CHARACTERIZATION OF OROBANCHE SPECIES OCCURRING IN BANGLADESH
Manisha Shaha and I. H. Mian. 2013. Characterization of Orobanche species occurring in Bangladesh. Bangladesh J. Plant Pathol. 29 (1&2): 39-46.
A survey was conducted in the district of Jessore, Kushtia, Pabna and Rajshahi to identify and characterize Orobanche species occurring in those locations of Bangladesh. Two distinct species of Orobanche were found to occur in the country. The first one was found to attack mustard, cabbage and cauliflower, and the second one attacked sunflower, tomato and tobacco. Orobanche sp. collected from mustard showed higher shoot length, canopy diameter, sepal number, petal length, ovary length, ovary diameter, and style length. Flowers were with 2 bractlets, calyx divided at the base posteriorly, and entire anteriorly, stamen number was 4 (all equal), with wooly anthers. On the other hand, Orobanche sp. collected from tomato had higher stem girth, leaf length, leaf width, sepal length, stigma diameter and stamen length. Flowers were without bractlets, calyx divided at the base anteriorly as well as posteriorly, stamen number was 4 (2 longer than other 2), with glabrous anthers. The morphological characters clearly showed that the two Orobanche observed on two hosts species were distinctly different species. The morphological characters were compared with a standard key and Orobanche collected from mustard was identified as O. aegyptiaca L. and the parasite attacking tomato was identified as O. cernua L.
Orobanche, commonly known as broomrape, is a genus of parasitic flowering plants. It is a holoparasite affecting several hundred crop species including rapeseed-mustard, tobacco, potato, brinjal, cabbage, cauliflower, turnip and many other Solanaceous and Cruciferous plants (Musselman 1980). They lack chlorophyll and their leaves are vestigial. Above-ground stems are produced only for the purpose of flowering and setting seeds. After ripening of the fruits, seeds of Orobanche shed in soil, they undergo a conditioning period of several weeks. After conditioning, seeds germinate in response to host stimulant and radicle (germtube) emerges. Germination is followed by haustorial initiation, attachment of the host roots, and lastly penetration of the vascular tissues (Musselman 1980, Kreutz 1995).
Species of Orobanche are aggressive and damaging parasitic weeds which have a tremendous impact on agriculture causing considerable yield losses, in many countries, especially in the drier and warmer areas of Europe, Africa and Asia (Dhanapal et al. 2009). The losses may vary from 5 to 100% (Stewart and Press 1990). The average crop loss, across all broomrape species, is approximately 34% (Linke et al. 1989, Diaz et al. 2006).
A comprehensive review published in India (Dhanapal et al. 2009) reveal that practices to control
broomrape include physical methods (weeding, soil tillage, flooding, irrigation, pola-rization, flaming), chemical methods (soil fumigation, herbicide application, use of germination stimulants), bio-logical methods (use of resistant or tolerant varieties), cropping systems with trap and catch crops, intercropping, and biological control (with insects or fungi).
In Bangladesh, Orobanche attack different crops. However, list of the susceptible crops are not available in the country. At at least two species of the pest namely O. aegyptiaca and O. cernua occur in Bangladesh (Begum and Huq 1983). Comprehensive report on crop loss due to the pest is not available in the country. A recent hand book on agricultural technologies published by Bangladesh Agricultural Research Institute reveals that Orobanche is a common pest of mustard. In Bangladesh, recommendations to control the pest include removal of Orobanche plants before flowering, avoidance of growing mustard in known infested fields, destruction of the weed with 2,4-D and use of appropriate amount of TSP fertilizer (Islam et al. 2004). Alam et al. (2008) identified some resistant germplasm of mustard in a screening test conducted during 2007-2008.
The present paper reported the results of an investigation conducted to characterize the species of Orobanche attacking crop plants in Bangladesh.
MATERIALS AND METHODS
The investigation was conducted in the district of Jessore, Kushtia, Pabna and Rajshahi. Orobanche infested fields of mustard, sunflower, tomato, cabbage, cauliflower and tobacco were visited and host range of the parasite was recorded. Apparently healthy and matured Orobanche plants were collected from each of the host crops. Canopy diameter covered by Orobanche plants and growth habit of shoot were recorded.
To collect data on plant characters, the selected Orobanche plants were uprooted by digging with a spade along with root systems of respective hosts. They were washed with tap water carefully to minimize root damage of host plants. Whole plants and their individual parts were checked with naked eyes or under a hand lens or using a compound microscope according to necessity. Data on characteristics of shoot, leaf, flower and fruit were recorded.
