By Nick Sakovich
Q: Arriving in Hilo from Europe several years ago, we were presented with several Norfolk pine trees in a pot to use as our first Christmas tree. … We noticed that several branches had gone brown/died off. We did notice, also, some very small webs at the base, though are unsure if this has any significance? Expecting it to recover in dappled sun conditions with plenty of water and some fertilizing; we noted recently that the browning has continued, though the trees have continued to grow. Any ideas what is causing the browning of branches (we notice some of the keiki Norfolk trees in pots in the garden have similar browning)? Any advice/assistance would be gratefully appreciated as this coming Christmas is a particularly special one, with a reunion of loved ones from afar. — R & A
A: The Norfolk Island pine or Australian pine (Araucaria heterophylla) is a southern hemisphere conifer, native to the Norfolk Islands and Australia. It may reach 200 feet in height and is salt tolerant. Propagation of Norfolk Island pine is by seed which germinates fairly rapidly in 10-15 days.
Now to address your question about the browning of the branches, here are some troubleshooting tips:
1. In their native environment, Norfolk Island pines enjoy moderate temperatures, moist air and bright light.
Therefore, dry air, or a relative low humidity, dry soil or infrequent watering and low light can cause the needles to turn brown and drop, leading to the loss of the lower branches.
2. A fungal disease called anthracnose will cause needles to dry up and slowly die. As a result, whole branches will turn brown. The infection begins as small necrotic (dead) spots on the needles. Fungal fruiting bodies can be seen with the naked eye in these dead areas; they appear as tiny black specks. When watering, keeping the water off the foliage will help prevent the disease from taking hold.
3. Root rot can cause a browning of the branches. Plants infected with root rot grow slowly or not at all. This disorder, although caused by a fungus is promoted by overwatering.
In addition, insect pests can present further problems. They include mealybugs, scale, and thrips. Mealybugs appear as white, cottony masses in leaf axils, on the lower surfaces of leaves and can also infest the roots. Honeydew and sooty mold are often present.
Q: My question is regarding a wiliwili tree that was attacked by the gall wasp. I had cut it back to ground level, and since then it will grow to about 3 feet tall with nice growth, but any higher the leaves are all infected. Is there anything I can do to help this tree recover? — Bob in Hawaiian Shores
A: Yes, there are two things to help the wiliwili tree recovery. First, you can purchase a systemic insecticide with the active ingredient Imidacloprid. This material can be foliar or soil applied and should last at least one month and possibly longer. If you are dealing with just a few small trees, the cost will be relatively low.
Second, it is important to note that a parasite specific to the gall wasp has been identified and released. A team of entomologists from the Hawaii Department of Agriculture and University of Hawaii’s CTAHR traveled to Africa searching for natural enemies of this pest. They were successful in finding a tiny parasitic wasp which has proven efficient in controlling the gall wasp; it will not attack any other insect or plant. The first release was made in November of 2008 in Honolulu. Even though it has spread throughout the Big Island, it will take time for the parasite to establish and for the gall wasp population to diminish. Biological control is never instantaneous.
As with most biological control measures, this beneficial wasp will never completely rid the area of the gall wasp. Wiliwili trees will always show some damage but they should be able to grow into good sized, fully leafed trees.
Hilo resident Nick Sakovich is a professor emeritus of the University of California. He has worked in the field of agriculture for 30 years, and is a member of University of Hawaii Master Gardeners. E-mail your questions to Sakovich at firstname.lastname@example.org.