Control Measures
The following is an outline of recommended control measures for Sclerotium rot of vanilla in Indonesia. Most of these measures are generic for Sclerotium-induced diseases and are hence applicable in other regions.
1. Regular monitoring and early and accurate detection of the disease.
2. Physical removal of diseased tissue, including surrounding plant parts and soil which may contain the sclerotia of the pathogen.
3. Application of biological control agents Trichoderma harzianum, T. lactae, and B. pantothenticus (Tombe, 2007), mixed with organic mulch or compost. The application of T. harzianum mixed in rice grain and sterilized soil, spread on the soil surface of vanilla nursery, was shown to suppress the disease by up to 54% (Kasim and Prayitno, 1993).
4. The use of eugenol as a natural fungicide has also been reported to suppress the disease (Sukamto et al., 1996; Tombe et al., 1997). Ground leaves and flower buds of cloves were spread around the base of vanilla stems or mixed with growing media in the nursery. Eugenol (approx. 10–20% active ingredient) can also be applied by spraying or stem dipping before planting.
5. Application of synthetic fungicides, such as carbendazim, mancozeb, and benomyl, especially in conjunction with the above measures.
Phytophthora Rot of Shoot and Bean
This disease has been reported in several vanilla-producing regions in the world, including Indonesia, Madagascar, Puerto Rico, Polynesia, but is generally not as significant as Fusarium rot and anthracnose. However, in Polynesia shoot rot is a serious threat to vanilla production, causing seedling death (Tsao and Mu, 1987). In Indonesia, Phytophthora shoot and bean rot occurs in nurseries and plantations in Bali, Sumatra, and Java (Manohara, 1993), but the disease has not become a serious problem.
Symptoms and Damage
Infected shoots turn light yellowish brown and become completely necrotic in later stages (Figure 8.4c). In Indonesia, disease lesions are usually restricted to the young parts without progressing to the older internodes. The diseased young shoots often die and fall off, after which the older parts are often observed to be further infected with Fusarium rot, which proceeds to cause serious damage (Manohara, 1993). On the shoot, Phytophthora rot is usually characterized by a yellowish brown color on diseased tissue, while Fusarium rot is more typically darker brown (Taufik and Manohara, 1998).
On the bean, Phytophthora rot develops from the tip, forming a water-soaked lesion, which slowly extends toward the pedicel, becoming darker green. The eventual necrosis extends to the whole bunch of beans, sometimes exhibiting abundant external growth of mycelium (Anandaraj et al., 2005). In India and Indonesia, the occurrence of this disease on vanilla plants is heavier during the rainy season (Rachmadiono et al., 1982; Manohara, 1993; Bhai and Thomas, 2000).
Causal Organism
There have been three species of Phytophthora reported to attack vanilla plants, namely Phytophthora palmivora in Polynesia (Tsao and Mu, 1987) and Thailand (Sangchote et al., 2004), Phytophthora capsici in Indonesia (Andriyani et al., 2008), and Phytophthora meadii in India (Bhai and Thomas, 2000).
Morphological characteristics of P. capsici from vanilla in Indonesia include the presence of sporangia, formed sympodially, which vary in shape from ovoid to obpy-riform (Figures 8.5a and b), 35–125 μm long and 17–58 μm wide, and clearly papil-late at the tip. Chlamydospores are also present and formed in the middle of the hypha (Figure 8.5c). Colony morphology varies and vegetative growth (Figure 8.5d) is observed at an optimum temperature range of 25–35°C (Andriyani et al., 2008). The isolates found in Indonesia are heterothallic, belonging to the A1 mating type. The oospores are produced in oogonia with amphygenous antheridia. The three species of Phytophthora reported on vanilla are most easily distinguished from each other based on the sporangial pedicel length: P. capsici with the longest pedicel (10–200 μm) (Figure 8.5a), P. meadii intermediate (10–20 μm) and P. palmivora the shortest (<5 μm) (Figure 8.5b) (Tsao and Mu, 1987; Erwin and Ribeiro, 1996). Further studies on the etiology of this disease are warranted due to the taxonomic confusion of this genus based solely on morphology.
FIGURE 8.5 Morphology of Phytophthora isolates: (a) Sporangial long-branching pattern of P. capsici; (b) sporangial short-branching pattern of P. palmivora; (c) chlamydospore; and (d) mycelium of P. capsici generated on V8 juice agar. (From Andriyani, N. et al. Jurnal Biologi Indonesia, 5, 227–234, 2008. With permission.)
Control Measures
Various recommendations for the control of Phytophthora shoot and bean rot of vanilla include phytosanitation (removal of infected plant parts, implementation of farm hygiene, etc.), control of humidity by pruning shade trees, application of botanical fungicide, such as eugenol (Tombe et al., 1993; Andriyani et al., 2008) and the use of phosphonate compounds, systemic fungicides effective against a wide range of plant diseases caused by many Phytophthora species (Drenth and Guest, 2004).
Concluding Remarks
There are other minor fungal diseases reported on vanilla in various growing regions but these are not well documented, not verified, or only reported in isolated cases. Some of these diseases include dry rot (caused by Rhizoctonia sp.) (Anandaraj et al., 2001), brown rot (caused by Cylindrocladium quinqueseptatum) (Bhai and Anandaraj, 2006), vanilla rust (caused by Puccinia sp. and Uredo scabies) (Correll, 1953; Augstburger et al., 2000), and other foliar diseases caused by Vermicularia vanillae, Guignardia traverse, Macrophoma vanillae, Pestalospora vanillae, and Physalospora vanillae (Correll, 1953). Reports on vanilla diseases are often confined within local scientific journals and extension reports, which are not easily accessible internationally. Fungal diseases of vanilla are generally not well studied. Much work is required to understand the biology, distribution, pathogen structure and spread, and disease management of these diseases.