For recording leaf characters, 20 fully open leaves were selected randomly from the plants. Lamina length from base attached with the stem to the tip, lamina width at the widest point and lamina shape were recorded.
A total of 20 fully open flowers were checked and data on flowers types, flowers arrangement (solitary or clustered), color of petals and anther, number and length of sepal, petal and stamen, length of style and ovary, diameter of stigma and ovary, and shape and diameter of pollen were recorded.
RESULTS AND DISCUSSION
Orobanche and their hosts identified
During the investigation, mustard, sunflower, tomato, cabbage, cauliflower and tobacco were found to be infected with Orobanche. The morphological appearance of the pests infecting sunflower, tomato and tobacco was distinctly different from the parasites that attacked mustard, cabbage and cauliflower (Plate-IA&B). Morpho-logical characters of stem, leaf and flowers recorded during the investigation were compared with a standard key and Orobanche from mustard family was identified as Orobanche aegyptiaca L. and the parasite infecting other crops was identified as Orobanche cernua L (Lie Dang Shu 1998).
In mustard fields, O. aegyptiaca infected up to 100% host plants. In severe cases, infected host plants died before maturation (Plate-IC). In infected tomato fields, Orobanche shoot emerged in cluster showing up to 42% host plants infection. Infected tomato plants gradually died and weathered before maturation of fruits (Plate-ID).
Both O. aegyptiaca and O. cernua were found to grow closed to the stem base or a little far from it depending on length of root systems of their hosts. The shoot of O. cernua attacking tomato is robust as compared to O. aegyptiaca that attacks mustard. The appressorium of the Orobanche developed at the base of stems was thicker on tomato than those infecting mustard (Plate-IE&F). The number of parasite shoots per square meter was higher in mustard fields than tomato fields. It might be due to the fact that the parasite attacking mustard has higher tendency to branch than those of tomato.
Patrick and Miller (1994) also found similar results. They reported that in some species (such as O. aegyptiaca, O. ramosa) branching of the shoot is normal, whereas in others (O. cernua, O. minor, O crenata) branching is rare.
Reports from various sources reveal that seeds of Orobanche germinate in response to the host stimulant and the radicle, also known as germtube, emerges. If the radicle reaches the host root, its tip thickens and attaches itself to root surface. The thickening is known as appressorium. Finally, haustorium initiates from the appressorium, which engulfs the host roots and penetrates the vascular tissues of the host and makes parasitic relationship with the host (Plate-IIA&B). With this organ, the parasite withdraws water, minerals and organic compound from the hosts (Visser and Dorr 1986, Stewart and Press 1990, Nun and Mayer 1993).
Morphology of Orobanche aegyptiaca
Plant: The canopy diameters of O. aegyptiaca in mustard fields ranged 1.40-3.80 cm with a mean of 2.44 cm and standard deviation 0.65 cm. Stems were erect, roundish, thinner, purplish white and glandular pubescent. The shoots were branched above the middle point but some were unbranched. They were smooth having crowded leaves and flowers. The plant height ranged 17.00-31.00 cm with a mean of 23.88 cm and standard deviation 3.71 cm. The range of stem girth at the widest point was 1.00-2.00 cm with a mean of 1.69 cm and standard deviation of 0.35 cm (Table 1).
Leaf: The leaves were alternate, acute, and lanceolate. The base of leaves was wide and tip was pointed. Stem under and around a leaf was light greenish yellow. Leaf length ranged 0.60-1.10 cm with a mean of 0.77 cm and standard deviation 0.15 cm. The maximum leaf width ranged 0.30-0.60 cm with a mean of 0.41 and standard deviation 0.09 cm (Plates I&II and Table 1).
Flower: Flowering initiated in January and ended in March. Inflorescence was spicate, bract ovate-lanceolate, with 2 bractlets, flowers sub-sessile, bisexual, alternate, axial and attached with the stem at an acute angle (Plate I&II).
nulate with 4 sepals, divided at the base posteriorly, entire anteriorly, light greenish yellow colour, sepal length 0.80-1.20 cm with a mean of 1.01 cm and standard deviation 0.16 cm. Corolla was funnel shaped with five lobed petals, usually rounded, tip of petal free and blue purple and purplish white along the tube, petal length 1.50-2.00 cm with a mean of 1.73 cm and standard deviation 0.20 cm. Stigma diameter ranged 0.10-0.20 cm with a mean of 0.12 cm and standard deviation 0.04 cm. Ovary was bicarpelary with terminal single style, ovary length
ranged 0.40-0.80 cm with a mean of 0.65 cm and standard deviation 0.14 cm, ovary diameter at widest point ranged 0.25-0.40 cm with a mean of 0.35 cm and standard deviation 0.05 cm. Number of stamens was 4 (all equal), with wooly anthers, length ranged 0.60-0.80 cm with a mean of 0.70 cm and standard deviation 0.08 cm. Style length ranged 0.05-0.60 cm with a mean of 0.48 cm and standard deviation 0.04 cm. Pollen diameter ranged 6.24-10.92 µm with a mean of 8.05 µm. Pollen shape was round with spines on the surface (Table 1).
