Common Landscape Plant Health Problems in Hawaii Dr. Scot C. Nelson Associate Specialist (Plant Pathology) University of Hawaii at Manoa College of Tropical Agriculture and Human Resources Cooperative Extension Service Hilo, Hawaii October 22, 2009 1. General landscape problems a. Site establishment b. Landscape installation c. Landscape maintenance 2. Disease and pest problems ▪ Nutrient deficiencies ▪ Fungal diseases ▪ Nematode diseases ▪ Insect pests ▪ Phytotoxicity ▪ Abiotic factors (lightning) ▪ Rats http://www.ctahr.hawaii.edu/oc/freepubs/pdf/PD-67.pdf Visit our new website – “The Plant Doctor” http://plant-doctor.net General landscape problems General landscape problems a. Site establishment 1. Land preparation 2. Poor fill soils 3. Selecting wrong plants for site 4. Grass and shrubs in same irrigation zone 5. “Parking lot” diseases of trees 6. Brackish irrigation water as source of irrigation 1. Land preparation Bulldozer scraping can break up and remove large rocks, deepen the rooting zone level, and remove weed growth. It can have long-term, detrimental effects on plant growth. Bulldozing near ‘ōhi‘a lehua trees can cause their death by crushing, breaking or damaging roots. Scraping off topsoil with bulldozers can expose hardened, nutrient-poor subsoil layers. Plants do not grow well in clayey hardpans due to poor aeration or plant nutrient imbalances, and may die from root, stem disease, or nutritional deficiencies. 1. Land preparation Solutions: - Keep bulldozers at least 10–20 feet away from ohia. - Harrowing preserves soil structure and topsoil. - Uptake of water and nutrients requires oxygen, so ensure good drainage during site preparation. - Compost improves the properties of clay soils. - Use herbicides rather than soil scraping. Land preparation - drainage problems Good drainage Poor drainage 2. Poor fill soils Do not use heavy subsoils collected during the process of acquiring good topsoils. Heavy subsoil is usually high in clay, poorly drained, poorly aerated, and lacking nutrients. The nutritional, physical (compaction, poorly draining), and disease problems associated with these poor subsoils used as topsoil can persist for many years. Soils used to prepare and fill new landscapes can also be infested with plant root parasites known as root-knot nematodes (Meloidogyne species). 2. Poor fill soils Solutions: - Use only topsoil for landscapes, no subsoil. - Test and treat for plant-parasitic nematodes. - Add gypsum and compost to improve soil structure, organic matter content, and plant nutrients. - Because most topsoils and original soils do not drain well, one would save money by adding large amounts of compost to the original soil. Compost is best but is also required in large amounts. 3. Selecting the wrong plants for a site Plants may not thrive where they are planted in microenvironments (consisting of various degrees of sun, shade, soil, pH, etc.) to which they are not well adapted. They may grow weakly, decline, and die, resulting in “disposable landscapes” that must be replanted continually. 3. Selecting the wrong plants for a site Solution: - Match the plants intended for a landscape to the environments that are best suited for their growth. Important factors: Moisture / Rainfall Soil or growth medium (physio-chemical) Sunlight / temperature / shade level Elevation 4. Grass and shrubs in same irrigation zone Shrubs tend to receive far too much irrigation water when placed on the same irrigation schedule as grasses. Grasses are more shallow-rooted than shrubs and may need more frequent watering in hot and sunny environments. If plants with a wide range of moisture requirements occur in the same irrigation regime, the ones requiring the least amount of water may develop problems due to loss of soil structure, root suffocation, nutrient deficiencies, and root rot diseases. 4. Grass and shrubs in same irrigation zone Solution: - Keep grasses on a separate irrigation schedule from shrubs, where possible. 5. “Parking lot disease” of trees Trees may suffer from what I refer to as “parking lot disease.” The problem usually occurs where trees are planted in relatively small openings within large paved areas. Sometimes irrigation is not installed or trees are planted too deep. Roots can rot or suffocate or trees suffer from drought and nutritional deficiencies. Oil or gas leaks from vehicles also damage plants. 