Cey. J. Sci. (Bio. Sci.), Vol. 24, No.2, 1995 
CULTURAL, MORPHOLOGICAL AND PATHOGENICITY 
STUDIES ON SOME PHYTOPHTHORA ISOLATES FROM 
COCOA IN SRI LANKA 
M.A.P.K. Seneviratne*, A. De S. Liyanage4 
Rubber Research Institute of Sri Lanka, Agalawatta 
and 
N.K.B. Adikaram 
Department of Botany, Faculty of Science, 
University of Peradeniya 
A detailed study was carried out on colony and sporangial morphology, growth characteristics, mating 
behaviour and pathogenicity of 110 Phytophthora isolates obtained from infected cocoa pods collected 
from different cocoa growing regions in Sri Lanka. The study revealed that about 95% of the isolates 
resembled Phytophthora palmivora morphological Form 1 and belonged to A2 compatibility. A small 
percentage (5%) was compatible with a known Ai type isolate. Isolates within locations showed 
significant variability in sporangial number, sporangial length/breadth ratio, pedicel length, chlamydospore 
number and their size. Variability in isolates in respect of sporangial length/breadth ratio and 
chlamydospore number was observed among locations. In general there was a variability in vitro growth 
pattern among isolates and also the lesion development on artificially inoculated pods. 
INTRODUCTION 
Cocoa (Theobroma cacao L), a native of the rain forest of Tropical America, has been grown in 
Sri Lanka primarily as an intercrop in rubber (Hevea brasiliensis Muell. Arg) in the districts of 
Kurunegala, Matale, Moneragala and Kandy. Intercropping generally provides an additional income and 
eases the financial burden of the grower during the long immaturity period of rubber which lasts for about 
5-6 years before trees are brought into tapping. It also helps to arrest soil erosion especially where the 
terrain is steep, through the accumulation of leaf litter on the ground. 
However, one of the major constraints for intercropping rubber with cocoa is the possibility of 
damage caused by Phytophthora spp. that affect both rubber and cocoa. P. meadii arid P. palmivora have 
been identified as casual organisms of the Phytophthora disease syndrome of rubber, the former being the 
predominant species (Dantanarayana et.al. 1984). P. heveae (Thompson, 1929; Turner, 1968) and P. 
botryosa (Chee and Wastie, 1970) have been reported from both rubber and cocoa. 
Present address: 
* Department of Export Agriculture, Bambarakelle Housing Complex, Nuwara Eliya, Sri Lanka. 
** Department of Primary Industries and Fisheries, Davonport/Tasmania 7310, Australia. 
60 
M.A.P.K. Seneviratne et al. 61 
P. palmivora was known to be the fungus attacking cocoa wherever it is grown. Several 
morphological groups within P. palmivora have been identified (Orellana, 1959; Turner, 1960; Chee, 
1969; Waterhouse, 1974a). Four morphological forms of P. palmivora have been recognised viz. MFi, 
MF2, MF3, and MF4 (Griffin, 1977). The isolates obtained from West Africa categorized as MF3 were 
identified as a new species P. megakarya (Brasier and Griffin, 1979). The isolates of MF4 collected from 
Central and South America resembled P. capsici (Kaosiri, 1978; Zentmyer et. al. 1979). Besides these P. 
citrophthora (Kellam and Zentmyer 1981) and P. katsurae are also pathogenic on cocoa (Liyanage and 
Wheeler, 1989). However, little information is available on the species, type and the distribution of 
Phytophthora species in Sri Lanka on cocoa, a prerequisite for recommending cocoa as an intercrop with 
rubber. 