Morphology of Orobanche cernua
Plant: The stems were erect, robust, rigid, thicker, light brown at immature stage, dark brown at mature stage, glabrous near stem base or minutely pubescent from middle upwards, and unbranched with crowded leaves and flowers. The plant height ranged 14.00-32.00 cm with a mean of 23.13 cm and standard deviation 5.27 cm. Maximum stem girth ranged 1.70-4.00 cm with a mean of 3.26 cm and standard deviation 0.85 cm, and the canopy diameters ranged 1.40-2.00 cm with a mean of 1.68 cm and standard deviation 0.25 cm (Plate-III and Table 2).
Leaf: The leaves were scale like, alternate, acute, ovate-lanceolate along with bracts, bractlets and calyx. The base of leaves was swollen due to development of flower bud, and pointed tip. Leaf length ranged 1.00-1.50 cm with a mean of 1.27 cm and standard deviation 0.20 cm. The maximum leaf width ranged 0.40-0.70 cm with a mean of 0.58 and standard deviation 0.10 cm (Plate III and Table 2).
Flower: Flowering initiated in January and ended in March. Inflorescence was spicate, bract ovate-lanceolate, without bractlets, acuminate, flower
sub-sessile, bisexual, alternate, axial and attached with the stem at an acute angle. Calyx was campanulate with 2 distinct sepals, divided at the base both anteriorly and posteriorly, greenish brown, length 0.7-1.0 cm with a mean of 1.88 cm and standard deviation 0.11 cm. Corolla was funnel shaped with five lobed petals, usually rounded, tip of petal free and white with blue tint at the tip, purplish white along the tube, length 1.30-2.00 cm with a mean of 1.68 cm and standard deviation 0.25 cm. Stigma diameter ranged 0.10-0.20 cm with a mean of 0.16 cm and standard deviation 0.05 cm. Ovary was bicarpelary with terminal single style, length ranged 0.30-0.70 cm with a mean of 0.57 cm and standard deviation 0.13 cm, diameter 0.20-0.30 cm with a mean of 0.27 cm and standard deviation 0.03 cm. Number of stamens was 4 (2 longer than other 2), length ranged 0.70 -0.90 cm with a mean of 0.81 cm and standard deviation 0.07 cm. Style length ranged 0.04-0.50 cm with a mean of 0.48 cm and standard deviation 0.04 cm. Pollen diameter ranged 7.80-10.92 µm with a mean 9.05 µm and standard deviation 1.57 µm. Pollen of O. cernua collected from tomato was round (Plate III and Table 2).
Comparison of O. aegyptiaca and O. cernua
Morphological differences between two Orobanche specimens collected from mustard and tomato are shown in Table 3. Orobanche collected from mustard showed higher shoot length, canopy diameter, sepal number, petal length, ovary length, ovary diameter, and style length. Flowers were with 2 bractlets, calyx divided at the base posteriorly, and entire anteriorly, stamen number was 4 (all equal), with wooly anthers. On the other hand, Orobanche collected from tomato had higher stem girth, leaf length, leaf width, sepal length, stigma diameter and stamen length. Flowers were without bractlets, calyx divided at the base anteriorly as well as posteriorly, stamen number was 4 (2 longer than other 2), with glabrous anthers. The morphological characters clearly showed that the two Orobanche observed on two hosts species were distinctly different species. The morphological characters were compared with a standard key published in China for identifying them up to species level (Lie Dang Shu 1998). According to the key, Orobanche from mustard was identified as Orobanche aegyptiaca L. and the parasite attacking tomato was identified as Orobanche cernua L. Previous reports published by Bangladesh National Herbarium also reveal that Orobanche aegyptiaca and Orobanche cernua occur in Bangladesh (Begum and Huq 1983). The pests are widely distributed in Southern Europe, Asia and Africa. Orobanche aegyptiaca is also known as O. indica, and Phelipaea aegyptiaca (Musselman 1980, 1986).
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