|Pearl Millet Diseases - Introduction|
Pearl millet (Pennisetum glaucum (L.) R.Br.) has traditionally been an important grain, forage, and stover crop primarily in the arid and subtropical regions of many developing countries. As pearl millet cultivation expands into non-traditional areas in temperate and developed countries, production constraints from diseases are assuming greater importance. Dissemination of accurate information on diseases of the crop has not kept pace with the increased interest in pearl millet as a viable crop in non-traditional areas.
The literature concerning pearl millet diseases is often confused and contradictory. Many pathological treatises are composed of information on diseases of Amillet@, which is a broad category of any of a number of small-seeded grasses. Millets include pearl millet (Pennisetum glaucum), proso, brown top, or broomcorn millet (Panicum miliaceum), little millet (P. sumatrense), foxtail millet (Setaria italica), finger millet or ragi (Eleusinecoracana), teff (Eragrostis tef), fonio (Digitaria spp.), Guinea millet (Bracharia deflexa), barnyard or Japanese millet (Echinochloa crusgalli), jungle rice millet (E. colonum), Kodo millet (Paspalum scrobiculatum), and Job=s tears (Coix lachryma-jobi). Many diseases among the different millets are quite host-specific, particularly those caused by obligate parasites. Considerable diversity also exists within the genus Pennisetum, which consists of over 100 species having chromosomes numbers in multiples of x=5, 7, 8, or 9 (Oliver 1934). It is not unusual that a pathogen is designated simply as causing a disease on APennisetum@, without a specific host designation. Pearl millet itself has undergone several changes in nomenclature, which can also lead to some confusion. Throughout the literature it is variously referred to as P. glaucum, P. typhoides, P. americanum or other names depending upon the accepted nomenclature at the time. It is also known by several different common names including cumbu, bajra and cattail millet. Because of all these variables, attempts to identify the diseases of Amillet@, without strict differentiation of the host, have resulted in sometimes confused and misinformed quarantine and regulatory policies.This bulletin was written in an attempt to provide some scientific clarity for use in policy-making decisions.
Most of the following information has been derived from the published scientific literature. When possible, I have examined the original publications rather than relying on conclusions and information attributed to earlier scientists by others in more recent publications. Because of the purpose of this document, descriptions of pathogen characteristics and the diseases they cause are necessarily brief. For positive identification of pathogens, reference to the appropriate citations is advised. Designated host ranges can be inconsistent among pathogens. Cross-inoculation studies have not been performed with most of these pathogens, and host specificity and strain specificity are difficult to determine from the literature. Common names of additional hosts were sometimes used instead of binomial nomenclature, and some binomial nomenclature has been changed since publication of the original works. Geographic distributions may vary depending on whether the pathogen has been observed on pearl millet or on other hosts. The accuracy of the geographic distribution on all hosts depends on the degree of host-pathogen specificity, which, as addressed above, is not well-defined for most of these pathogens. For the most part, information on seed transmission of diseases does not exist. Seed infection is well documented for several pathogens, however, transmission to the seedling has not often been demonstrated.
Various regulatory agencies have been concerned about some pathogens which are not well-documented in the literature and their actual role in causing diseases of pearl millet is likewise vague. These pathogens are also discussed in an attempt to address these concerns where questions of thoroughness may arise.
The contributions and comments of L.E. Claflin, Kansas State University, G. Gillaspie, USDA-ARS, S.B. King, ICRISAT, G. Lovell, USDA-ARS, J. Petit de Mange, USDA-APHIS, S.D. Singh, ICRISAT, and A. Tschanz, USDA-APHIS, throughout the development of this document are gratefully acknowledged.