Black spot on Den? Sunspot?

[Forever-mango]

I noticed recently some large black spots on some of the leaves of my Dendrobiums. I've asked my local orchid club when they had a show on at the shopping mall, they said it might be from the sun. Its gets direct sunlight in the morning till the afternoon. Im making a shade house kind of place for the orchids later on, but thats the only place that has sunlight. I moved most of my cyms. there and they love it, just the Dens. got some spots.

After it gets a few spots, the leaves go yellow and falls off. On my mounted Den. the same problem.

[digitalgate]

i believe those are fungus infection spot. Quite infectious so pls cut the infected part away. You have cut half the leave away. spray fungicides and increase air movement. Keep plant leave on the dry side to reduce the problem.

[Zsophia]

What fungicide would you recommend?
Can I use it all the time to prevent such problems?

[digitalgate]

I use a variety of fungicides. captan is mild for household use. Mancozeb is also not bad. Remember some of the fungicides are carcinogen.

[Forever-mango]

Thanks DG

Just wondering, what is the causes for these Fungus infections? I tend to get Den. that randomly get one spot.

[digitalgate]

Wet .. humid... That how orchid like it. You can increase air movement so that the water does not stay on the plant for long. But dry wind is devastating for Phalaenopsis bellina.

[bill_asia]

Without a lab test it is hard to diagnose the problem accurately. If the spots were on the bottom of the leaf I would tend to agree with a guess it could be a fungus issue. But because it is on top of the leaf and in direct sunlight I suspect it is caused by water that was heated by the sun which burned a hole into the leaf. This happens often when orchids in direct sunlight. Watch it closely and see if it dries up. It may grow about 15 – 25 % larger before it stops and has completed the die off. If you see other spots are appearing on the bottom of the leaf that are unrelated to the existing spots, and then take appropriate action with fungicide. It never hurts to treat with fungicide on a regular basis (say monthly for an outdoor plant). If it is water damage, change your watering times to a period before or after the direct sunlight is on the plant.