The oil palm (Elaeis guineensis jacq.) is one of the most economically
important crops in southern Thailand. The total area of oil palm production
in Thailand is approximately 625,600 ha and of this about 528,400 ha is in the
south (Chavananand, 2011). Oil palms are enriched with
numerous nutrients, fatty acids and vitamin A and E, which can be used for consumption
in the food industry, in cosmetics production and for renewable energy. Oil
palm planting needs high rainfall to gain high yield production. As high levels
of humidity can induce germination of some phytopathogenic fungi, oil palms
are faced with several diseases in all stages of growth, from seedling nursery
stage to mature plant.
A major cause of leaf blight and leaf spot of oil palm is caused by fungi.
In general, leaf blight and leaf spot symptoms caused by fungi are mostly similar
(Elliott, 2005) and the disease can hardly be distinguished
by symptom alone. The disease primarily affects young seedlings up to 3 months
old or seedlings which have recently been transplanted (Turner,
1981). In Africa, anthracnose caused by Botryodiplodia palmarum,
Melanconium sp. and Clomerella cingulata resulted in severe damage
with important economic input, while in southeast Asia Curvularia leaf
blight (Curvularia eragrostidis) and Leptosphaeria leaf spot (Pestalotiopsis
spp.) were common (Aderungboye 1977).
Leaf spot disease is considered to be a minor problem but it can spread rapidly
in oil palm growing areas of Thailand, Papua New Guinea, India and Sri Lanka
(Aderungboye, 1977). The Cercospora leaf spot
caused by Cercospora elaeidis is the most widespread foliar disease of
nursery seedlings and young oil palms in west Africa (Rajagopalan,
1973). The host range of leaf spot disease includes not only oil palm but
also other plants. For example, Pestalotiopsis palmarum is one of diseases
of coconut widespread in many countries (Ohler, 1999),
Curvularia eragrostidis yam leaf spot is a serious concern among the
northeast Brazilian yam growing areas (Michereff et
al., 1994) and petal blight of dendrobium is a problem in the northern
Territory (Duff and Daly, 2002).
Only few previous studies have been conducted on leaf blight and leaf spot
disease and their distribution in Thailand. Therefore, the objective of this
study was to examine these diseases in the nursery stage of oil palm and to
prepare a distribution map of these diseases for southern Thailand.
MATERIALS AND METHODS
Field inspection and disease sample collection: Surveys were conducted to diagnose
leaf spot and leaf blight in oil palm seedlings in planting areas of eleven
provinces around southern Thailand during 2011 and 2012. Five infected leaves
per plot were placed in plastic bag and stored in a cooler box. The collected
samples were brought to the laboratory for isolation and identification of the
causal agent of leaf blight and leaf spot disease.
Isolation of the pathogens from suspected leaf spot and leaf blight symptoms:
The infected leaves samples showing leaf blight and leaf spot symptoms were
washed in sterilized water and placed on sterilized tissue paper and incubated
in a moist chamber at 25°C for 3 days to induce sporulation. Single spore
isolations were made within 24 h of cutting. Single spores collected from leaf
blight and leaf spot were cultured in Corn Meal Agar (CMA) (HiMedia, Mumbai,
India.) and incubated at 25°C for 10 days. Characterization of leaf blight
and leaf spot of oil palm seedlings was examined based on morphological analysis.
Spores/conidiophores were examined using light microscopy (Olympus CH30RF200,
Disease incidence: The disease incidence of oil palm seedlings was analyzed
under nursery conditions. A total of 277 leaf samples was used as the denominator,
while the number of each isolate was used as the numerator. Disease incidence
was calculated based on the following equation:
where, x1 is the total number of each isolate of fungi and x2 is the total
number of collected plants which showed leaf blight and leaf spot symptoms.
RESULTS AND DISCUSSION
The total of 277 samples of symptomatic leaf blight and leaf spot were collected
from nurseries in eleven provinces of southern Thailand. The most common symptoms
appearing during nursery stage were leaf blight and leaf spot. The leaf blight
showed as small circular and translucent yellow to brown necrotic tissue diffusely
scattered on leaves, while leaf spot was demonstrated by dark brown pin point
on the leaves. The collected samples were subjected to single spore isolation
on CMA and maintained in Potato Dextrose Agar (PDA). The leaf blight and leaf
spot causal agents were examined by compound microscope and were identified
as the genera Curvularia and Colletotrichum leaf spot based on
morphology. The most common fungal isolates found in this study in decreasing
order were Curvularia 149 isolates, Colletotrichum 48 isolates
with disease incidence in decreasing order of Curvularia leaf spot (61.01%)
and Colletotrichum leaf spot (22.38%) (Table 1).
