Abstract: Curvularia species causing black kernel of rice have been tested in 150 samples of three varieties of boro rice seed. Five species of Curvularia, viz. Curvularia geniculata, Curvularia oryzae, Curvularia lunata, Curvularia eragrostidis and Curvularia intermedia have been detected. The seed infection was higher in case of Curvularia lunata (5.5 %) than other species. Species were detected by studying the growth habit of fungus on seeds, shape, diagnostic characters and appearance of conidia under compound microscope.
Introduction
A number of Curvularia species have been reported as causal agents of leaf spots, leaf blights, kernel rot, root rot, seedling blights, grain discoloration, grain lesions and grain deformation (Benoit and Mathur, 1970). Twelve species of Curvularia viz C. eragrostidis, C. intermedia, C. siddiquii, C. oryzae, C. lunata, C. pallescens, C. trifolii, C. clavata, C. geniculata, C. inequalis, C. uncinata and C. cymbopogonis have been detected from rice seed (Benoit and Mathur, 1970). Curvularia lunata is a seed-borne pathogen of rice (Parimelazhagan and Francis, 1999) and it can cause leaf blight (Kim and Lee, 1998). Curvularia protuberata is a new seed-borne pathogen and it can cause germination failure and is an associated microorganism of spotting of rice grain (Sisterna and Dal Bello, 1998). Knowledge on the association of fungi with rice seed is important for controlling seed-borne diseases and producing high quality healthy seeds. species and determine the association of Curvularia, the pathogen occurring on boro rice seeds in northern district of Bangladesh is the aim of project.
Materials and Methods
150 seed samples of three varieties of boro rice namely BR 1 (V1), BR 26 (V2) and BR 28 (V3) were chosen and collected from of Dinajpur district (Sador, Birgonj, Chirirbandor, Birol and Shetabgonj) and tested by blotter method based on 400 seeds of each sample during the period of 1998-99. The seeds were incubated on moist blotter at day temperature under alternating cycles of 12h defused daylight near glass windows and 12h natural darkness for a period of 8 days. Prevalence of fungal species and their morphological studies and have been done based on the nature of conidiophore, the arrangement of conidia on conidiophores and the shape, size and colour of the conidia.
Results
Habit characters of Curvularia on Rice: The characteristic used in the identification of different species of Curvularia are tabulated in Table 1, while details on the morphology of conidia under compound microscope are summarized in Table. The descriptions are made on the basis of colonies on seed, after 8 days of plating.
The species observed were C. geniculata, C. oryzae, C. lunata, C. eragrostidis, C. intermedia. The infection percentages varied from 0.0-6.2. Curvularia lunata had a most predominant infection of 5.5, followed by Curvularia oryzae 3.7, Curvularia geniculata 3.0, Curvularia eragrostidis 2.6 and Curvularia intermedia 0.6 the least occurring fungus. The range of seed infection slightly varied from 3.5 to 4.5% place to place and 3.3 to 4.3% variety to variety.
Discussion
Consulting the relevant literatures identified the Curvularia species. The recorded species were difficult be distinguished from each other because of more or less similar characteristics. Moreover the arrangement of conidia on conidiophore of the Curvularia geniculata, Curvularia eragrostidis, Curvularia intermedia are acropleurogenous types where the appearance of conidia was broad in the middle. The median septum of Curvularia eragrostidis and Curvularia intermedia are thick whilst in Curvularia geniculata have bigger cell in the middle of the conidial body. Compound microscope observation result revealed that in Curvularia eragrostidis the conidia were mostly straight whilst and in Curvularia intermedia were curved (Table 1).
| Table 1: | Characters of Curvularis species on rice seed under stereomicroscope-Blotter test |
| Table 2: | Conidial structure under compound microscope |
| Table 3: | Frequency of seed infection of different locations on different rice varieties |
| Legends: Sador = L1, Birgonj=L2, Chiribandor = L3, Birol = L4 and Shetabgonj = L5 | |
| Table 4: | Frequency of seed infection of Curvularia species on different rice varieties |
The conidia of Curvularia lunata were more easily distinguished from others because the conidia in this case were mostly curved, curvature at the bigger cell towards the tip and third cell from the base were bigger and darker than other end cells with rounded tips.
Curvularia oryzae have mostly pyriform conidia and arranged on conidiophore singly. Moreover the second cell from the base was found larger and the third septum from the base was darker (Table 2). The above taxonomic studies of different species of Curvularia supported by the Subramanian (1953), Ellis (1966), Boedijn (1933) and Sivanesan (1987). Their identifications were mainly based on cultural studies. In routine health testing of rice seed, the same criteria as employed for rice in the identification of the different species were opened by Benoit and Mathur (1970). The colour of the conidia was mostly black which was not helpful for identification (Table 2).
The result showed that the range of infection percent of locations and varieties were not much higher (Table 3, 4) probably because of present less inoculum potential and cultivation of tolerant cultivars against Curvularia.
The present investigation provides the information on the occurrence and distribution of the above five species of Curvularia. The information is important in respect of breeding programme and planning for disease management. Further investigation regarding Curvularia species on rice seeds is required for more species suggested on different varieties of rice in different districts of Bangladesh.