|Pearl Millet Diseases - Questionable or Poorly Described|
Bacterial Leaf Spot - Bacterial Leaf Blotch
Bacteria Xanthomonas penniseti (Rajagopalan and Rangaswami 1958) and Xanthomonas annamalaiensis (Rangaswami et al. 1961a), isolated from diseased pearl millet in India, and Xanthomonas rubrisorghi, isolated from sorghum and pathogenic to pearl millet (Rangaswami et al. 1961b) have been determined to be Erwinia herbicola (Qhobela and Claflin 1988). These bacteria are gram negative, non-capsulated, non-spore forming, short rods with a single monotrichous polar flagellum. Colonies are dull shiny yellow on nutrient agar and no soluble pigment is formed. Because standardized inoculation procedures were not used in prior assays, the assumption that Erwinia herbicola is pathogenic to pearl millet should be reexamined (L.E. Claflin, personal communication). A later report suggests that Pantoea agglomerans (=Erwinia herbicola) is pathogenic to pearl millet in Zimbabwe (Frederickson et al. 1997)
Yellow leaf blotch, caused by a bacterium tentatively identified as a Pseudomonas spp., was reported on pearl millet, maize, sorghum, Johnsongrass, wheat, and other unidentified grasses in Cameroon, Ghana, Niger, Nigeria, and Senega (Zummo 1976). Lesions can develop in the seedling stage, and are cream yellow to light beige and distinct, with a tendency for streaks to follow the veins. Lesions in pearl millet usually formed at leaf tips. Young infected plants can be stunted. Some differences in symptoms exist between this and other diseases caused by Pseudomonas spp., suggesting it may be a different pathogen. The observation that no bacterial exudate is associated with the lesions is generally not characteristic of bacterial diseases. In addition, the genus of the bacterium was not positively identified. Additional studies on the etiology of this disease is required.
Alternaria alternata (Fr.) Keissler was isolated from pearl millet samples (either leaf spots or head mold, not clearly stated) and inoculated back onto pearl millet. Scattered oval to irregular shaped lesions developed on the leaves. Lesions were copper red with black in the center, and varied from 5 to 20 mm in diameter. This disease has been reported once (Gaikwad and Rane 1977) and may be an artifact of controlled inoculations.
Charcoal rot, caused by Macrophomina phaseolina has been reported on "millet" (Botswana 1987). This report is subject to question, since the "millet" referred to in the report was not specifically identified as pearl millet, and several pathogens of pearl millet (for example, Puccinia penniseti, Tolyposporium penicillariae, and Claviceps fusiformis) were identified as causal agents of diseases of maize. Further verification will be useful before confirming that charcoal rot is a disease of pearl millet in Botswana.
A disease called dry stalk rot of pearl millet was described from India and reported to be caused by Macrophomina phaseolina (Singh et al. 1997). The disease caused blakening or browning of the stem usually above the first node and extending beyond the second or third node. Tillering was reduced and plants often died before flowering. Pycnidia developed on infected internodes. Pith tissue was disintegrated and vascular bundles were free. Infected plants sometimes lodged. Inoculations were performed to confirm pathogenicity.
Konde et al. (1980) has reported isolation of R. bataticola [=Sclerotium bataticola Taub (teleomorph: Macrophomina phaseolina (Tassi) G. Goid.)] from seed.
Small, dull-brown, round to irregular spots on pearl millet foliage were observed in India in 1967. After controlled inoculations, small watersoaked lesions developed within 48 hours of incubation. During the subsequent 48 hours lesions turned from light yellowish green through yellowish grey, light grey and finally dark grey, due to profuse sporulation of the pathogen. Lesions eventually covered large areas of the leaf and ultimately caused drying of the entire leaf. The pathogen was identified as the condial stage of Cochliobolus bicolor (anamorph: Bipolaris bicolor) (Misra and Singh 1974). Additional occurences of infection of pearl millet have not been reported, however, see Table 2 for other Bipolaris species reported to be pathogenic to Pennisetum.
A single diseased panicle of pearl millet grown next to foxtail millet (Setaria italica Beauv) was observed in India in 1975. Florets in the lower half of the affected panicle were greyish and glued and pressed toward the rachis. Spores were acicular hyaline and measured 15-24 x 1.4-1.8 um with an average length of 19.26 um, which is very close to spore size of E. oryzae on S. italica.This disease was an atypical occurrence that does not appear to have been observed again. E. oryzae has also been reported on Pennisetum hohenackeri and P. alopecuroides (Reddy and Channamma 1976). Balansia is the teleomorph of Ephelis. Balansia claviceps Speg. has been cited as a pathogen of pearl millet in Senegal by Ramakrishnan (1971), however, the source citation referred to in that publication is incorrect and thus is unavailable for examination. Other reported hosts include Echinochloa crus-galli, Setaria italica, Isachne elegans and Eragrostis tenuifolia (Venkatakrishniah 1952).