|| Disease incidence of leaf spot and leaf blight of oil palm
The major pathogens which caused leaf blight and leaf spot on oil palm seedling
belonged to Curvularia and Colletotrichum genera. The obvious
symptom of Curvularia leaf spot was the appearance of small yellow spots
on the leaves. The progress of symptoms started from light brown, changing to
dark brown and covering almost all parts of the leaves. Finally, the infected
leaves turned black (Fig. 1a). The colony of Curvularia
was dark brown (Fig. 1b). Characteristics of conidiophores
and codinia were observed by compound microscope. The conidiophore was single
with one conidium, simple or branched, straight or flexuous and brown to dark
brown. Conidia were 3-distoseptate, approximately ovoid, obclavate or almost
elliptical with the second cell from the base the largest and 24-40x12-22 μm
in size. Based on morphological analysis, this was identified as Curvularia
oryzae (Fig. 1c).
Oil palm anthracnose on nursery stage was caused by Colletotrichum gloeosporiodes.
This disease begin as small brown spots and water soaked spots. The infected
areas expand into circular spots with tan to light brown centers. As the spots
expand, lesion centers lighten to very light tan to cream color with some spots
developing brown margins around the centers (Fig. 1d). The
colony of C. gloeosporiodes was white to grey color with delicate and
thin mycelia (Fig. 1e). The conidia cylindrical shape was
smooth and 3-5x9-14 μm in size (Fig. 1f).
Furthermore, Pestalotiopsis-like leaf blight and Cercospora-like
leaf spot symptoms were also found in this observation. The Pestalotiopsis-like
leaf blight lesions appear black and sunken on leaves. Old lesions have somewhat
grey borders and may contain spore clusters in the center (Fig.
1g). The Cercospora-like leaf spot symptom were the appearance of
small yellow spots with a brown point in the middle. They spread and transform
to brown and then are surrounded by a yellowish halo. At a later stage, most
spots became brown speckle scattered over the leaf (Fig. 1h).
Nevertheless, the isolation of minor disease on oil palm seedling was not done
in this study.
Leaf spot and leaf blight diseases are commonly found in nursery stage oil
palms. Leaf spots due to C. oryzae and C. gloeosporiodes
are major diseases of oil palm seedlings in nursery stage and have been recorded
as causing moderate to severe damage in wide areas of Malaysia, Sabah, Sumatra
and Thailand (Turner, 1981).
||(a)Symptom of Curvularia leaf spot, (b) Colony of
Curvularia oryzae incubates on CMA agar for 10 days, (c) Conidiophore
and conidia of C. oryzae, (d) Symptom of anthracnose causal by Colletotrichum
gloeosporioides, (e) Colony of C. gloeosporioides on CMA agar
for 10 days, (f) Conidia of C. gloeosporioides, (g) Pestalotiopsis-like
leaf blight on oil palm and (h) Cercospora-like leaf spot, Bar =
It has been also reported that C. oryzae caused leaf spot on mature
oil palm in Krabi Province, southern Thailand (Doungsa-ard
et al., 2011), while C. lunata and C. maculans have
been documented as causal agents of leaf spot in oil palm in Malaysia (Englert
et al., 1999). Leaf blight and leaf spot diseases can be caused
by several fungal genera, such as Annellophora, Bipolaris, Cercospora,
Colletotrichum, Calonectria (Cylindrocladium), Exserohilum,
Gliocladium, Pestalotiopsis, Pestalotia, Phaeotrichoconis,
Phyllachora, Pseudocercospora and Stigmina (Elliott,
2005) in Florida. In Venezuela, Pestalotiopsis leaf blight was caused
by Pestalotiopsis palmarum and Pestalotiopsis glandicola (Labarca
et al., 2006). Suwannarach et al. (2013)
recently reported leaf spot disease on oil palm in Chiang Mai Province, northern
Thailand, caused by Pestalotiopsis theae.
The results from our study are the first report of leaf blight and leaf spot
disease dispersal of oil palm seedling in southern Thailand (Fig.
2). These diseases are caused by at least two genera of fungi; Curvularia
and Colletotrichum. The major fungal genera causing leaf blight and leaf
spot are Curvularia and Colltotrichum.
|| Epidemiology map of leaf blight and leaf spot in southern
Thailand, Symbols represent the diseases
Knowledge of the distribution of pathogenic fungi in oil palm nurseries is
likely to be most important for farm advisors to guide farmers in disease management
and control. Further studies on fungal species, their morphological, biological
and molecular characterization and their management are needed.
This study was supported by (1) National Research University Project of Thailand,
(2) Grant from Prince of Songkla University (Grant No. NAT560393S) and (3) The
Center of Excellence in Agricultural and Natural Resources Biotechnology, Prince
of Songkla University.