Janice Y. Uchida Department of Plant and Environmental Protection Sciences University of Hawaii at Manoa

Janice Y. Uchida Department of Plant and Environmental Protection Sciences University of Hawaii at Manoa

Phytophthora species Some of the most destructive pathogens The genus has a very wide host range; Some species have limited host range while others are broad. Limited: Phytophthora colocaisae (taro) Phytophthora kasturae (coconut, chestnut) Broad: Phytophthora nicotianae (P. parasitica) Phytophthora palmivora Phytophthora cinnamomi

Typical Disease Cycle in the Tropics Sporangia: released from the fruit surface by splashing water, and blown to new host tissue; Air movement only will not release the spores of Phytophthora palmivora. The sporangia lands on a leaf surface or fruit.

Numerous lemon-shaped sporangia

Sporangia germinates by producing germ tubes (threads) that penetrate the host. Moisture is needed for germination. Sporangia stained

Or if water is present, zoospores are formed. They swim to new sites on the plant, encyst, and penetrate through stomates or grow between cells. Zoospores also accumulates on wounds and root tips.

Root tip Wound on a root Cloud of zoospores

Disease cycle cont. Disease symptoms develop: After Phytophthora penetrates the host, leaf spots, fruit spots and rots, trunk rots, rot roots, plant decline, toppling or death occurs.

Abundant sporangia formed on the fruit surface

Canker rots are formed on the trunk of the tree.

Disease cycle cont. Sporangia form on the outside of infected fruits and are splashed or carried to healthy trees by insects, snails, birds, etc. (Epidemiology).

Disease Cycle Continued: Chlamydospores are round (spherical) spores that are formed on the inside of infected fruits and eventually end-up in the soil after the fruit decomposes. They have thickened walls. Survival spores. When the new crop is planted, the chlamydospores germinate, and infect roots. Phytophthora causes root and collar rots, and kills young plants. Surviving plants are very weak and fall-over easily. Known as the replant problem. New land was continually cleared for growers who needed new uncontaminated fields.

Chlamydospores

Decomposing papaya; Chlamydospores remains in the soil. Sporangia survive for only a short period of time.

New disease discovered on Breadfruit. Symptoms looked similar to Phytophthora palmivora on papaya.

Isolated Phytophthora palmivora Internal infection within the fruit

Mummified fruits in the trees

fruits Inoculated fruits with the Pure culture and measured the diameter.

8.00 7.00 Lesion size (cm) 6.00 5.00 4.00 3.00 2.00 1.00 0.00 T1 T5 T13 T17 T37 T43 T49 T58 T67 T70 T195 T52 Isolate number * Average lesion size with standard deviation bars Different isolates of P. palmivora collected from Diseased breadfruit

Massive clean up of the field, gathering all infected fruits and plant parts. Has been successful in greatly reducing disease levels.

With a decline in sugarcane production there was an effort to diversify agricultural production in Hawaii. 1986, cacao seeds were imported from Costa Rica and planted in Hawaii by a private company. Seedlings developed leaf blight and damping-off.

Fresh seeds provide the best germination. Dry seeds have greatly reduced germination. When seeds are collected from infected pods, some seeds are infected. These seeds were sent to Hawaii.

Infected seed: Outside healthy, internally infected

Stem rot and leaf blight of seedlings developed from infected seeds.

Phytophthora palmivora was isolated; introduction of a new strain was immediately suspected. 1. Cacao has been grown on several islands; All pod rots were caused by Lasiodiplodia; no Phytophthora was ever isolated. 2. Chlamydospores of the Phytophthora palmivora from cacao were larger. 3. Previously when cacao pods were inoculated with P. palmivora obtained from papaya, no disease developed on the pods.

Prohibiting the further importation of diseased cacao was needed. However, economic decisions prevented this. Tremendous pressure for the State to diversify and for new business to establish a cacao plantation quickly. Foreign experts (mainland consultants) argued: P. palmivora is already in Hawaii on papaya, thus there should not be a great concern about P. palmivora on cacao.

Without a ban on the importation of seeds, P. palmivora on cacao, the worst pathogen of cacao continued to arrive in Hawaii. Without eradication, infected trees continued to grow and epidemics were common. The pathogen is now established on cacao trees on the Big Isle. The disease cycle is similar to that of P. palmivora on papaya.

Typical black pod disease Sporangia

Pod rots and mummified fruits with chlamydospores internally.

Mummified fruits on trees

Stem canker on young plants.

Research was needed to determine if there is host specificity in some populations of P. palmivora. That is, will the P. palmivora from papaya infect cacao and will the P. palmivora from cacao infect papaya or are they specific to their host. A large collection of P. palmivora was made in Hawaii: (1) from papaya (2) from cacao (3) ornamentals and misc crops.

Each group was tested on papaya, cacao, and orchid plants. RESULT: P. palmivora collected from diseased papaya does not kill cacao; P. palmivora collected from diseased cacao does not kill papaya; Thus, there is host specificity in P. palmivora.

Sample inoculated plants P. palmivora from papaya P. palmivora from cacao

A1 A2 A1 A1 Sexual spores, called oospores are produced when 2 compatibility types are grown together.

Papaya Cacao Papaya Papaya A1 A2 A1 A1 All P. palmivora from papaya were A1 All P. palmivora from cacao were A2

Oospores

Example of oospore Oogonium (female part) Oospore Amphigynous Antheridium Unique to Stramenopila Antheridium (male part)

In Hawaii: P. palmivora is a pathogen of: orchids, palms, Catharanthus (periwinkle), Draecena, Macadamia, Breynia disticha (snow bush), Cucurbita, Ricinus, Hedera, and a few other plants. Elsewhere is has been reported on coconut, rubber, citrus, Bougainvillea, Hibiscus, Black pepper, Petunia and many other host.

END

. Hormanal stimulation of antheridial and oogonial development. Female penetrates the male antheridium and expands 3. Male penetrtes the Female oogonium Double Penetration

Induced Homothallism A1 Compatability Type Polycarbonate membrane A2 Compatability Type

All isolates collected from cacao were of the A2 compatibility type. All isolates from papaya were of the A1 type. All isolates from orchid were A1 except one isolate. Not surprising as orchids are imported into Hawaii yearly.

Thus, based on: Host specificity. Compatibility type. Historical absence. Slight morph. difference. A new strain of P. palmivora was introduced into Hawaii. Disease cycle on cacao is similar to that on papaya.

Concepts: Phytophthora palmivora is listed with a wide host range and is reported to attach many plants. However, there is some host specificity in this species and all isolates will not cross infect all hosts. For the cacao industry, they have lost the golden opportunity to grow this crop without the most serious pathogen attacking cacao. For the ornamental industry, the new strain may attack ornamental plants that had not been attacked before.