Fungal rots can be an occassional problem when growing vandas outdoors. When they attack, treatment is challenging and the plant usually dies. The truth is, any orchid that has died on me did so because of rot, whether the disease was fungal or bacterial. Here are two of the worst kinds to find on vandas.

Dry rot

This one is most devastating because when it attacks, the plant never recovers and death follows swiftly. It’s frustrating too, because I could not identify the pathogen. Here are the symptoms:

1. It starts at the roots (as always, right?). First, there is a shriveling of the tips. It might look like fertiliser burn, but it’s not.

Vanda root rot-2

A shriveling of a root tip is a first suspicious sign. This is NOT chemical burn.

2. When you water the roots until they are green, you will see a yellowish discolouration where the shrivelled tip meets the water-soaked green velamen. This yellowing advances up the root very quickly and the whole root shrivels and dies. It’s inevitable.

Vanda-root rot

A dry infected root (left) and the same root wet (right). Note the yellowing points (circled) that advance upwards rapidly.

Cutting the root off an inch above the yellow point does not help. The yellowing and die-back continues upwards from where I made the cut, which tells me that whatever pathogen that’s causing it is already in the plant’s system.

3. Soon, the leaves turn yellow and fall off one-by-one, starting from the bottom-most leaf until it reaches the crown.

Vanda rot

Leaves will turn yellow and fall off in no time.

4. The plant collapses and dies. Another one bites the dust!

My guess is the disease, pythium or phytophthora, but I’m not entirely sure. There is none of that ‘black rot’ symptom that is typical of this fungal attack. Furthermore, fungicides developed for this treating disease, like Alliette or Subdue, do not seem to have an effect.

Could it be the dreaded Fusarium? Perhaps, but there is none of the tell-tale purple rings when I cut the stem.

The best thing to do is isolate the plant, pray for the best, and discard it immediately after it has gone kaput.

I’ve bid RIP to many vandas because of this dry rot. If anybody can identify this disease kindly drop me a note.

 

The ‘fish-egg disease’, sclerotium

Next, is sclerotium rolfsii, or the ‘fish-egg disease’, so called because of how the sclerotia resemble ‘fish-eggs’. This is another bad, bad rot. Usually, it starts at the base of the stem (see pic below) and sometimes, white silky strands of hyphae appear. It spreads rapidly.

Sclerotium

Tiny fish-egg-like spots are a common sign of Sclerotium rolfsii. 

To treat, cut the stem about an inch above the rot, making sure that the top half has at least one healthy root. Sterilise your cutting tools beforehand of course, by flaming it with a lighter.

Soak the whole plant in fungicide for a few minutes, throw away all the old media (if any), and repot. The plant may come clean and recover, or it may not.

The disease could also make a reappearance on a ‘cured’ plant. In which case, unless the vanda is a prized one, it’s best to burn or discard the whole orchid to prevent spreading of the disease.

The good news is that sclerotium rolfsii rarely makes an appearance on my orchids, being a soil-borne pathogen. Maintaining drier, clean environment with some balance of organic microbes can help keep it in check.

 

How to minimise these rots

As you may have guessed, prevention and good cultural practice is key to keeping infections at bay. In my limited experience, here’s what I find works best:

1. Don’t over-fertilise. I think heavy feeding, especially with inorganic fertilisers, kills mycorrhizae or other natural protective microorganisms that thrive at the roots and in the media. Less is more certainly holds true.

2. Apply microbial innoculants. There are good fungi that protect plants by edging out the bad ones through competition. Trichoderma-based ones work best, although they are really hard to find and have short shelf-lives. I’m trying to cultivate some, which I’ll document my progress in due course (or not, if I’m unsuccessful).

3. Give good water. I try to avoid water straight from the tap nowadays because of the chlorine content, unless I can’t help it. It’s ok, tap water won’t kill plants, but I think the chlorine is bad for the good germs in the long term. I prefer letting the water sit overnight before using it. And of course, rain water’s the best.

In my experience, commercial fungicides aren’t very effective against many of these rots, even as a preventive. They are good against leaf spots and bud wilts, but for rots – sadly, no. I’ve given up spending $ on them. That’s not to say that you can’t try them; use them by all means, if it works for you.

So keep an eye on your plants, don’t feed them too heavily and let rots be gone.

My Rhynchovanda Sri-Siam – lost to sclerotium infection.