THE PLANTS ARE BUGGED
Even the most constant gardener needs to be on guard for disease and infestation
There’s a sad footnote to the story about Petchsuporn Rapley’s date palms in Doi Saket, Chiang Mai (Green Fingers, March 6). For those who did not read the article, Ms Petchsuporn planted some 100 date palms as an experiment a little over three years ago. A year later two trees started to flower, followed by a few more last year. Braving sharp-as-nails giant thorns, she and her workers cross-pollinated the trees manually and these successfully bore fruit for the first time last year.
This year, 36 Deglet Noor, known as the queen of dates, and mixed Barhee/Deglet Noor varieties were in bloom. However, shortly after the article about her date palm cultivation experience was published, Ms Petchsuporn noticed that some of her trees had droopy leaves. Upon investigation, she realised that snout beetles had surreptitiously laid eggs inside the palms, boring in from underground.
“The upshot was that, yesterday, we had to fell three beautiful date palms that were about to flower, with one female that was also deeply infected, still being closely observed. It was heartbreaking,” she wrote in her email to me.
Not one who easily loses heart, Ms Petchsuporn visited an agricultural fair and bought small trees to replace the ones she had lost.
Meanwhile, Daniel Campion wrote to say that he thinks there are bugs in his Terminalia ivorensis, or huu krajong, tree. “The leaves usually fall off this time of year, then new ones come in. But this year no new leaves are coming in, even though another tree has gotten its leaves,” he wrote.
“It looks like some bugs have gotten into the tree through some old knots where branches were cut off in the past. I’m afraid the tree may die. Do you have any suggestions?” Photos that he sent show tell-tale signs that stem borers are maybe sapping the life out of his tree.
Insect pests, and to a lesser extent, diseases, are the bane of gardeners and orchard growers. I was very happy when seven avocado trees I grew from seeds grew to about three metres tall. During a visit to the farm I noticed that two of my trees had disappeared; the caretaker said they died due to root rot. However, I still had five other trees growing beautifully so I did not think much about it.
Today, not one of my trees is left; they have all become victims of root rot caused by Phytophthora, a genus of plant pathogens. Had we lived on the farm, I would have noticed what was eating my trees at the first sign of the disease and applied a fungicide that kills soil-borne diseases.
Plant diseases caused by Phytophthora are difficult to control chemically. To guard against the disease, I could have mixed the soil with the bio-control agent Trichoderma, which will prey on the fungus causing the disease, before planting my avocado trees. But we had no problems with the longan and lychee trees planted years earlier so I did not anticipate the root rot attack.
The lesson I learned is to either treat the soil before planting a species vulnerable to root rot, or plant resistant cultivars like lychee, longan and mango trees. However, I haven’t given up on avocado and will try again at the start of this coming rainy season.
Some years ago, hundreds of thousands of coconut trees in Phetchaburi, Prachuap Khiri Khan, Chumphon and elsewhere died due to a coconut rhinoceros beetle attack. The insect lays its eggs in compost heaps around coconut trees and decaying palm trunks and stumps.
The eggs hatch after eight to 12 days and the entire larval stage is spent inside the breeding medium. As adults they live up to six months or more, and feed by boring into the growing point or meristem of coconut trees, causing loss of yield and death in coconut trees of all ages. Similar attacks stunt growth of young oil palms.
Researchers tried several methods of controlling the pest, including use of insecticides, both systemic and non-systemic. Systemic insecticides are applied to the soil and absorbed by the plant so that when insect pests feed on the plant, they ingest the insecticide. These are used to reach pests that are otherwise hard to kill, such as rhinoceros beetles on top of tall coconut trees. Non-systemic or contact insecticides are effective against both chewing and sucking pests and will kill virtually all insects upon direct contact.
Another method used was the application of the synthetic chemical attractant ethyl dihydrochrysanthemumate (chrislure) to the breeding site to trap and kill the insect pest. Both chrislure and drenching breeding sites with insecticide effectively suppressed the adult and larval stages, respectively, of coconut rhinoceros beetle and I wonder if they are also effective on Ms Petchsuporn’s date palms. Researchers at the Biological Control Division of the Department of Agriculture, Ministry of Agriculture, may be able to give her some advice, but one thing palm growers should do is clear their orchards of decaying tree parts that could serve as breeding grounds for insect pests.
Meanwhile, injecting Bacillus thuringiensis (commercial preparations are available in gardening supply stores under several trade names) or the insecticide Malathion into the holes made by stem borers may save Mr Campion’s huu krajong tree. To kill the pests, he may have to cap the holes with clay after application, and repeat the process at least three times every 10 to 14 days.
Pesticides are poisonous to humans so handle with care and follow the directions on the label. Wear protective clothing and avoid contact with the skin, and cover your nose and mouth to prevent inhalation of the chemicals. Spray when there is no wind, wash and store all equipment beyond the reach of children, and wash your hands thoroughly and immediately after using the chemicals.
When pruning a tree, cut off the branch beyond the branch collar, which is a swelling of the bark around the base of the branch where it joins the tree. The branch collar is part of the trunk and must remain intact; it is where the tree will naturally heal after the branch has been cut off. If you remove the branch collar you cut into the trunk, and fungal disease or stem borers could get into the wound and eventually kill the tree.