 1980).
Inoculation tests on five plants with F. oxysporum indicated that the vanilla isolates were pathogenic to vanilla and not pathogenic to melon, cucumber, tomato, and cymbidium, whereas isolates from tomato were nonpathogenic to vanilla (Nurawan, 1990). In a recent study, Xia-Hong (2007) isolated 87 strains of F. oxysporum from vanilla showing stem rot in seven provinces of China. Among these strains, 81 were tested for pathogenicity and only 43 were found pathogenic.
Vegetative compatibility groups (VCG), sometimes called heterokaryon compatibility groups, are a useful tool to identify different genetic groupings in a population of fungi. Each VCG is unique in the sense that the members of one group are not compatible with the members of any other groups. The Indonesian isolates of F. oxysporum f. sp. vanillae have been grouped into two VCGs with another four VCGs represented by single isolates (Tombe et al., 1994b). In Indonesia, isolates within the same VCG were not necessarily restricted to the same region or location (Tombe et al., 1994). These results were similar to those reported in F. oxysporum f. sp. tuberasi from potato (Venter et al., 1992) and Fusarium proliferatum from asparagus (Elmer, 1991).