Morphological and molecular characterisations of Phytophthora palmivora infecting cocoa in Pahang and Perak, Malaysia
Phytophthora spp. are one of the most destructive plant pathogens worldwide. Many economically important crop species such as cocoa, rubber, jackfruit, durian, taro, papaya, pepper, coconut, potato, citrus and forest trees are susceptible to various diseases caused by Phytophthora spp. Agronomist...
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| Format: | Thesis |
| Language: | English |
| Published: |
2015
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| Subjects: | |
| Online Access: | http://psasir.upm.edu.my/id/eprint/68110/1/FP%202015%2092%20IR.pdf |
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| Summary: | Phytophthora spp. are one of the most destructive plant pathogens worldwide. Many
economically important crop species such as cocoa, rubber, jackfruit, durian, taro,
papaya, pepper, coconut, potato, citrus and forest trees are susceptible to various diseases
caused by Phytophthora spp. Agronomists and plant pathologists have been aware of the
economic importance of diseases caused by Phytophthora in Southeast Asia. However,
there is a lack of published information on Phytophthora spp. in the tropics. Although
many studies have been conducted over the past few decades to overcome diseases
caused by Phytophthora spp., the general information is often not outlined with details
while specific information of occurrence and economic impact are dispersed in many
different publications in various languages. Black pod disease of cocoa caused by
Phytophthora palmivora is a serious problem of economic importance to cocoa growers
in Malaysia. It is also known to cause stem canker disease where their control in infected
cocoa plantations is very difficult. This research was undertaken to study the cultural
characteristics, reproduction, and pathogenicity as well as molecular characteristics of
fourteen isolates of Phytophthora spp. randomly obtained from infected cocoa pods in
two different states, Pahang and Perak in Peninsular Malaysia. The cultural
characteristics of these isolates were assessed on four types of agar medium namely
PDA , CA, VJA and CMA incubated at 25 ± 2°C showed various textures of colonies as
fluffy with irregular margin on PDA, cottony with uniform margin on CA, slightly
cottony with regular margin on VGA and flattish with regular margin on CMA. It did
not show specific colony pattern on all four types of medium and each medium
demonstrated its own morphological appearance. Growth rates of colonies were
measured after three days of incubation on four types of agar media for one week and
demonstrated that isolates grew significantly faster on CMA with 13.3mm/day, followed
equally by VJA and CA with 10.0 and 10.2mm/day respectively, and the poorest growth
was on PDA with 6.1mm/day. Dimensions of sporangia on CMA medium were
measured averaging 43.48μm in length, 29.37 μm in width, 3.53 μm in pedicel length
and 5.31 μm in size of exit pore. On culture media, the sporangia were of different shapes. Sporangia of isolates were all caducous. Zoospores were induced on four types of
medium CA, VJA and CMA. Isolates on CA medium produced the largest number of
zoospores than on VJA and CMA medium. Chlamydospores were produced on VJ broth
ranging between 25.21 to 21.55 μm in diameter. Molecular identification using ITS
region of rDNA confirmed that all isolates were Phytophthora palmivora. Eight isolates
were selected randomly for pathogenicity tests on green healthy pods of KKM22 cocoa
cultivar. The test results confirmed that isolates RB03 and RB04 from Pahang
significantly more virulent with size of lesion 129mm and 117.75mm correspondingly
compared to other isolates after five days of inoculation. Hence, complete brown to black
lesions covered all inoculated after nine days. |
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