Preventing and Managing Fusarium
Courtesy of David Gardella: www.rainforestvanilla.org
Vanilla is a magnificent tropical crop, but it comes with challenges. Most vanilla producers fall in love with vanilla as she draws everyone under her spell. Vanilla requires no special fertilizers or pesticides. It thrives on a simple mulch of leaves, twigs, coconut husks, etc. While it does have problems with insects in its native habitats, these problems are generally not severe, especially if the vines are getting the right balance of sun and shade and protection from wind and heavy downpours. It does require adequate water and a dry spell during pollination. Otherwise, it is a very pleasant crop to grow.
There is only one significant disease in vanilla; root rot disease caused by the pathogen Fusarium oxysporum. All other diseases are insignificant in importance or merely minor pests with little economic affect on yield. Fusarium is “the scourge” of vanilla. It can and does completely destroy vanilla plantations in a matter of weeks-to-months. When it appears in your plantation there is nothing you can do to stop it. Plant another crop or… take the following recommendations to heart and start all over again but this time manage your plantation correctly.
Fusarium isn’t actually the culprit in this drama. Fusarium is just an opportunistic fungus that arrives after the damage has already been done. The theory of vanilla root rot disease goes like this:
Vanilla has a symbiotic fungi or mycorrhiza living in its root hairs. It’s a species of Rhizoctonia sp. Many species of Rhizoctonia are pathogens that cause damping off disease in seedlings. Most mycorrhiza help the host plant absorb fixed or insoluble phosphorous in highly acidic soils. This particular mycorrhiza is actually an extension of the vanilla plant’s root system. Mycelia filaments extend up to 4 inches (10cms) out from the root hairs in search of organic matter. Once the fungus makes contact with a fallen leaf, it decomposes it and passes the nutrients on to the vanilla plant. In exchange, the fungus is bathed in metabolites derived from photosynthesis. It’s a perfect balance between a fungus that breaks down dead organic matter and a plant that makes metabolites from the sun’s energy. However, when the balance is broken everything falls apart.
When the vanilla plant is stressed, the mycorrhiza switches from being a mycorrhiza and becomes a root pathogen. Instead of “eating” photosynthetic metabolites, it starts eating the vanilla plant’s root system. This opens the door to fusarium, a killer that lurks in the soil and is harmless until the plants are stressed.
The vanilla plant withstands having its root system destroyed and has sufficient reserves to send another root down the tutor tree. As soon as this root touches the leaf mulch or soil it also becomes infected with fusarium and dies. The dead roots appear as hollow straws clinging to the tutor tree. In a last ditch attempt to survive, the vanilla plant sends a second root down the tutor, but this also dies. At this point the vanilla plant has now exhausted all of its reserves, lacks a root system to absorb nutrients or even moisture, and finally dies. From the first appearance of symptoms to final death of the vanilla plant can take less than two months.
Did fusarium wipe out your plantation? No, poor management of the plants killed the vanilla. Fusarium just did what it always does, i.e. consume dead and dying plant material.
The main factors that stress vanilla in order of importance include:
Not enough leaf mulch to feed the mycorrhiza and the vanilla plant, and also conserve moisture during dry periods.
Not enough shade to protect the vanilla plant from excess sunlight that continuously destroys chlorophyll (the light absorbing compound that gives plants their green color). Plants need to continuously renew their chlorophyll which is continuously destroyed in the presence of sunlight.
Over pollination that produces an excessive load of beans that the vanilla plant has insufficient reserves to support. Over pollinating vanilla overwhelms the plant’s ability to produce a crop.
Genetic material that isn’t disease or drought resistant.
Use of planting distances that are too close and put plants in contact with each other by way of their root systems.
An El Niño year with a prolonged dry season or drought.
Recommendations for controlling Fusarium root rot:
Use genetic material that is disease-resistant or tolerant. The Sarapiquí 1 (S1) variety is both Fusarium resistant and drought resistant. S2 is Fusarium tolerant (won’t succumb to Fusarium if managed correctly) and its beans have high levels of vanillin (a natural antibiotic).
Correct shade management.
Adequate mulching. Don’t starve your plantation to death with insufficient mulching.
Planting densities that don’t put individual plants in contact with each other through their root systems.
Choose a planting site that has tropical rainforest and short dry seasons (not more than 3-4 months).
As a final note and not to end so pessimistically, vanilla is normally a “weed” that grows like one in the right environment. Pick the correct planting site and use adequate cultural practices and you won’t get into trouble. Good luck and have a great vanilla year!