5. “Parking lot disease” of trees Solutions: - Allow a large enough opening in the asphalt for tree roots to breathe and receive sufficient rainwater, irrigation and fertilizer. - Pave the lot before planting. - Protect planting holes from oil or gas runoff from the parking lot. Protect trees from ramming by vehicles. Large space for root aeration and rain water reception Soil cover for weed control Barrier for runoff control and for foot and vehicle traffic management 6. Brackish irrigation water The sodium in the rather brackish water in the North Kona district of the island of Hawai‘i can destroy soil structure by breaking down and dispersing soil materials into finer and finer sized particles. This causes soils to eventually lose aeration, become hard packed and drain poorly. The salinity may also cause root rot. 6. Brackish irrigation water Solutions: - Do not over-irrigate plants with brackish water. - Use mulch and compost to protect soil structure. - Aerate soils periodically. - Treat brackish water where possible before irrigating with it. General landscape problems b. Landscape installation 1. Trees planted too deep 2. Plants placed in wrong growth medium 3. Planting pot-bound plants 4. Large plants too close to a structure 5. No compost or mulch installed 1. Trees planted too deep If tree stems are planted too deep and then over- irrigated, their roots can suffocate and rot and the stem can decay and even girdle beneath the soil line, leading to plant decline. This commonly can affect coffee, longan, monkeypod, shower trees, and other valuable woody plants in landscapes . 1. Trees planted too deep Solutions: - Make sure the hole is not too deep and fill it in with soil and pack the base slightly before transplanting. The proper depth to plant a tree is to place it in the soil so that the soil line is even with the top of the first lateral root connected to the stem; - Do not place irrigation emitters immediately adjacent to stems for long, but place them at or near the tips of the roots (the drip line of the plant) Planting too deep 1. Trees planted too deep Solutions (continued): - If over time the new transplants sink vertically, dig soil away from the stem until you see the first lateral root. Also: Use herbicide, mulch, or compost to keep a grass-free zone near the plants. However, do not allow contact between mulch and tree stems, because stem decay can occur when the mulch becomes hot during its composting. 2. Plants placed in wrong growth medium When plants are grown in the wrong growth medium, they can develop root and stem diseases and have nutritional deficiencies that predispose them to unthrifty growth. 2. Plants placed in wrong growth medium 3. Planting pot-bound plants A pot-bound plant is one whose roots encircle excessively within a pot, having grown for too long in the pot. The plant essentially becomes too large for the pot. Pot-bound plants may eventually die or not grow well after they are transplanted into landscapes. The curling roots may not expand in the soils and may “choke” the stem of the plant. 3. Planting pot-bound plants Solution: - Avoid planting pot-bound plants in landscapes; inspect them before purchase or use and discard them. - Some growers may choose to clip or loosen the pot-bound root systems with shears before planting to interrupt the root curls. Installing pot-bound plants No compost or mulch installed Excessive irrigation that occurs in many professional landscapes can destroy soil structure and later its chemistry, making it very difficult to grow plants successfully. Mulch or compost, considered to be unsightly by some people, can help preserve soil structure and improve nutrition. General landscape problems c. Landscape maintenance: 1. Excessive irrigation - root and stem rots - loss of soil structure 2. Nutrient management 3. Over reliance on pesticides 4. Soil pH problems 5. Irrigation emitters position left unaltered 6. Failure to recognize and treat problems early 7. Poor pruning 8. String trimmer injury 1. Excessive irrigation Too much irrigation can cause root and stem rot and can destroy soil structure, creating a “disposable landscape” phenomenon. Solution: -Do not water on a schedule, but as needed by plant and weather factors. - Avoid sprinkler irrigation impaction on woody plant stems and disease-susceptible foliage Xanthomonas bacterial leaf blight A common disease where these plants receive overhead sprinkler irrigation. Irrigation problems with trees (impact) Sprinkler irrigation impaction on woody plant stems or over-watering can cause stem bark diseases. Stem canker on shower tree – irrigation was excessive 2. Nutrient management Nutrient deficiencies are common in landscapes. Plant species differ in their nutritional requirements; hence it is important to supply important plants with the proper blend of macro and micro elements. Some common deficiencies in Hawaii: - Potassium - Iron - Boron - Nitrogen - Magnesium Nutrient Deficiencies for Palms Potassium, boron magnesium & nitrogen are most common Potassium regulates the opening and closing of stomata Potassium deficiency on Kentia Potassium deficiency Potassium deficiency