Testing for the resistance of newly generated hybrid cacao germplasm in the gene pool of Cocoa Research Institute of Nigeria (CRIN) against Phytophthora megakarya pathogen causing black pod disease of cocoa

Black pod disease caused by P. Palmivora and P. megakarya is an important disease affecting cacao in West Africa which produces 70% of the world output. Resistance to the pathogen is therefore, an important factor to be considered when breeding for high yielding and earliness in fruiting. Resistance to the pathogen using Leaf Disc Test (LDT) was conducted at the Plant Pathology Laboratory, Ibadan, on detached 15mm diameter discs of two-month-old leaves obtained from nineteen newly generated cacao hybrids in the gene pool of the Institute. Attached Whole Leaf Test (AWLT) was also conducted on few selected hybrids in the screen house at the same institute. Ten μl zoospores suspension of P. megakarya pathogen adjusted to inoculum strength of 3×105 zoospores/ml using haemocytometer was used for the inoculation of the cacao LD. Using 0-5 disease rating scale, data was taken on the 5th and 7th day after inoculation for LDT and on the 6th day after inoculation for AWLT. Data obtained were subjected to Analysis of Variance and significant means were separated using Student Newman Kuels Test at p<0.05. The LDT classified the hybrids into five groups namely: Resistant (hybrid 001, 003 and 005); Moderately Resistant (hybrid 006, 007, 008,013,015,018 and 019) Moderately Susceptible (hybrid 004, 014 and 017); Susceptible (hybrid 002, 010, 011 and 016) and Highly Susceptible (hybrid 009 and 012). Scores from LDT significantly correlated (r= 0.92**) with that of AWLT. It was observed from the study that cacao hybrid 001, 003 and 005 showed traits of resistant and therefore, could be incorporated into the national breeding programs for the development of high-yielding and resistant cacao cultivars. The moderately resistant hybrids could be conserved for future germplasm enhancement program.

to the pathogen using Leaf Disc Test (LDT) was conducted at the Plant Pathology Laboratory, Ibadan,

26
on detached 15mm diameter discs of two-month-old leaves obtained from nineteen newly generated 27 cacao hybrids in the gene pool of the Institute. Attached Whole Leaf Test (AWLT) was also 28 conducted on few selected hybrids in the screen house at the same institute. Ten µl zoospores 29 suspension of P. megakarya pathogen adjusted to inoculum strength of 3x10 5 zoospores/ml using 30 haemocytometer was used for the inoculation of the cacao LD. Using 0-5 disease rating scale, data 31 was taken on the 5 th and 7 th day after inoculation for LDT and on the 6 th day after inoculation for

32
AWLT. Data obtained were subjected to Analysis of Variance and significant means were separated 33 using Student Newman Kuels Test at p<0.05. The LDT classified the hybrids into five groups namely: with that of AWLT. It was observed from the study that cacao hybrid 001, 003 and 005 showed traits Introduction Ekiti, Edo, Abia, Adamawa, Taraba, Akwa Ibom, Delta, Kogi and Kwara were reported to produce

127
Pure cultures of the Phytophthora species obtained were inundated with 20ml of cold sterile water at 128 10 0 C. This was gently swirled and then the culture surface was scrapped with a sterile scalpel and filtered 129 through 2-fold cheesecloth into a 50ml beaker to remove sporangia and agar debris.,It was further refrigerated at Leaf Disc Inoculation

146
The LD was inoculated the same day on which the leaf discs were prepared later in the afternoon.

147
Inoculation was generally carried out by using sterile automated repeatable dispenser attached to a micro-148 syringe (Eppendorf). This was used to display 10µl droplets, discharge of the zoospore suspension in the middle 149 of each leaf discs, at a right angle to the direction in which the discs of the genotypes were displayed in the 150 trays. The light-proof trays were sealed hermetically and placed in a humidified chamber at air temperature of 151 26 ± 2 o C and relative humidity 80-100% to incubate.The zoospore suspension was continuingly agitated prior 152 and during the inoculation process to ensure even distribution of the zoospores.

153
The trays were inspected for lesion appearance, 72 hours after inoculation and data were collected for 154 leaf discs reaction to black pod pathogen. Inoculation droplets that have not been absorbed yet were dried with

172
The inoculated leaves were inspected for lesion appearance, 6th day after inoculation and data were recorded on 173 their reaction to black pod pathogen using a five-point disease assessment scale as described above.

233
A good correlation was found between leaf discs and attached whole leaf score means at p<0.05 (r= 0.93**),

284
The study also showed that seven (7)

317
The insignificant isolate x host interaction has been reported from the findings by several scientists (2, 318 25, 38-40) established Phytophthora species non-specificity with cacao genotypes. This indicates that cacao 319 hybrids resistance in this study to P. megakarya could be applicable to other Phytophthora species causing pod 320 rot of cacao in the other areas of cocoa production in the world.

321
It was observed also from the study that C77 cacao genotype which was one of the resistant controls 322 (28, 29) showed moderately susceptibility to the P. megakarya pathogen. This finding points to the fact that