MATERIALS AND METHODS 
Isolation of Phytophthora from cocoa 
Infected pods were surface sterilized in 70% alcohol and washed in several changes of sterilized 
distilled water. Small pieces (1cm2) were cut aseptically from the leading edge of the lesion and placed on 
2.3% Lima Bean Agar plates (LBA, Difco). Pure cultures of Phytophthora thus obtained were maintained 
on LBA slopes under sterile liquid paraffin, at room temperature 28 ? 2?C. One hundred and ten isolates 
were obtained from cocoa growing districts of Sri Lanka viz. Matale, Kandy, Kurunegala, Badulla, 
Moneragala, Kegalle, Kalutara, Gampaha and Galle. Isolates were numbered serially, but separated by 
district of location as: Badulla B-l to B-18, Galle G-l to Gl-7, Gampaha Gm-1 Gm-8, Kalutara Kl-1 to 
Kl-9, Kandy Kn-1 to Kn-20, Kegalle Ke-1 to Ke-2, Kurunegala Ku-1 to Ku-13, Matale Ma-1 to Ma-23, 
Moneragala Mo-1 toMo-10. 
Growth rate and colony morphology on LBA 
A mycelial plug (4mm diameter) cut from the leading edge of an actively growing 3-4 days old 
culture was placed at the centre of a LB A plate. The plates were incubated at 28 ? 1 ?C in the dark At 24h 
intervals two measurements of colony diameter were taken at right angles to each other, each passing 
through the centre of inoculum plug. Four replicates were used for each isolate. For colony morphology 
studies cultures grown in total darkness for 6 days were examined against a black background. Colony 
morphology was assessed in terms of aerial mycelium and presence or absence of petaloid characters. 
Spnrangial morphology, dimensions and pedicel length 
Suspensions of sporangia were prepared by shaking mycelial plugs cut from 7-day-old cultures of 
LBA (grown in dark at 28?1?C) in distilled water. Drops of the suspensions stained with 
lactophenol/cotton blue were examined at a magnification of x 300. The sporangia! length and breadth and 
pedicel length of 25 sporangia from each replicate were measured using an eye piece graticule calibrated with 
a stage micrometer. Two replicates cultures were used for each isolate. 
Sporulation 
Isolates were grown for 7 days on 2 LBA plates at 28 ? 1 ?C in the dark as above. Eight mycelial 
plugs (4mm diameter) were removed from each plate from positions 1.5 and 2.5cm away from the centre of 
the colony. Each plug was placed in a drop (0.05 ml) of lactophenol/cotton blue on a slide which was 
subsequently heated gendy to melt the agar. A coverslip was placed over, the preparation by applying gentle 
pressure and the number of sporangia were counted in 3 randomly selected microscopic fields (magnification 
of x 300). Sporulation of each isolate was thus assessed in a total of 48 microscopic fields. 
Chlamydospore number and size 
Cultures prepared as above were incubated in the dark at 20?C for 1 month. Eight mycelial plugs 
(4mm diameter) were removed using a cork borer and the number of chlamydospores was assessed by the 
same procedure used above for counting sporangia. The diameter of 50 chlamydospores was measured from 
each replicate at a magnification of x 600. Two replicate cultures were used for each isolate. 
62 
Phytophthora in Cocoa 
Compatibility type 
To determine the compatibility type of the isolates, they were paired separately on LB A in the dark 
with a known Ai culture (P. meadii isolate No. 126 from rubber maintained at the Rubber Research 
Institute). These were left at room temperature, until mycelium of both isolates made contact with each 
other. The plates were subsequently transferred to a 20?C incubator and kept for a week before examination 
of plates for sexual reproductive structures. 
I Asinn development nn detached cocoa nods 
Healthy, green cocoa pods (about 15-18 cm long) were harvested carefully from a cocoa plantation 
having uniform plants of a single variety for artificial inoculation with Phytophthora isolates in the 
laboratory. The pods were arranged in metal trays lined with moist filter papers supported on glass rods. 
Mycelial plugs (12mm diam) cut from the leading edge of 7-day-old cultures grown on LBA at 28 ? 1?C in 
the dark were inserted into 1cm deep holes cut in the pod with a sterile 12mm cork borer. After inserting 
the mycelial plug the tissue disc removed was replaced. Three replicate pods were used per isolate tested. 
The trays containing the inoculated pods were covered with black polythene and incubated at room 
temperature (28?C). Commencing from two days after inoculation the developing lesions were traced onto a 
tracing paper for 3 consecutive days and the area of lesions was measured using a planimeter. 
