Healthy plants can usually ward off diseases but in our warm, wet summers plant disease pathogens can multiply quickly. Of course, the best disease treatment is prevention. Start by growing plants in soil that drains well and is rich in organic matter. Maintaining an adequate water and nutrition level can keep them healthy. Avoid “loving” your plants to death with too much water or fertilizer.
Healthy plants can usually ward off diseases but in our warm, wet summers plant disease pathogens can multiply quickly. Of course, the best disease treatment is prevention. Start by growing plants in soil that drains well and is rich in organic matter. Maintaining an adequate water and nutrition level can keep them healthy. Avoid “loving” your plants to death with too much water or fertilizer.
Even healthy plants will sometimes succumb to diseases. Accurate diagnosis is paramount to remediation. Many visible symptoms of plant problems are not actually caused by disease pathogens, however. Knowing the symptoms of a few common ailments so that you can diagnose problems early and remedy them is your best defense.
University of Hawaii Plant Pathologist Dr. Scot Nelson suggests using a systematic approach in the diagnostic process. To begin diagnosis you need to know the normal appearance of a plant and its environmental preferences. If it appears normal and is in a suitable spot, you need to determine if the symptoms you are seeing are noninfectious or infectious.
Though many problems begin in the soil, some symptoms are visible above ground. Brown leaf tips, for instance, are usually a symptom of excess mineral salts in the soil. By watering with rain water and reducing fertilizer applications this problem can be remedied. Wilting or yellowing leaves can indicate numerous plant issues. Often this is also a noninfectious issue that may only require a change in your watering or nutritional practices.
Many external issues can cause noninfectious plant problems. They include weather stresses, acid/alkaline imbalances (pH 6.5 is optimal), physical injuries or chemical damage from herbicides or pesticides as well as practices including over or under watering and fertilizing. Noninfectious symptoms are usually distributed over a large area and may affect several different plant species.
Once you have eliminated noninfectious issues, you want to look for disease symptoms. These are changes in the plant’s normal appearance caused by the presence of a pathogen. If the pathogen is attacking the plant above ground you may see some specific symptoms. For instance, rusts and powdery mildews can be easily identified by their appearance. Leaf mosaics are usually associated with viral diseases. Spots and lesions could be symptomatic of a fungal or bacterial disease.
Root or internal stem diseases are harder to detect until they have severely compromised the plant’s ability to absorb and circulate adequate water and nutrients. Leaves dying, plant wilting or extremity stem death can be indications of a possible internal or underground pathogen.
Contrary to public opinion, diseases are not the primary cause of plant decline. Plant diseases can only develop when there is a susceptible host, a disease pathogen and a favorable environment. Infectious disease symptoms are most often restricted to a single species and usually do not spread. Diseases can be bacterial or viral but most are caused by fungi which are absolutely dependent on moist conditions.
One of the most common diseases occurring this time of year is fungal root rot. Infected roots will begin to rot, causing diminished water and nutrient uptake. This can lead to yellowing and wilting of the leaves and can eventually cause plant death. The phytophthora and pythium species, which are responsible for root rot in many plants, are oomycete pathogens and may not respond to treatment with standard fungicides. Infection of roots by root-knot nematodes and bacterial diseases may also have similar symptoms. Maintaining plant health and good soil drainage can help prevent root rot.
Anthracnose is another common fungal disease that thrives in wet conditions. It starts as small dark, sunken spots on the leaves, flowers or fruit and may spread. Infected leaves, flowers and fruit will often drop. Several copper products can help prevent the disease from spreading but eradication is nearly impossible in wet weather when the spores spread easily.
Damping off is a fungal disease that can kill seedlings as they emerge. Several anecdotal prevention methods seem to work. Topping a seedling tray with a thin layer of vermiculite provides some anti-fungal protection. Maintaining a moist, not wet, seeding mix can help and spraying the emerging seedlings with chamomile tea, which is also anti-fungal, seem to prevent the disease.
Once you have identified the problem, treatment options can be found online or by calling one of the U-H helplines. If you prefer to research your plant problems online, Nelson has loads of information about plant problems in Hawaii at hawaiiplantdisease.net.
Diana Duff is a local organic farmer as well as a plant advisor and consultant.
Tropical Gardening Helpline
Email plant questions to konamg@ctahr.hawaii.edu for answers by Certified Master Gardeners. Some questions will be chosen for inclusion in this column.
Joshua asks: I want to put in a hugelkultur. I have some Italian cypress logs that have been sitting for over a year. Would they be okay to use? How about albezia, African tulip or palm logs?
Tropical Gardener Answer: A hugelkultur is a type of raised bed. Translated from German, the word means hill or mound culture. A true hugelkultur can include many compostable elements but always contains tree logs. Leaves, grass clippings, small branches, straw, cardboard, newspaper or any other biomass can be used as well. A hugel bed has logs at the base with smaller branches and other elements stacked up into a tall hill covered with a few inches of topsoil and mulch and then planted. In these beds the gradual decomposition of the elements provides a consistent, long-term source of nutrients. The elements also are good at storing water and providing moisture in dry times and they sequester carbon in the soil. With the logs at the base, beds can be built high where they are easier to work.
The only reason not to use Italian cypress logs for a hugelkultur is they can be slow to rot and are rather acidic. Albizia is also nontoxic. It is a nitrogen fixing tree with soft wood that should break down well. African tulip is also nontoxic and has soft wood as well. The fibrous trunks of palms may be helpful in hosting organisms which help the composting process. For more information on this technique go the Permaculture Research Institute’s website at permaculturenews.org.
Gardening Events
Monday: “Growing Moringa in Hawaii” meeting of Hawaii Tropical Fruit Growers will be from 7 to 9 pm at 81-6393 Mamalahoa Highway in Kealakekua, a half mile south of Konawaena Elementary School Road. Dr. Ted Radovitch from University of Hawaii College of Tropical Agriculture and Human Resources, Manoa will discuss the potential of this plant as a Hawaiian crop. The meeting is free and open to all. For more information, contact Brian Lievens at 895-8753 or greenwizard@hawaii.rr.com.
Farmers markets
Farmers markets
Wednesday: Hooulu Farmers Market, 9 a.m. to 2 p.m. at the Sheraton Kona Resort & Spa at Keauhou Bay
Wednesday: Sunset Farmers Market, 2 p.m. to sunset at the north makai corner of the Kmart parking lot
Saturday: Keauhou Farmers Market, 8 a.m. to noon at Keauhou Shopping Center
Sunday: South Kona Green Market, 9 a.m. to 2 p.m. at Amy B. H. Greenwell Ethnobotanical Garden in Captain Cook
Monday to Saturday: U-Pick greens and produce, 10 a.m. to 4 p.m. Tropical Edibles Nursery in Captain Cook
Plant advice lines
Anytime: konamg@ctahr.hawaii.edu
Thursday: 9 a.m. to noon at UH Cooperative Extension Service in Kainaliu, 322-4892.
Monday, Tuesday and Friday: 9 a.m. to noon at UH CES at Komohana in Hilo, 981-5199 or himga@hawaii.edu.