A phycomycete resembling a species of Achlya was isolated from leaves of kikuyu grass (Pennisetum clandestinum) in Australia. Leaves become uniformly yellow with brown flecking. Roots become yellowish-brown and rotted. No information is available concerning its pathogenicity to pearl millet or other Pennisetum species (Wong 1975).
Leptosphaerulina trifolii (Rost) Petr was isolated from pearl millet samples (either leaf spots or head molds, not clearly stated) and inoculated back onto pearl millet. Initial infection of leaves developed as small light brown spots which later turned papery with perithecia in the center. Ascospores have muriform septation. This disease has been reported once (Gaikwad and Rane 1977) and may be an artifact of controlled inoculations.
Pestalotia disseminata Thuman was isolated from pearl millet samples (either leaf spots or head molds, not clearly stated) and inoculated back onto pearl millet. Initial infection of leaves developed as minute brown spots which subsequently developed to oval to elliptical brown lesions with dark copper red margins surrounded by a faint yellow halo, varying from 5 to 20 mm diameter. This disease has been reported (Gaikwad and Rane 1977) and may be an artifact of controlled inoculations.
Information on smut of pearl millet is quite confused in the literature. Several apparently unique species have been reported or implicated in the cause of smut diseases of pearl millet. The following paragraph is derived from Rachie and Majmudar (1980).
AThe fungus Tolyposporium senegalense Speng. (sic), T. bullatum Schroet., and Sorosporium bullatum Schroet. was first reported from Africa (Corbetta 1954); but Mundkur (1940) reported its occurrence in the region of Poona, India. Hirschorn (1941) suggested the name T. bullatum, as T. senegalense was only a synonym of the same species. It also attacks Echinochloa crusgalli in Germany.@
Corbetta's (1954) paper is in Italian, and from what I can understand from it, he is attempting to clarify the nomenclature of the smut pathogen of Panicum crus-galli and Panicum erectum. I believe that his final conclusion is that Tolyposporium bullatum, Sphacelotheca destruens, and Sorosporium panici-miliacei should be named Sorosporium bullatum. Pearl millet, or an associated smut pathogen of pearl millet, is not mentioned in the paper.
The argument for synonymy of Tolyposporium senegalense and T. bullatum was made by Hirschorn (1941). The paper is written in Portuguese (?) and I believe that Hirschorn promoted lumping these two species based upon morphologic similarities. Cross-inoculation studies were unsuccessful, but he attributed this to physiological differences. Although similar morphology may be adequate to include the fungi into the same genus, examination of sporidial cross compatibility should probably be examined before including concluding the fungi are identical, particularly since the two fungi are reproductively isolated from each other because of host specificity. Vanky (1977) split several species out from Tolyposporium into a new genus, Moesziomyces. After examining the type specimens it was concluded that T. penicillariae and T. senegalense were the same species, M. penicillariae, which differed from M. bullatus. Mordue (1995) reinforced the observation that M. penicillariae and M. bullatus are morphologically similar, but there appears to be biological differences in host specificity and possibly in germination.
Mundkur (1940) described a fungus named Ustilago penniseti. This specimen was described from infection on Pennisetum fasciculatum, and was not been demonstrated to be pathogenic to pearl millet, Pennisetum glaucum. Subsequent observations of the disease have not been reported.
Likewise, Mundkur (1939) also stated that he considers Tilletia pennisetina and Neovossia barclayana as synonyms. These type specimens were collected from Pennisetum alopecuroides and P. orientale, respectively, and thus cannot be considered to be pathogenic to pearl millet without completing Koch's postulates. Subsequent observations of the disease have not been reported.
Mundkur's (1939) report of Tilletia ajrekari as a pathogen of pearl millet should also be examined closely. In the original description (1939), he stated that a single infected ovary was found from greenhouse-grown plants. Examination of other plants in the greenhouse and field failed to identify any other similar sori. In this case, a new species was described based upon spore characteristics of a single infected ovary. In a later publication (Mundkur and Thirumalachar 1952) the description is the same, however, here it is implied that several ovaries are infected. Because of the non-systemic infection and other characteristics of the spores he stated that it may be a species of Neovossia, however, "This can be definitely decided only after the germination of the spores has been obtained". In this reference he also states "germination unknown" regarding the spores. This leads me to believe that no inoculations were ever performed. This fact, plus the discrepancy between a single sorus infected as described in the 1939 paper and several sori in the 1952 reference leads me to question whether this work is valid or can be reproduced.
In India, black, fluffy fungal growth was observed on the auricles of older leaves on either side of the leaf sheath. The fungus was identified as Microxyphiella hibiscifoia Bat., Nasc., & Cif. Elongate to linear pycnidia, black to dark olive in color, attach superficially to epidemal cells. Although described as a pathogen (Singh and Grover 1968), sooty mold fungi generally colonize insect honeydew or plant exhudates, and are not considered to be plant pathogens.
Information concerning stalk rots of pearl millet is currently limited. Fusarium poae (Peck) Wr. was isolated from diseased stalks and inflorescences in India (Ramakrishnan and Subramanian 1952). Inoculations to verify pathogenicity were not performed.