In Vitro Propagation
Vegetative growth proceeds slowly following establishment, usually taking at least one year before the plants reach the flowering/fruiting stage. Plants established from in vitro propagules are more phenotypically uniform and healthier, and consequently yield higher than vegetative cuttings. The cost and care of in vitro propagation is prohibitively expensive for many vanilla farmers.
Preparation and Disinfection of Cuttings
Cuttings are prepared prior to planting. The three most basal leaves are removed by hand by twisting at the petiole and taking care not to tear into the stem where open wounds can facilitate the spread of pathogens.
In order to prevent stem rot, caused primarily by F. oxysporum, stem cuttings are disinfected prior to planting. The basal portion of the cutting is submerged for 2–5 min in a fungicidal solution. The solution may consist either of carbendazim (2 g/L) or Bordeaux mixture (1 kg lime + 1 kg copper sulfate in 100 L of water), the latter being less effective but authorized for the production of organic crops. Fungicidal solutions are handled with rubber gloves to avoid harmful exposure to the human body.
After disinfection, cuttings are hung separately on a structure 1–1.5 m tall, in a shaded and well-ventilated area for a period of 7–15 days. The cuttings slightly dehydrate allowing for more flexible material for planting. Calluses form over areas of the cuttings that were damaged during leaf removal.
Establishing Cuttings—Timing
Cuttings are planted when support trees have developed sufficient foliage to prevent the young vanilla plants from sunburns. With shade cloth, cuttings are planted immediately after the establishment of support trees.
Cuttings can be planted practically at any point during the year given a sufficient availability of water. In the winter, new vegetative shoots are slower to develop and may be “burned” by low temperatures, which slow the overall development of the plant. In the rainy season, excess humidity can cause up to 50% of cuttings to suffer some degree of rotting.
The best conditions for planting cuttings are in humid substrates during warm, dry months preceding the onset of the rainy season (Ranadive, 2005). This timing favors a high percentage (>90%) of successful establishment of cuttings since high temperatures are conducive to the emergence of new shoots and roots.
Establishing Cuttings—Planting
Cuttings are planted in the following manner: adjacent to the support, a shallow ditch is dug 5–10 cm deep, into which the cutting is placed horizontally (but only the part that has had the leaves removed). The cutting is then buried with 3–5 cm of organic material and/or fertile soil or leaves, which will serve as mulch and as a source of nutrients. The extreme basal end of the cutting (2–3 cm) is left uncovered to prevent rot (Lepierre, 1988; Wong et al., 2003; Ranadive, 2005), especially when the substrate is humid. Some cuttings are established without making ditches, and are placed on top of a humid substrate.
Once planted, the rest of the cutting (with leaves, ca. 4–5 nodes) is positioned vertically on the support and fastened with biodegradable material such as banana leaves, tree bark, or henequen fiber.
Under optimal conditions of humidity and temperature, and with vigorous, healthy cuttings, the first roots begin to emerge the first week after planting and during the first shoots in about one month. This relatively early rooting negates the need to purchase rooting hormones or products.
Techniques/Practices for Vanilla Cultivation
After cuttings are successfully established, several activities and practices are necessary to ensure the development and optimal production of the vanilla crop.
Irrigation
Water is the main factor in the growth and development of vanilla. Sufficient availability of water is most critical during flowering/pollination. Irrigation has three benefits: (1) encouraging growth and development of the plant, (2) increasing fruit yield and quality, and (3) prolonging the productive life of the plant.
The most frequent form of irrigation in Mexico is the use of microemitters to moisten the mulch layer of the vainillal.
One criterion for irrigation is to maintain at all times a moist layer of mulch without reaching saturation levels. The frequency and amounts of irrigation therefore depend on the type of mulch, phenological stage of the vanilla plants at the time of watering, and the prevailing climatic conditions such as rain and solar radiation as well as the amount of shade covering the vainillal. Generally, in the dry season, watering is performed once to twice per week.
Nutrition
The primary source of nutrition for vanilla in cultivation is organic material (humus) that results from the natural decomposition of vegetable/animal residues (mulch), composting (via microorganisms), or vermiculture (worm-mediated breakdown of organic material).
In addition to apportioning nutrients, mulch has the following benefits: (1) it helps maintain soil humidity, (2) serves as a porous substrate, aiding in soil aeration and permitting the unrestrained development of roots, (3) maintains an adequate temperature, and (4) decreases the incidence of weeds.
Mulches increase the biological activity of microorganisms and promote the development of mycorrhizal associations (Porras Afaro and Bayman, 2007). For example, the fungus Rhizoctonia sp. is known to establish a mycorrhizal symbiosis with vanilla roots, from which the vanilla derives a better absorption of nutrients and water while the fungus gains carbohydrates produced by the plant (Wong et al., 2003). Other fungal species belonging to the genera Ceratobasidium, Thanatephorus, and Tulasnella also positively effect seed germination and plant growth (Porras-Alfaro and Bayman, 2007).
The most common mulch for vanilla is from decaying leaf litter deriving from leaf fall from trees, pruning, and from herbaceous plants in the vainillal. Decomposed and rotting tree bark is also used in the form of sawdust.
When there is not sufficient organic plant material found in the vainillal, mulch can be made from stubble of kudzu (Pueraria phaseolides), elephant grass (Pennisetum sp.), or Guinea grass (Panicum maximum) (Bouriquet, 1954; Ranadive, 2005).
Coconut fiber is also a popular mulch because it is porous, lightweight, and has an excellent capacity for retention of humidity and for conserving an appropriate micro-climate for the promotion of root growth.
Sawdust in a fresh state may contain toxic substances to plants such as phenols, resins, terpenes, and tannins. Fresh manure or manure that is not well decomposed can also burn or cause root rot and mortality. It is important that these materials are composted before they are applied to the vanilla plants.
The mulch should be 10–20 cm deep and approximately 50–100 cm wide, depending on the extent of root growth. In areas of high humidity and deficient drainage, the depth is less, in order to mitigate the development of fungal pathogens, which can favor root rot.