Problems with Dendrobium orchids, Fiji – April 2021
The company affected grows and supplies orchids mainly Dendrobium orchids - for the hospitality and tourism industries. Based in Nadi, Fiji, it has been in the business for over 20 years. It has several large screenhouses where thousands of plants are kept on raised benches.
The company from time to time imports new orchid plants from biosecurity certified producers overseas, including Hawaii and Thailand, following the prescribed quarantine regulations.
Dendrobium orchid plants in one of the screenhouses have increasingly been showing loss of individual flower buds mostly in the middle ranges (flowers No. 5-8) of the sprays, as well as some dieback and bud drop from the tip. Common symptoms also include a curling of the tip of the spray (flower spike), and showing a brown scarring mark on the main stem. Affected flower buds initially show a yellowing / browning at the base of the flower stem, rapidly spreading over the bud. A few buds (< 1%) show fungal infection, but this could be a secondary infection. Some deformation of flower buds was also observed, but it does not appear widespread. Early flowers (No. 1-4) are developing and flowering normally.
Staff mentioned that plants of the white Dendrobium variety
‘Uniwai Mist’ (parent ‘Jacklyn Thomas UH 800), and of a variety with yellow
flowers that was imported with several more varieties from Thailand 4 years ago are most severely affected.
Closer inspection of the sprays and the individual flowers with the naked eye does not indicate any fungal infection, nor insect damage. There are no signs of thrips nor mites.
The problem was first observed some 4 years ago. Around that time a shipment of Dendrobium orchids was imported from Thailand. Remarkably, the problem is largely contained to this specific screenhouse where these plants are kept, however nearly all plants and sprays are affected. With the value of the spray determined by the number of consecutive flowers, flower production is practically worthless.
Dendrobiums kept in a second large screenhouse located several 10’s of meters away have been far less affected. Also, staff mentioned that when Dendrobium plants with newly developing sprays are moved to a location away from the screenhouse the development of the flowers is normal, without symptoms of flower drop.
The plants are treated with insecticides following an alternating pattern, mostly for control of spider mites but also for orchid beetle and other pests, as follows:
Orthene at 10-day intervals for 3 weeks;
Malathion at 3-day intervals for 1 week;
Diazinon at 3-day intervals for 1 weeks.
Other pesticides used are Robar, Multi Guard, Super Guard – mainly for control of thrips.
Management informs that tools (pruning scissors) are cleaned and dipped in sterilizing solution after use on individual plants.
Comments and suggestions much appreciated!
Wilco Liebregts, Suva, Fiji
If Bob's recommendation does not work out, I would be looking into those deformed plants. You do say that deformed plants are rare, but you mention "
There a internet mention of bacterial disease in Hawaii here:
More photos of the entire plants would be good
Some of those buds certainly look like they have small lesions and one at least a rot of some kind. My first guess would be anthracnose (Colletotrichum sp.) but it would need lab confirmation. I suggest you give some to Mereia Lomavatu at Koronivia to incubate up and have a look at.
I note the spray programme does not seem to include fungicides.
As a start I would suggest:
1. Hygiene. Remove all dead and dying material from the shade house and bury or compost – old leaves, dropped flowers and buds etc. If it is a fungal problem that will help remove sources of inoculum.
2. Spray developing inflorescences with a fungicide such as a DMI + strobilurin mixture e.g. Amistar Top (difenoconazole + azoxystrobin) or equivalents (there are many) . Fungicides containing prochloraz are also effective against Colletotrichum. Mereia will be able to advise on what is available. Otherwise let us know what fungicides you can find and we can advise from there.
Hope this is helpful.
From the samples taken last week, there were a lot of larvae about 3mm long exiting from infested bloom samples stored in ethanol. We will need to get more samples to rear to adult stage but believe that this is possibly the cause any likely the Orchid blossom midge (Contarinia maculipennis) or similar. Refer to CABI factsheet attached for more details. We will get better pictures of the larvae and hopefully adult for authentication but attached a picture of larvae for now. The webbing that was observed all over the blooms indicates a likely severe spread to the whole area and a clean up will be necessary.