I make it a habit to inspect my orchids regularly for buds, new growth or more importantly, signs of ill health. Plants of good vigour can display tell-tale signs of disease which if not caught early, can prove devastating.
For instance, this young strap-leaved vanda looks healthy and well established in its pot.
However, a closer look at its base reveals something sinister: dried and ‘woody’ lower roots, and what appears to be some kind of infection at the bottom of its stem (minute earwigs probably). This is definitely not a case of under-watering.
This ‘dry rot’ will advance upward quickly, and eventually kill the plant when all the roots have been dessicated.
I strongly suspect that the disease is Fusarium, a devastating fungus that causes plants to weaken, wilt and eventually die.
There is no available home gardening fungicide that can satisfactorily ‘cure’ a plant of Fusarium. Thiophanate methyl may to work to some extent, but the fungus tends to make a comeback in a plant that has been infected before.
The best way to help a Fusarium infected plant is to catch the disease early. In this vanda’s case, I cut the base of the stem up until where the cross-section is clear of the brown, sometimes purple woody decay. It is important that the remaining top-portion of the cut has at least one healthy root (one root goes a long way).
The healthy cutting is repotted in a clean, sterilised growing medium and pot. It is also important that the bottom tip of the cut stem does not come in contact with any potting medium.
The diseased stump is discarded or burned to prevent further spread of the disease.
I’ve had my battles with Fusarium rots, and find that the best way to prevent them is through a monthly watering of gardening lime solution to raise the pH levels (acidity). Sometimes, I sprinkle some lime dust onto the crowns of the plant and let it run-off the crown down the stem on rainy days. Gardening lime also provides calcium, a good mineral for growth.
Secondly, I don’t fertilise too often. Frequent fertilising can increase incidence of Fusarium attacks, possibly because fertilisers tend to reduce pH levels (acidity) around the plant. A lower pH favours the disease.
Lastly – and this is something which I’ve started with good results – I use a trichoderma based protectant every 2 months or so. There are many articles written about the trichoderma fungus; more can be read here.
I do believe that cultivating an environment with naturally occurring beneficial bacteria and fungi may do wonders in preventing diseases. Good microbes can compete and edge out the bad ones. In light of this practice, I have also reduced the need for commercial fungicides.