RESULTS 
Colony growth of isolates 
Colonies of the 79 isolates cultured on LBA grew at different rates. Diameter of 6-day old colonies 
of the isolates tested ranged between 3-6cm. However, about 78% of the isolates attained colony diameters 
between 4-5cm after 6 days. In colony growth there was a significant interaction (P<0.001) between the 
isolate and time. It was not possible to give a common growth curve for all the isolates tested. It was also 
difficult to determine as to which isolate would have a common equation without examining the graphs of 
each isolate. 
Colony morphology on LBA 
Of the total number of isolates examined, 35% produced stellate colonies on LBA. The majority 
(about 56%) of the isolates, however, produced 'chrysanthemum' pattern colonies, while a few of them (B-
 10,, Kl-6, Kl-7, Kn-16) produced colonies of broad petaloid type resembling the 'Rose' pattern. About 8% 
of the isolates produced 'uniform' colonies without distinct shapes. 
Eight percent of the isolates produced moderate to sparse aerial mycelium over the entire colony. 
Another 13% produced colonies with sparse aerial mycelium, while 8% of the isolates produced colonies 
with moderate aerial mycelium. Isolates No.Kl-3 and Kl-8 produced abundant to moderate aerial mycelium. 
The aerial mycelium of the rest of the isolates was mostly confined to the colony centre, though some 
isolates produced more aerial mycelium over the entire colony and grew particularly dense towards the 
centre. i ... 
Generally, isolates produced colonies with sharply defined leading edges while isolates Kl-7 and B-
 II produced wavy margins. The margins of the colonies of isolates B-12, B-13, Kl-4 were diffused. 
Sporangial morphology 
Most of the isolates produced sporangia with prominent papillae. These were mostly ovoid to 
ellipsoid in shape, the sporangia were often symmetrical, usually with a rounded base but sometimes with a 
slightly tapering base. Occasionally spherical sporangia and sporangia with two papillae were also seen. 
All the isolates produced sporangia with short, fairly broad and occluded pedicels. Sporangia were 
caducous in water. A small percentage of asymmetrically positioned pedicels were seen on some sporangia. 
In each isolate, sporangia without any obvious pedicel were also seen. Further all the isolates produced 
chlamydospores readily and freely on LBA, at 20?C. 
M.A.P.K. Seneviratne et al. 63 
Compatibility types 
The isolates B-3, B-8, B-17, Mo-2, Mo-4 and Mo-8 produced sex organs when paired with a 
known A] compatibility type (Phytophthora meadii, isolate No. 126). However, even with these isolates 
only a very few oogonia were observed in cultures. The rest of the isolates did not produce any sex organs 
when paired with the Ai compatibility type isolate. 
Sporangia!, chlamvrinspnre and pedicel dimensions 
Seven parameters were considered, viz. sporangial length, sporangial breadth, sporangial 
length/breadth ratio, pedicel length, sporangia number, chlamydospore number and chlamydospore diameter, 
to study their variation at different locations (Badulla, Galle, Gampaha, Kalutara, Kandy, Kurunegala, 
Matale and Moneragala) and also their variation within a location. Mean sporangial dimensions and pedicel 
length of isolates obtained from different locations are given in Fig.l 
3-5 
MCD(/Am) 
3 0 
3-5 
?
  3 - 0 
M l ' U / i m ) 
2-5 
1-6 
1-5 
MSL IB \U 
1-3 
30 
MSB(/Ok m)2<? 
26 
45 
MSL(/lm)40 
1-22 
tO-63 ? 092 ? 0-86 jO-59 .1-73 ?052 i_f_ 
tO-77 
. 026 
iO-09 * 0-13 .0-09 
? 0-16 Q 
? 0-11 >o-i; 
.0-06 
0 02 0 _ , 0 0 3 t 0 0 2 * ? ?
