Fungal Diseases
This is one of the most destructive diseases observed in all the vanilla-growing countries and is severe in India too. It was first reported in India in 1980 (Philip, 1980). Later it was shown to be caused by Fusarium oxysporum sp. vanillae (Thomas et al., 2002).
Symptomatology and Etiology The disease is observed during warm humid conditions. The fungus colonizes the root and causes yellowing of the vines with degeneration of the root system. It also causes dark brown rot on the stem as well as on the leaves (Figure 20.2). The rotting starts at any part of the plant. All parts of the plant are prone to infection, but more often at the nodal region. Later it spreads upward causing rotting, resulting in drying up of the distal portion beyond the point of infection. Severely affected gardens appear dry from distance, typical of wilting. The affected stem exhibits vascular browning, typical of fusarial wilts (Y.R. Sarma, unpubl. data).
FIGURE 20.2 Fusarium stem and leaf infection.
Epidemiology
Although detailed epidemiological investigations have not been carried out, ambient temperature around 25–28°C favored the disease incidence and spread. The disease occurs individually and in certain cases overlaps with Phytophthora rot. Hence, mixed infections are common. Abundant sporulation is observed in the affected tissues. Disease spreads through rain splashes within the plant and across the vines. Detailed investigation on the disease progression curves in relation to environmental condition is called for. The fungus survives on dead dried-up debris and the dead and the dried-up vines left over from gardens are the perennial source of inoculum.
Disease Management
Since the available cultivars are all highly susceptible and as such, no disease resistance exists. Recent report of variability of seedling progenies of V. planifolia to stem rot is important and needs intensive field evaluation (Minoo et al., 2008b).
Cultural Practice
A regular monitoring of the disease is essential for early identification of the disease to take corrective measures. Phytosanitation involving collection and removal of infected vines from the garden is essential to reduce the pathogen inoculum in the soil.
Chemical Control
Phytosanitation followed by spray and soil drenching with 0.2% carbendazim periodically, depending on the intensity of the disease, has been suggested (Thomas et al., 2002).
The disease was first recorded in vanilla plantation of Manalaroo Estate in Nelliampathi and Arnakal Estate at Vandiperiyar during 1998 and also in a stray vine at Directorate of Arecanut and Spices Development Office, Calicut (Y.R. Sarma, unpubl. data). It was later observed in Idukki and Kottayam districts of Kerala, specially in Moovattupuzha and Koothattukulam areas (Bhai and Thomas, 2000).
Symptomatology and Etiology
The disease incidence coincides with the southwest monsoon in Kerala. The disease occurs as water-soaked spots. Observations at Manalaroo and Arnakal Estates showed that Phytophthora infection occurs on leaves, stems, and beans causing rotting (Figures 20.3 and 20.4). All parts of the vine are prone to infection. On leaves, the water-soaked spot enlarges up to 0.5–1.5 cm with a translucent advancing margin. On stems, it is typically a wet rot unlike that of fusarial rot, and affected portions become soft and rot completely. In vanilla fruit bunches, infection starts at any point of beans either from the tip end or from the stalk region. When the infection starts at the tip, it gradually spreads upward resulting in varying degrees of rotting. If infection occurs at the stalk or peduncle region, either the infected bean drops off or dries resulting in beans hanging to the peduncle region (Y.R. Sarma, unpubl. data).
FIGURE 20.3 Phytophthora stem and leaf infection.
FIGURE 20.4 Phytophthora bean rot.
The causal agent was identified as Phytophthora meadii. Under artificial inoculation, infection was recorded 5–8 days after inoculation. The fungus on carrot agar medium appears cottony white. The sporangial size of 39.6 × 20 μm with an LB ratio of 1.98 was recorded. The cultures were heterothallic. Also, intercalary chlamy-dospores in the mycelium were observed (Bhai and Thomas, 2000).
Epidemiology
No detailed epidemiological investigations have been conducted. However, the disease coincides with the southwest monsoon with continuous rainfall of 15–20 days in a month with intermittent showers during the July–August period of the year. On the infected beans, fungal growth occurs, which harbors abundant sporangia. Disease spreads through rain splashes both in the vine and across the vines.
In Kerala and Karnataka, P. meadii has been reported on rubber, small cardamom, and arecanut. It is important to investigate their pathogenic relationship at molecular level and the possibilities of crossing and development of new races/ biotypes of P. meadii.
Disease Management
Cultural practices: As mentioned earlier in the case of fusarial rot/wilts, phytosani-tation measures consisting of systematic collection and pulling out weeds of infected and dried up vines is important.
Chemical control: Spraying of the vines with 1% Bordeaux mixture and soil drenching with copper-oxy chloride (0.25%) was reported effective (Bhai and Thomas, 2000). Prophylactic spraying of the vines with 4–5% of potassium phos-phonate (Phytophos 40), a systemic chemical, at 15 days intervals did control the disease but if the wet spells continued, the disease control was poor (Y.R. Sarma, unpubl. data).
This disease, caused by Sclerotium rolfsii, was reported from the Ramamangalam area of Kerala in a survey carried out during 1999 (Thomas and Bhai, 2000).
Symptomatology and Etiology
Rotting of either one or two beans or all the beans in a bunch was observed. Whitish mycelium spreads on the bean in a fan-like fashion. The infected beans showed rotting symptoms with deep sunken areas that are reddish brown. On leaves and stems, the fungus spreads whitish thick mycelial threads and later produces cream-colored to brownish grain-like sclerotial bodies (Y.R. Sarma, unpubl. data).
This is a minor disease observed in Chempukadavu area of Kozhikode district of Kerala in a mixed cropping system of coconut, arecanut, clove, and vanilla. The disease is caused by Cylindrocladium quinquiseptatum. It starts as water-soaked spots on beans, which later enlarge into a sunken spot of 1–10 mm. The lesions coalesce forming reddish-brown sunken lesions resembling anthracnose. Either a single bean or all beans in a bunch get affected. Leaf infection with sunken lesion was also observed (Bhai et al., 2006).
This is another minor disease of vanilla observed in Kozhikode district of Kerala, caused by Colletotrichum vanillae. The disease is characterized by premature yellowing of the beans with dark-brown sunken lesions that gradually expand resulting in rotting of the beans (Figure 20.5). This leads to premature bean shedding (Bhai et al., 2006). In a survey during April–May, 2003, premature bean shedding was recorded. It was reported that high incidence was due to high temperature and low relative humidity (Bhai and Dhanesh, 2008).