just a couple of more cents from long time vanda growing in South Florida.... Under watered vandas will ALWAYS start dropping the older leaves (at bottom of plant). Plant will drop leaves to compensate for lack of moisture and will continue to do so until it reaches the balance of water intake and amount of leaves that the reduced water can support. This frequent phenomenon among new vanda growers generates what we call "palm trees". Imagine a 2 foot tall vanda with 3 pairs of leaves at the top, a long bare trunk and roots at the bottom of the plant, roots will be mostly dried and dead. These palm trees got that way because of not enough moisture,

The point being that vanda DO NOT DROP leaves from lack of water from their middle section unless there is a problem in that area.

I don't know...

I'm not so sure it's Fusarium. Fusarium does spread all over, and it does so extremely fast. Since it is showing signs of blackened areas closest to where the leaves connect with the petiole, near the stem, eventually, not only will it cause the leaves to drop, but it will turn that thick stem the Vanda's got into mush. That's how it lives up to its name "wilt".

The thing with Fusarium is that it does exist. It can be a problem, but in the grand scheme of things, it's kind of rare. Most people don't experience it all that often, and when it does, people usually remember what it does to the plant.

I've had it infect a Huntleya wallisii of mine in the past. I had grown it in an area where it was warm, but not too warm, where it had very little light and was humid. Those are perfect conditions for Fusarium to take hold. Come to find out, these conditions did not favor the orchid as well, (it needed to be grown much brighter). This species has a thick rhizome, once the Fusarium infected that rhizome, it turned it into mush and it snapped in half. I could see that characteristic watery purplish-pink ring around where the vascular tissues were. Ive only seen this disease maybe once or twice in my years of growing orchids. The catalyst that starts it is a weakened plant with a weakened immune system.

If deanna wants to take precautions, it is best to use a systemic fungicide. Contact fungicides don't do much if the disease organism has already infected the host.