Tropical Gardening Helpline: Summer heat breeds pesky garden problems

  • Powdery mildew develops on leaves that get moist but then dry out. (Photo courtesy /

  • Leaf spot diseases usually leave black or dark brown spots on a plant’s leaves. (Courtesy photo /

Ray asks: I have noticed lots of new problems in my garden lately. I don’t think I have made any maintenance changes, what could be going on?

Tropical Gardener answer: Welcome to summer in Kona. Though we may not notice the subtle seasonal changes, our plants as well as insect and disease cycles are often affected by the slight shifts in temperature, rain frequency and day length.


Longer and warmer days will often increase insect activity and reproduction. Increased humidity definitely encourages the growth and spread of bacterial, fungal and viral diseases.

If you look closely and can identify insects, you can proceed to control them. The insects that suck plant juices like aphids, scale, mealy bugs and whitefly require contact by oil and soap to die. Usually spraying several times with a mix of neem oil and Safer soap will knock out a small infestation. Using ant stakes to control these “farmers” of sucking insects will also help.

If you have sucking insects and ants you probably also have the disease called sooty mold which appears as a black coating on the leaf surface. Blast it with water, rub it off with a soapy glove and get rid of the ants before it gets too established.

If you have shot-holes in your leaves or chewed edges you probably have either Chinese rose beetles or caterpillars feeding on the leaves. The rose beetle feeds right after sunset. Go out then to look for them and collect them. If you put lights on the plants during the night, they will probably not feed there. If you find or suspect caterpillars, try Bacillus thuringiensis, known as BT or Dipel. It kills caterpillars in a few days.

Of course, as insects move among your plants they can spread disease organisms that will develop into a problem under the right conditions. High humidity provides the ideal conditions. You can reduce humidity by avoiding over-watering and ensuring that irrigation water goes into the soil rather than on the leaves. Additionally, careful pruning that encourages good air flow between branches can be helpful.

Fungal diseases often occur in rainy or high humidity conditions. The names gray mold and black spot actually describe the symptoms of these two fungal diseases that thrive in warm, wet environments. Powdery mildew appears as a gray or white powder on leaf surfaces but actually prefers drier situations to thrive.

All can be treated with fungicides. You can make your own by mixing 1 tablespoon of baking soda, 1 tablespoon of oil and 1 of liquid soap in a gallon of water and spraying it on the affected leaves. Garden copper or sulfur formulas are also effective and low tox. A product called Serenade is organic and effective against lots of common diseases.

The best way to prevent insect or disease attacks is to keep your plants healthy, increasing their ability to ward off pests.

Email plant questions to for answers by Certified Master Gardeners. Some questions will be chosen for inclusion in this column.

Diana Duff is a plant adviser, educator and consultant living part time in Kailua-Kona.

Gardening Events

Saturday: “Work Day at Amy Greenwell Garden” from 9 a.m. to 12:30 p.m. Meet at the Garden Visitor Center across from the Manago Hotel in Captain Cook. Volunteers will be able to help with garden maintenance and are invited to bring a brown bag lunch. Water and snacks provided. Call Peter at 323-3318 for more information.

Wednesday: Gerrit Takasaki of Hawaii Hybrids will be discussing dividing and repotting cattleya orchids. A potluck starts off this month’s Kona Daifukuji Orchid Club meeting at 6:30 p.m. at the Kona Daifukuji Soto Mission in Honalo. Free.

Farmer Direct Markets

Wednesday: “Hooulu Farmers Market” 9 a.m. to 2 p.m. at Sheraton Kona Resort &Spa at Keauhou Bay

Saturday: “Keauhou Farmers Market” 8 a.m. to noon at Keauhou Shopping Center

“Kamuela Farmer’s Market” from 7:30 a.m. to 1 p.m. at Pukalani Stables

“Waimea Town Market” from 7:30 a.m. to noon at the Parker School in central Waimea

“Waimea Homestead Farmers Market” from 7 a.m. to noon next to Thelma Parker Gym in front of Thelma Parker Library.

Sunday: “Pure Kona Green Market” 9 a.m. – 2 p.m. at Amy Greenwell Garden in Captain Cook

“Hamakua Harvest” 9 a.m. to 2 p.m. at Highway 19 and Mamane Street in Honokaa


Plant Advice Lines Anytime: Tuesdays &Thursdays: 9 a.m. to noon at UH-CES in Kainaliu – 322-4892

Mon., Tues. &Fri: 9 a.m. to noon at UH CES at Komohana in Hilo 981-5199 or

Leave a Reply

Your email address will not be published. Required fields are marked *


By participating in online discussions you acknowledge that you have agreed to the Star-Advertiser's TERMS OF SERVICE. An insightful discussion of ideas and viewpoints is encouraged, but comments must be civil and in good taste, with no personal attacks. If your comments are inappropriate, you may be banned from posting. To report comments that you believe do not follow our guidelines, email