 6 
OLD 
? 003 tO-011 t0-03 
o 
0 , 5 2
  . 1 0 4
  + 0 - 7 9 
.0-74 I 0" 5 * 
K7 
1O66 
O U 
10-69 
tO-59 
T i?H, . 0 8 7 t O - n '0-56 
t1W .0-73 
t 0 * 9 ,0-77 
r ~ ] r~i 
B Gl Gm Kl Kn Ke Ku Ma Mo 
Location 
Fig.l. Mean sporangial length (MSL), Mean sporangial breadth (MSB), Mean 
sporangial length/breadth ratio (MS L/B), Mean pedicel length (MPL), Mean 
chlamidospore diameter (MCD) of isolated from different locations 
(B - Badulla, Gl - Galle, Gm - Gampaha, Kl - Kalutara, Kn - Kandy, Ke -
 Kegalle, Ku - Kurunegala, Ma ? Matale, Mo - Moneragala). 
A significant variation (P<0.001) was present between isolates within a location for all 7 variable 
studied. Also there were significant differences between locations on 5 out of 7 variables studied, viz. 
sporangial length (P<0.01), sporangial breadth (P<0.01), sporangial length/breadth ratio (P<0.001), pedicel 
length (P<0.05) and chlamydospore diameter (P<0.01). 
There were no significant differences (P<0.05) between locations on sporangia number and 
chlamydospore number. 
The range of mean sporangial length, mean sporangial breadth, mean sporangial length/breadth 
ratio, mean pedicel length and mean chlamydospore diameter were 45.14 ? 0.58 - 42.48 ? 0.87um, 29.91 ? 
0.52 - 27.4 ? 0.74p.m. 1.59 ? 0.031 - 1.395 ? 0.029, 3.59 ? 0.26 - 2.73 ? 0.14pm, 39.99 ? 1.22 - 34.43 
? 0.77um, respectively. 
64 
Phytophthora in Cocoa 
The genetic type correlation among variables 
The genetic type coefficient of correlation among the 7 variables including colony diameter and 
mean lesion area were computed through the ANOVA using the between location component the ANOVA. 
These are not the usual "Environmental correlations" derived from the error component of the ANOVA, 
instead these correlations can be assumed to reflect relationships between any 2 variables, and be free of the 
environmental effect. 
Significant positive correlations were seen among the variables, sporangial length and sporangial 
breadth (P<0.001), sporangial breadth and pedicel length (P<0.05), sporangial length and square root of 
chlamydospore number (P<0.01), sporangial breadth and square root of chlamydospore number (P<0.001), 
sporangial length and colony diameter (P<0.01). These results show that when one variable increases the 
other also increases significantly. 
Significant negative correlations were observed between variables, sporangial breadth and 
sporangial length/breadth ratio (P<0.001), sporangial 1/b ratio and pedicel length (P<0.05), sporangial 
length and square root of sporangia number (P<0.01), pedicel length and chlamydospore diameter (P<0.01) 
chlamydospore diameter and colony diameter (P<0.001) and square root of sporangia number and lesion area 
(P<0.05). 
Lesion develqpment on pods inoculated with different isolates 
When the isolates were inoculated into pods and incubated they developed lesions characteristic to 
black pod disease in cacao. The rate of lesion expansion varied among isolates. Lesion growth by different 
isolates showed strong evidence of a positive linear effect of the rate of growth. The rate of increase in the 
lesion size is constant over the 3 days considered. But this constant seemed to vary with the isolates as 
shown by a significant (P<0.001) interaction between isolates and day in the linear component. 
DISCUSSION 
The pedicel length of the sporangium has been widely accepted as a stable character having a high 
diagnostic value in the identification of Phytophthora species (Waterhouse, 1974 b; Griffin, 1977; Tsao, 
1977; Tsao and Tummakate, 1977; Zentmyer et al. 1977). This character has also been found stable under 
different environmental conditions (Kaosiri, 1978). In a previous study Dantanarayana et al. (1984) has 
shown that the pedicel length is a criterion that separates Phytophthora species in rubber from those of 
cocoa in Sri Lanka. The isolates tested in this study produced caducous sporangia with short, broad pedicels 
less than 5 u in length. There was a variability in the pedicel length of isolates between different locations. 
The isolates thus resembled P. palmivora morphological form 1 in pedicel length, occluded nature of the 
pedicel, caducous sporangia with short (<5um) and broad pedicel as given in the key of Newhook ei al 
(1978). The caducity has been well recognized as a taxonomic character dividing species of Phytophthora 
into two groups, one with caducous and the other with persistent sporangia (Waterhouse 1974b; Al-
 Hedaithy and Tsao 1979; Saleh et al 1979). 
