Dendrobium with black spots on leaves

[Jling09]

I bought a couple of Dendrobiums from my favorite mail-order orchid place, and liked them so much I bought another few. The plants in the first shipment seem healthy.

In the second shipment, though, a couple of the plants had black spots on the leaves and yellowing (although, curiously, not around the spots themselves).

The first two pictures are of a Dendrobium aurantiflammeum; the third picture is of a Dendrobium tobaense var. giganteum.

What's going on with these plants? I repotted them in fresh soil (a seedling mix with small orchiata, Turface MVP, and some small cork chunks) but have not otherwise done anything to them but water with a half-strength solution of Jack's 30-10-10. Have otherwise not used any Physan or antifungals.

Should I throw them out, or can they be saved? Am I watering them too much? How do I know when they should be watered?

Detailed instructions would be super helpful - I seem to have a hard time keeping orchids alive. Thanks!

[estación seca]

Welcome to the Orchid Board!

They both looked OK to me in their original little pots. I would definitely not have repotted these until they began making new growth. That might not be until next spring. In appears to me the Hausermann's small seedlings I have bought have probably been repotted into the arrival pots within 6 months of sale, and the medium is good for at least another year. I don't repot them until they are outgrowing the little pots.

Some leaf yellowing and dropping may happen to warmth and humidity loving Dendrobium seedlings when shipped. Small black spots like that are relatively common on thin-leafed orchids in less than perfect conditions, like spending time in a dry dark box. I would just watch it. If it does not progress I would not worry about it.

You can read more about Den. tobaense here using the Search feature. It's a member of a group called the nigrohirsute or Formosae Dens. They prefer to be humid, moist and brightly lit all year. This one likes moderate temperatures. They require a lot of air at the roots. Notice the plant on arrival was potted in large chunks of bark, to ensure the roots had good air. It might not be easy to provide all this in a centrally heated Midwestern home in winter. Den. tobaense is not the easiest of this group to grow. You can find threads here on Orchid Board from people growing it.

I always look up species I'm contemplating buying on the Internet Orchid Species Photo Encyclopedia. I read the information about its requirements. I then come here to Orchid Board and use the Search feature to see how people here grow it. Some orchids are much easier to grow than others.

In my opinion, repotting most seedlings on arrival when they're not in active growth is not a good idea. I realize other people say they repot all orchids on arrival. I disagree with this practice most of the time. I only repot when plants are beginning to make new roots. Seedling roots are especially easy to damage when repotting.

Dendrobiums (and many other orchids) only make roots when a new growth is forming. When you repot you almost always damage existing roots. A small seedling with damaged roots may struggle to take up enough water until it grows again, and makes new roots. These two Dens might not grow again until next spring.

I haven't grown Den. aurantiflammeum. IOSPE says it is a relative of Den. cruentum and Den. faciferum, which I have grown. Den. aurantiflammeum is a warm to hot growing plant from lowland, very humid evergreen forests. This group of Dens needs very large air pockets around the roots, and frequent watering. H&R in Hawaii ships Den. faciferum seedlings in small plastic baskets with only a few chunks of extra-large bark. I don't know whether a smaller-particle potting mix is a good idea for either of these species. If you pay close attention, and don't keep them sopping wet, is can certainly work. But remember they both need plenty of air at the roots. I hope people here with experience growing them join the conversation.

In terms of what you should do now - they would prefer higher humidity than you probably have in your home. Low humidity will predispose to leaf spots and spider mite attack. They are small enough you could set an ultrasonic humidifier to blow on them. Or you could put them into a terrarium to raise the humidity. Both would do well outside in a typical Midwestern summer, in bright shade.

I probably would not repot them again. Water them and lift with your hand. Note the weight. When they are getting almost dry they will be a lot lighter. Don't let either dry out completely, especially the aurantiflammeum. Next spring, or when they next begin making new roots, I would consider repotting into something with larger chunks.

They won't need fertilizer in winter when they're not growing.

The tobaense will probably be happy at typical home temeratures. The aurantiflammeum will probably be fine too, though it might prefer warmer.

As for light - a typical Midwestern south-facing window is probably OK in winter, but don't let them get too close and freeze. Extra electric lighting would probably be appreciated.