Disease

a. Phytophthora root rot (Phytophthora cinnamomi) is the only serious disease of avocado in Hawaii; it also affects pineapples, pomegranates, macadamia nut, and many other uncultivated species such as Ohia. Disease symptoms are trees with sparse foliage, some branches completely lacking leaves, branch leaves only at the tips, and pale green to yellow leaves frequently with the dead (necrotic) leaf margins, caused by increased sensitivity and accumulation of chlorides. Wilting is common.

Dispersal is by infected nursery stock and other plant material, and soil and water movement. Infection can occur at any age. The fungus attacks the roots resulting in a lack of new growth, dieback, and even death of the tree in severe situations.


b. Many avocado orchards appear to have low-level infestation of Phytophthora root rot; however, the environmental conditions in Kona -- moderately acid soil pH, high organic matter, and good drainage -- help to suppress disease development.


c. Excess soil moisture leads to development of the disease. Serious problems arise in orchards on soils with poor surface and/ or internal drainage.


d. The disease organism can persist in the soil for very long periods of time, even in the absence of a living host.


e. Control measures should consist of a combination of practices:

  1. Use of clean nursery stock grown in soil that has been steam-heat sterilized or fumigated with methyl bromide.

  2. Modification of soil moisture and pH to 6.2 to 6.5 on a Wahiawa clay soil resulted in disease suppression as shown by CTAHR research (M.F. Falcon, R.L. Fox and E.E. Trujillo. 1984. Interaction of soil pH, nutrient and moisture on phytophthora root rot of avocado. Plant and Soil 81:165-176.). Gypsum supplied calcium without raising the soil pH too high under these conditions when growth was reduced at soil pH above 6.6. However; specific calcium recommendations have not been developed for Kona conditions.

  3. High levels of organic matter, partially by contributing ammonium nitrogen (NH+) and other organic compounds, suppress growth of the fungus. Mulching or grass ground covers should be encouraged, but not disking or mechanical weeding. The shallow surface-feeding roots of avocado are easily damaged and wounds are quickly infested with root rot.

  4. Avocado does not tolerate high level of chlorides (Cl-) in the soil or irrigation water. Phytophthora-infected trees are more susceptible to chloride toxicity than healthy trees. Fertilizers with Cl- such as muriate of potash (KCl) should be avoided, if infection by root rot is possible. Soils known to have been previously irrigated with saline water or exposed to sea water (not likely to be a problem in mauka Kona) should be avoided. If well water is to be used for irrigation it should be checked for total salts content. Soluble salt levels greater than 120 ppm may result in chloride toxicity, particularly in areas on heavier soils where Phytophthora root rot is likely to be found or to become a problem.

  5. The fungicide metalaxyl (Ridomil 2E) is registered, but may not be practical or economical in the long-term for Kona farmers especially as its effectiveness deceases over time. Other more effective materials have not been approved yet for use on avocado in the U.S. such as Aliette and more recently the use of phosphoric acid and potassium hydroxide. Development of the new technology to trunk-inject disease phosphorous acid and potassium hydroxide has shown dramatic results in South Africa and Australia.

  6. Preventing the movements of soil and water from infected areas in the orchard to non-infected areas is very important, but it may not be a practical or economical control measure in Kona.


f. Moderately resistant rootstocks (e.g., G-6 and Duke-7) are used with some success in southern California. Preliminary studies show them to be less effective in Hawaii. Several mainland commercial nurseries sell grafted and ungrafted trees of clonal rootstocks. CTAHR has selections tentatively identified as being highly resistant and more vigorous than those from California. Further tests are being made with 'HAES 7315 Sel.l" under field conditions.


g. Other fungicides are registered for use on avocado in Hawaii:

Tri-Basic Copper Sulfate (WP)
Kocide 606 Flowable Agricultural Fungicide
Kocide 101
Clean Crop Micro Cop 1-2
Ortho Phaltan 50 (WP) (folpet)
Blueshield


Fungicides for which tolerance have been established though lacking a SLN (local needs) label for Hawaii are:

Chemical nameTolerance (parts per million (ppm) in fruit
benomyl 3
captan 25 (post harvest only)
terrazole0.15
thiabendazole (TBZ)10


h. Wholesalers, retailers, and consumers continue to experience postharvest disease losses. A research project was initiated by CTAHR with GACC funding after the 1985 Avocado Industry Analysis (no. 2) to investigate possible solutions. The scientist met with the Hawaii Avocado Assn. to develop a control strategy. He learned that few farmers would practice a preharvest spray program to control postharvest diseases because of the terrain, cost of equipment, labor, and time. The HAA Board recommended a research program to investigate the efficacy of postharvest applied fungicides that could be applied in the grower's or packer's facilities.


i. CTAHR determined that postharvest disease losses have been traditionally attributed to anthracnose (Colletotrichum gloeosporioides). In high rainfall areas of east Hawaii and probably other areas in the state 75% of the surface body rots commonly called anthracnose, are caused by C. gloeosporioides.

However, on 'Sharwil' fruit from west Hawaii (Kona) Dothiorella sp. is more commonly the cause of surface body rots. Furthermore. other species (Phomopsis sp., Dothiorella sp, and Stilbella cinnabarina) and not C. gloeosporioides are associated with postharvest wounds which cause stem-end rot, the next most frequent postharvest disease. Neither stem-end or surface body rots are not prevalent until the fruit soften.


j. Copper based fungicides registered for Hawaii applied in the field and cultural practices should control these diseases.


k. CTAHR is researching the development of disease control technology utilizing postharvest applied, fungicidal chemicals in fruit dips, sprays, and/or waxes to control postharvest body and stem-end rots. These unregistered fungicides--iprodione (Rovral), imazalil (Fungaflor) and thiabendazole (Mertect), and Chlorox (0.25% a.i.), and simply waxed fruit alone were tested by applying after harvest. All were as ineffective as no treatment at all.


l. Prochloraz (Sportak), used in Australia at 250 ppm, is not registered in the U.S. Preliminary results from CTAHR's initial tests with Prochloraz were promising. Subsequent results at rates 10 times greater than recommended in Australia have been erratic and ineffective. Presumably, these erratic results are related to different organisms in Kona. Prochloraz cannot be used with wax or non-ionic surfactants which interfere with its activity. The emulsifiable concentrate is superior to the wettable powder.


m. Research is continuing on Prochloraz, modified temperature control, organic forms of bromine and basic fruit and pathogen biology behavior during the critical postharvest period. Since these rots are not seen until the fruit ripens, refrigeration at 50F before ripening may be an important tool for delaying postharvest disease development.

Avocado insects, pests, and plant disease pathogens, Knowledge Master, CTAHR
Hawaii Pesticide Information Retrieveal System, CTAHR