Plant protection
Diseases
Vanilla is susceptible to various fungal and viral diseases. The symptoms and management strategies for various diseases are described below.
Bean rot
Two kinds of rot caused by two different species of fungi are recorded.
Phytophthora induced rot develops at the tips of beans, slowly extends towards the pedicel and the affected beans show water soaked lesion which become dark green leading to rotting of the beans. The rotting extends to whole bunch of beans exhibiting abundant external growth of fungal mycelium. In later stages of infection the rotting advances to the stem, leaves, aerial roots and extend to the entire vine.
Sclerotium induced rot is characterized by rotting of bean tips and affected portion shows white thick mats of fungal mycelium forming a mantle around the bunch of beans and leaves. Excess shade, continuous heavy rains, overcrowding of vines, waterlogged conditions and presence of pathogen inoculum in the field are the predisposing factors for bean rot.
Management
• Remove and destroy infected plant parts and mulch during rainy season.
• Regulate shade during monsoon period in order to prevent excess shade.
• Allow at least 30-50% light to fall on the vines.
• Spray bordeaux mixture (1.0%) and drench soil with 0.25% copper oxychloride 2-3 times depending on the severity of infection and as prophylactic measure.
• If rotting is due to Sclerotium, carbendazim-mancozeb mixture 0.25% can be sprayed twice at 1 5 day interval.
Premature yellowing and bean shedding
The disease is of relatively recent origin and is noticed in all vanilla plantations of Karnataka and Kerala especially during summer months. The disease initiates as dropping off of dried corolla from the tip of immature beans which otherwise remains attached to the beans till half way through maturity. As the dried corolla drop off, exudates from the beans accumulate at the tip, the beans turn yellow followed by brown discoloration from the tip upwards.
High temperature (more than 32°C) and very low relative humidity (less than 70%) prevailing during the months of February-May predisposes the plants to infection. Over crowding of the beans may also play a key role in immature bean shedding. Intensity of the disease is low under conditions of high altitudes where temperature and humidity are maintained under forest cover. Constant association of Colletotrichum vanillae and insect larvae inside the flowers are noticed.
Management
• Provide 50% shade in the plantation.
• Provide mist irrigation for at least 4-6 h during pollination till the onset of pre monsoon showers in order to maintain a relative humidity of more than 70%.
• Restrict the pollination to 15-18 flowers/inflorescence.
• Spray dimethoate or quinalphos 0.05% during flowering period thrice at 1 5-20 day interval and fungicides such as thiophanate methyl 0.2% or carbendazim - mancozeb (0.25%) at 1 5-20 day interval thrice from February up to May.
Stem Rot
The disease usually appears during the post monsoon period of November-February. The disease appears as yellowing and shriveling of the inter- nodal area extending to both sides of the stem. When the basal or middle portions of the vines decay and shrivel, the remaining distal portions of the vines show wilting symptoms. Stem rot and drying are generally observed at the basal portions above the ground level. The disease is caused by Fusarium oxysporum f. sp. vanillae.
Root rot/wilt
Initially the disease appears in the form of browning and death of underground and aerial roots. Aerial roots die before entering the soil resulting in flaccidity and shriveling of the stem and finally the vine droops. The disease is caused by Fusarium batatis Wollen var. vanillae.
Management of stem and root rot/wilt
• Remove and destroy infected plant parts (Phytosanitation).
• Foliar spray with carbendazim 0.2%, soil drenching with carbendazim (0.2%), copper oxychloride (0.25%) or a mixture of carbendazim-mancozeb (0.25%).
• Apply biocontrol agents such as Frichoderma harzianum and Pseudomonas fluorescens having a cfu of 108 g@50 g/vine.
Tip rot and die back
Visible symptom is the brown discoloration of the growing tip of the vine. The symptom starts at the collar region of the funnel like tip which extends to the inter nodal regions resulting in rotting of the tip. The disease may be caused either by Phytophthora meadii or F. oxysporum. In case of Phytophthora rot, thick white mycelia of the fungus cover the water soaked black lesion. But if the infection is due to Fusarium , the lesion is grayish in colour with large number of pin-head like encrustations of mycelia aggregate on the lesion. Such aggregation contains a large number of conidia.
Management
• Nip off the infected tip below the next node.
• Spray bordeaux mixture (1%) or mancozeb or carbendazim at 0.25% as prophylactic measure.
Viral infection
Mosaic disease
Various kinds of mosaic such as mild mottle, mild mosaic and mild chlorotic streak (could be seen when the leaf is held against light) are observed. In a few cases, such mosaics are also associated with leaf distortion with wavy margin. The size of the leaves also gets reduced and in advanced stages, leaves become brittle and show severe crinkling.
Stem necrosis
The disease is characterized by the appearance of brown necrotic patches on the stem region with shriveled appearance. The affected stem shows distinct necrotic lesions of varying length (few mm to several cm). This disease is different from the fungal induced stem rot. Stem necrosis can be distinguished from stem rot caused by fungi by the following :
• Stem rot affected region will be totally blighted and very soft which can be easily felt by touching the affected region, while stem necrosis (caused by viruses) affected region when touched appear very dry and hard and gives cracking sound when attempted to break open. Fungal diseases are commonly seen during the wet monsoon period whereas stem necrosis is seen all through the year.
• A close look at the stem rot affected region show a white cottony growth on the upper surface of the affected region while no such growth is seen with stem necrosis affected region.
• In case of stem rot the portion above the lesion often wilts with yellowing of leaves whereas in stem necrosis no wilting would be seen.
In a few cases, necrosis is also seen on the leaves at the lower surface in the form of scab. This often gives the appearance of sun scorch. The disease initially starts as a necrotic spot on the stem and slowly gets enlarged and encircles the stem. In an affected plant, necrosis may be seen only at one or few regions on the stem. Rest of the stem region looks apparently healthy without any visual symptoms. A few of the necrosis affected plants also show mosaic symptoms on leaves.