Anthracnose of Vanilla
Although anthracnose of vanilla is generally regarded to be present in most growing regions today, including Indonesia, India, the Comoros, Madagascar, Mexico, Uganda (Bouriquet, 1954; Tombe, 1993b Augstburger et al., 2000; Thomas et al., 2002; Magala, 2008), there are no good records of its distribution and history of its detection and etiology.
Symptoms and Disease Development
Anthracnose symptoms on vanilla appear, especially on older leaves and stems, initially as small gray or black spots, which coalesce or increase in size to become dark brown to black patches or lesions of varying sizes (Taufik and Manohara, 1998) (Figure 8.4a). The disease is also reported to infect young shoots, inflorescence, and beans in India (Anandaraj et al., 2001; Anilkumar, 2004). In advanced disease stages, these lesions may contain the fruiting structures (acervuli), which appear as tiny black dots (Tombe, 1993b). Under humid conditions infective propagules (conidia) are produced and released from the fruiting structures, observed as masses of salmon-pink spore ooze on the surface of the disease tissue (Taufik and Manohara, 1998). These conidia are dispersed by rain splash or carried on various body parts of insects that come into contact with the spore ooze. Infection on the leaves may occur by spore penetrating through stomata or direct penetration. Latent infection occurs when spores penetrate only into a few epidermic cells and does not develop continuously. Only under favorable environmental conditions the fungus will develop and form necrotic spots. Infection on older or senescing leaves does not usually cause significant damage to the crop. The best way to distinguish Fusarium rot from anthra-cnose is to examine the internal tissue where external symptoms are observed. The presence of internal discoloration, in particular within the vascular tissue is indicative of Fusarium rot. Early symptoms of anthracnose are confined to the epidermis or superficial layers while the vascular tissue appears healthy.
FIGURE 8.4 Symptom of (a) Anthracnose on leaf; (b) sclerotium rot (brown water-soaked leaf tissue and white mycelium and sclerotia on soil; (c) Phytophthora stem rot on naturally infected shoot (Inset: symptom after artificial inoculation). (From Andriyani, N. et al. Jurnal Biologi Indonesia, 5, 227–234, 2008. With permission.)
Causal Organism
The causal organism of anthracnose on vanilla is Colletotrichum gloeosporioides Penzig et Saccardo (syn. Colletotrichum vanillae Scalia, C. vanillae Verplancke et Claess, Gloeosporium vanillae Cooke) (Tombe, 1993b; Anilkumar, 2004; Ratanachurdchai and Soytong, 2008). The pathogen is sometimes reported as the teleomorph Glomerella cingulata (Stoneman) Spauld et Schrenk [syn. G. vanillae (Stoneman) Sacc et Traverso], but the sexual stage has never been observed on the plant host. In brief, the morphological description of C. gloeosporioides includes
(1) acervuli disc or cushion-shaped, subepidermal, epiphyllous, 30–345 μm in diameter; conidiophores simple, short, hyaline, setae brown slightly swollen at the base and tapered to the apex on which conidia are occasionally borne 22.5–75 × 2.5–5 μm; and (2) conidia hyaline, unicellular, oval to oblong, 10–17.5 × 3–5 μm.
C. gloeosporioides has quite a wide host range and commonly exists in various kinds of conditions and places. However, the relationship between the vanilla isolates and those from other hosts has yet to be investigated.
Control Measures
Anthracnose on vanilla until now has not been reported to cause great losses to farmers (Taufik and Manohara, 1998; Magala, 2008). However, the disease indirectly affects the overall health of vanilla plants and hence vanilla production. This disease can be controlled using several methods, examples of which are outlined below.
1. Regular pruning of shade trees to control humidity and sunlight. Pruning is preferably carried out at the beginning of the rainy season. Drainage should also be improved at the same time.
2. Eradication of diseased plant parts (by burning for example) to prevent disease spread.
3. Application of fungicides (e.g., benomyl, mancozeb, and carbendazim) especially when disease is recurrent (Taufik and Manohara, 1998; Bhai et al., 2006).
4. Improve plant vigor (e.g., nutrition with organic matter).
Sclerotium Rot of Vanilla
This disease generally attacks during the rainy season when the humidity is high and is often found together with Fusarium rot. Sclerotium rot can be a serious problem in the nursery and, whenever associated with Fusarium rot, can result in the death of productive plants in the field (Tombe and Sitepu, 1987; Taufik and Manohara, 1998). The disease is reported in India and Indonesia (Tombe and Sitepu, 1987; Anandaraj et al., 2005), but there are hardly any records on the history and distribution of this disease in other growing regions, probably due to its frequent association and perhaps confusion with the more significant Fusarium rot.
Symptoms and Damage
The disease occurs on the stem base of vanilla, commonly restricted to 5 cm above the soil surface. The diseased stem base becomes water-soaked, brown to dark brown in color, then becoming necrotic and dies. White fluffy mycelium is often found on the infected stem and the surrounding soil surface (Matsumoto, 1993) (Figure 8.4b). Small spherical structures (sclerotia), light brown in color, 0.5–2 mm in diameter, are often observed in association with the mycelium. If the disease occurs in the cutting propagation nursery bed, damage is usually very heavy (Taufik and Manohara, 1998). Sclerotium rot is occasionally found on the vanilla bean, characterized by rotting of bean tips with thick white mat of fungal mycelium, which eventually covers the whole bean. Excess shade, continuous heavy rains, overcrowding of vines, waterlogged conditions, and the presence of the pathogen inoculum in the field are the predisposing factors for bean rot (Anandaraj et al., 2005).
Causal Organism
The causal organism of Sclerotium rot, Sclerotium rolfsii, does not form conidia or any other reproductive structures. However, dormant structures, the sclerotia, are commonly formed (Taufik and Manohara, 1998), which persist in the soil for years. These survival structures are easily spread by rain water splash and run off, contaminated soil, animals and farming equipment (Tombe and Sitepu, 1987), and thus also represent the dispersal and infective propagules of the disease. Under favorable environmental conditions, the sclerotia germinate and form mycelial mats, which colonize host tissue. Characteristic diagnostic symptoms and signs of the disease are the white mycelium around infested plant parts and the light brown sclerotia produced on basal stems and surrounding soil surface. The causal fungus grows well on artifi-cial medium in the laboratory but the teleomorph is never observed. Numerous scle-rotia are formed on the agar surface after 10–14 days. Deriving from the mycelium, these sclerotia are initially white in color and turn pale to darker brown as they mature and form an outer melanized rind.