Fusarium rot occurs on the roots, stems, fruit, leaves, and shoots of the plant.
However, stem and root infections cause the greatest amount of damage, leading to signifi cant yield loss, and hence the disease is often referred to as stem rot, foot rot, or stem and root rot. In Indonesia, Fusarium rot can infect all parts of the plant from nursery to productive plants but the attack on the stem is most harmful. In light of its signifi cance, a greater emphasis on this disease is placed in this chapter.
HISTORICAL REVIEWAND DISTRIBUTION
Fusarium rot, attacking the vanilla crop, has been reported since the crop was culti-vated commercially in producing countries, including Indonesia, India, Puerto Rico, the Seychelles, Reunion Island, Madagascar, and Polynesia.
A disease on vanilla was fi rst recorded by Zimmermann in Indonesia in 1903 (Tombe, 1994) when infections on stems and leaves were noted. In 1918, symptoms of a root disease were fi rst noticed in vanilla plantations in Mayagüez, Puerto Rico (Alconero and Santiago, 1969). Tucker (1927) reported that the vanilla root rot in Puerto Rico was caused by a soilborne pathogen called Fusarium batatatis var.
vanillae Tucker. In 1925, van Hall (in Tucker, 1927) mentioned the occurrence of a root fungus that caused death of vanilla cuttings in Java, Indonesia. However, the earliest reference associating a Fusarium species to vanilla stem rot in Indonesia was by Soetono (1962), who isolated a species of Fusarium from infected vanilla plants and identifi ed it as F. batatatis Wollenweber.
In the Seychelles Islands, where vanilla was formerly an important crop, the same root disease was present according to a report made by Dupont (1921). The disease description coincided quite closely with that in Puerto Rico (Tucker, 1927). Meinecke (in Tucker, 1927) also observed an undescribed Fusarium attacking the tender tips and young pods of vanilla. Averna-Sacca (1930) reported that the same species of Fusarium was isolated from the diseased vanilla growing in Brazil. However, he stated that the Fusarium pathogen only attacked leaves and stems without any men-tion of its occurrence on the roots.
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).
SYMPTOMSAND 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 fi eld. 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 defi ned 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 fi nally 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 fl accidity 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 (See color insert following page 136.) 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.
128 Vanilla
CAUSAL ORGANISM
Tucker (1927) was the fi rst 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
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.
infected part of vanilla; microconidia in false heads and short conidiophores, abun-dant, single-celled, oval, 4–9 × 2–3.5 μm; macroconidia usually 3- septated, occa-sionally 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.
The general morphological features of the Fusarium isolates from vanilla isolated in Indonesia, Reunion Island, the Comoros, and Central America today are all simi-lar to those of F. oxysporum described by Messiaen and Cassini (1981). They also fi t 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 fi ve plants with F. oxysporum indicated that the vanilla iso-lates 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 compati-bility 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 compat-ible 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).
Studies are currently being conducted on the genetic diversity and population structure of F. oxysporum f. sp. vanillae throughout Indonesia, including reference isolates from various parts of the world. These include the use of PCR-based fi nger-printing markers as well as phylogenetic analysis of multiple gene regions. Preliminary results indicate the presence of a relatively high number of haplotypes, some of which form large clonal groups. The 87 strains of F. oxysporum f. sp. vanillae isolated in China (Xia-Hong, 2007) belonged to 12 different VCGs with no correlation between VCG and virulence.
The isolation of nonpathogenic strains of F. oxysporum from vanilla roots led to investigations on the possibility of utilizing some of these strains as biocontrol agents against Fusarium rot of vanilla. Promising results have been observed under experi-mental conditions, but fi eld effi cacy has yet to be verifi ed.
CONTROL MEASURES
Although several vanilla relatives, such as Vanilla phaeantha, Vanilla aphylla, and Vanilla andamanica, have been shown to be resistant to Fusarium rot (Theis and Jiménez, 1957; Minoo et al., 2008), commercial varieties resistant or tolerant to Fusarium rot have not yet been described. Owing to the fact that Fusarium rot can
130 Vanilla infect various plant parts at all stages of growth, integrated control measures against this disease are paramount and should be implemented right from the cutting prepa-ration stage, throughout the vegetative and productive phases in the fi eld, until senes-cence (Hadisutrisno, 1996; Tombe et al., 1997; Anandaraj et al., 2005; Tombe, 2008).
Several components of reported control measures among others are the application of benomyl, carbendazim, and mancozeb fungicides (Matsumoto et al., 1992;
Anandaraj et al., 2005; Bhai et al., 2006), and biological agents such as Bacillus sp., Pseudomonas fl uorescens, and avirulent strains of F. oxysporum (Tombe et al., 1992c, 1997; Hadisutrisno, 1996; Anilkumar, 2004).
Integrated control measures have been developed in Indonesia, which involve the use of (a) pathogen-free and disease-tolerant cuttings inoculated with a nonpatho-genic F. oxysporum strain 10A–M (Figure 8.3) to induce resistance (Bio-FOB cut-tings), (b) fungicides, such as benomyl or mancozeb, by dipping cuttings for 20–30 min, (c) biological agents Trichoderma lactae and Bacillus pantothenticus (Figure 8.3), Bacillus fi rmus and T. lactae or P. fl uorescens (Tombe, 2008) premixed with organic material, and (d) botanical fungicide eugenol extracted from clove leaves or clove fruit stalks. Table 8.1 outlines the recommendations for the control of
FIGURE 8.3 Biological agents used in the integrated control of Fusarium rot in Indonesia:
(a) approximately 3-month-old vanilla cuttings treated with nonpathogenic Fusarium oxysporum strain 10A–M; (b) antagonistic test of B. pantothenticus to F. oxysporum f. sp.
vanillae; (c) B. pantothenticus on sucrose peptone agar; (d) Trichoderma lactae on potato dextrose agar.
TABLE 8.1 IDM Recommendations for Fusarium Rot of Vanilla in Indonesia IDM RecommendationsNew Plantation Plantation without Any Disease Symptoms Plantations with Low Disease Incidence and Mild Severity
Plantations with High Disease Incidence and Severity (before Replanting) Plant pathogen-free cuttings (Bio-FOB)+ Dip cuttings with fungicides (benomyl or mancozeb)+ Use organic fertilizer amended with biocontrol agents+++ Apply organic mulch, for example, hay, coconut husks, clove leaves+++ Follow local recommendations for land tilling, plant spacing, irrigation, and shade trees+ Regularly monitor for early symptoms and physically remove any disease tissue. Disinfect cutting tool+++ Regularly apply botanical or synthetic fungicide, especially after fertilizing, pruning, weeding, and harvesting+++ Regularly prune shade trees to control humidity and shade+++ Improve drainage especially during rainy season+++ Physically remove disease tissue and apply botanical or synthetic fungicide to cut wounds. Disinfect cutting tools++ Introduce crop rotation with other plants+ Completely eradicate diseased plants and implement rigorous sanitation measures+ Plough land thoroughly to improve soil solarization+ Grow annual crops such as corn and beans as well as crops believed to maintain antagonistic microorganisms (e.g., Welsh and red onion, Chinese chive, garlic)+
132 Vanilla Fusarium rot of vanilla in various scenarios based on an integrated disease manage-ment (IDM) approach in Indonesia.