Brasier and Griffin (1979) examined the colony morphology of S-type isolates of P. palmivora 
from both cocoa and other hosts and showed considerable variation within the type which were nonetheless 
highly characteristic, although a subjective character. Most of the isolates we studied were stellate and 
chrysanthemum with sharply defined leading edges. This provides further evidence for identifying these 
isolates as P. palmivora. Brasier and Griffin (1979) also observed that the aerial mycelium of S-type (P. 
palmivora MFi) was sparse, being more or less absent from the peripheral areas of the colony but more in 
the centre. Similar results were obtained from the present study too. 
Sporangial length/breadth ratio has often been used as a taxonomic parameter in P. palmivora 
(Ashby, 1929, Turner, 1960,1961, Chee, 1969, Waterhouse 1974 a), but is liable to alter with different 
environmental conditions (Ashby, 1929, Leonian, 1934). In P. palmivora the 1/b ratio is always >1.4 on 
a standard medium. Most commonly the 1/b ratio is 1.6 and may extend upto 2.0 or more (Waterhouse, 
1974 a). In the present study the 1/b ratio of all the isolates were >1.4 and most were around l.S 
confirming that the isolates belong to the species, P. palmivora MFi. 
The production of chlamydospores was a useful character in the separation of P. medii from P. 
palmivora, the latter producing chlamydospores freely under all conditions (Dantanarayana et al. 1984; 
Waterhouse, 1963). The production of chlamydospores by all the isolates is also a useful parameter in 
identification of isolates as P. palmivora. 
M.A.P.K. Seneviratne et al. 
65 
Liyanage and Threemanna (1982) studied 22 isolates of P. palmivora from different cocoa 
plantations of Kandy and Matale Districts and found that they were of A2 compatibility type. Later Bandara 
et. al. (1985) who examined Phytophthora isolates from cocoa in Kandy and Matale Districts concluded 
that the isolates were MFi type of P. palmivora and belonged to the A2 mating type. However, Zentmyer 
(1974) has reported the presence of both Ai and A2 types in Sri Lanka on cocoa. During the present study 
mainly the A2 type was found among the cocoa isolates collected from different locations, with a small 
percentage of Aj compatibility type. 
A significant variation was observed in the sporangial length, sporangial breadth, sporangial l/b 
ratio, pedicel length, sporangia number, chlamydospore number and chlamydospore diameter within a 
location. Among the isolates tested these characters varied between locations. This indicates that there is a 
high variability among isolates. It is also important to note that eventhough there is a significant variation 
in sporangial number and chlamydospore number in a location, this does not vary significandy between 
locations. 
When the genetic type correlation between the variables explained above and the colony diameter 
and mean lesion size produced on cocoa pods are considered, there is a highly significant positive correlation 
between the sporangial length and the sporangial breadth. This explains that when the sporangial length 
increases in an isolate, sporangial breadth also increases proportionately. There is no correlation between 
the sporangial length and l/b ratio, but there is a highly negative correlation between sporangial breadth and 
l/b ratio. This explains that the contribution of the length of a sporangial to l/b ratio is negligible when 
compared to that of the sporangial breadth. 
The "isolate x day" interaction on colony growth, shows variability of growth patterns of different 
isolates. The slow, medium and fast growing isolates can be seen in each location indicating the variability 
of growth rate in isolates of a location. 
On inoculation into cocoa pods all the isolates produced symptoms characteristic of black pod 
disease in cocoa. The rate of increase in the lesion size was constant over the three days, for individual 
isolates but varied with different isolates, indicating that the pathogenic ability was different among 
isolates. Further in laboratory inoculation studies these isolates produced lesions on the leaves, petioles and 
the pods of rubber. However, the fact that this compatibility type has never been isolated from rubber 
indicates that such infections do not take place in nature. 
ACKNOWLEDGMENTS 
This work was supported by a grant from the Rubber Research Institute of Sri Lanka. The authors 
thank Professor J.M.R.S.Bandara and Dr (Mrs) N.LS. Liyanage for their help in this study. 
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