Comparison of transmission abilities of four Cicadulina species vectors of maize streak virus from Nigeria

Four Cicadulina species [Cicadulina arachidis China, Cicadulina dabrowskii Webb, Cicadulina mbila (Naude), and Cicadulina storeyi China (Homoptera: Cicadellidae)] found during field surveys in 1997–1999 across five ecological zones in Nigeria were reared in screenhouses and females were used in a study to compare their abilities to transmit Maize streak virus (Geminiviridae: genus Mastrevirus) from maize to maize using the susceptible variety Pool 16. The results showed that for both acquisition access feeding periods (AAP) and inoculation access feeding periods (IAP), transmission efficiencies decreased in the following order: C. storeyi > C. mbila > C. arachidis > C. dabrowskii. The transmission efficiencies of these vectors increased with longer feeding periods, as the means of susceptible test plants that showed Mastrevirus symptoms for both acquisition and IAPs were higher for 24 and 48 h than for 1 and 2 h for all four species studied. The epidemiological implications of these differences in transmission abilities are discussed.     

Incidence and infection rate of Maize streak virus disease by Cicadulina triangular on maize plants and its distribution from the lowest diseased leaf under tropical conditions

Several programmes have been initiated for the development of maize varieties with resistance traits of Maize streak virus (MSV) by International Institute of Tropical Agriculture (IITA), Ibadan, and have been released to farmers and research scientists. Therefore, a survey was conducted in five states in the south west of Nigeria (Oyo, Ogun, Ondo, Ekiti and Osun) during the raining planting season to determine the incidence of MSV disease by visual examination and sero-diagnostic screening of symptomatic plants. The determination of infection rate of MSV disease by Cicadulina triangular on maize plant and its distribution from the lowest diseased leaf was also studied. The mean MSV disease incidence observed in these states was 35.95% which confirms the presence of MSV in the south west of Nigeria. Sero-diagnostic screening of virus-induced symptomatic leaf samples indicated that out of the 250 leaves sampled per state, 24.4% tested positive for MSV in Oyo, 25.6% in Ondo, 34% in Ogun, 19.6% in Ekiti and 38.8% in Osun. In two-week-old plants, symptoms developed on the leaves that were emerging at the time of inoculation, while in six-week-old plants, symptoms developed on the leaves directly below the emerging leaves irrespective of the number of C. triangular used. These suggest that the lowermost leaf with symptoms of the disease indicates the growth stage at which a plant was infected. There was a relationship between symptom expression and plant age which could be very effective when carrying out surveys to gather information for epidemiological studies. In addition, the 10 varieties of maize inoculated with MSV through C. triangular transmission showed no significant difference in disease severity over time irrespective of the number of C. triangular used.     

Assessment of yield losses as a result of co-infection by maize streak virus and maize stripe virus in Mauritius

The effect of co-infection by maize streak virus (MSV) and maize stripe virus (MStV) on plant growth and grain yield was investigated in a susceptible variety of maize (Zea mays), ZS 5206, in Mauritius. Under natural conditions MSV, transmitted by the leafhopper Cicadulina mbila, was normally established before MStV, which is vectored by the planthopper Peregrinus maidis; as a result, MStV symptoms were often partially or completely masked by those of MSV, making MStV detection by symptomatology very unreliable. MSV and MStV were diagnosed by ELISA and MStV by a novel method of detecting the MStV-coded non-capsid protein. The maize hybrid ZS 5206 was inoculated with either MSV, MStV or both, at two stages in the growth cycle (3–5 or 7–10 leaf stage). A greater reduction in plant growth was observed in plants inoculated singly with MStV (80% and 29% for first and second stage, respectively) than with MSV (50% and 23%, respectively). No cobs were produced by plants singly infected with MStV at the first stage, or co-infected with MSV and MStV at both stages; however, marginal grain production was recorded in plants singly infected with MSV at the first stage (91% reduction), or infected either with MSV or MStV, at the second stage (65% and 80% reduction, respectively). In maize hybrid ZS 5206, MStV is more virulent than MSV; co-infection by both viruses causes greater reductions in plant growth and grain yield than single infection by either virus at a given stage of plant development. In the event of co-infection by MSV and MStV, yield losses can be erroneously attributed to MSV only if the symptoms of MStV are masked by those of the former and if adequate methods for MStV detection are not used.     

Symptoms,epidemiology and management of finger millet mottle streak disease

Abstract

Initial symptoms of the mottle streak disease on finger millet were observed after only 45 days of sowing. Chlorotic mottling, streaking, striping and yellowing were the common symptoms on leaves and the diseased plants had more nodal branches and unproductive tillers. Among the weather parameters relative humidity and rainfall were positively and wind velocity was negatively correlated with disease incidence and the population of the vector Cicadulina bipunctella. A significant positive correlation existed between the vector population and disease incidence. In the disease management study seed extract of Harpullia cupanioides at 10% was found to reduce the mottle streak incidence by 61.42% with maximum yield of 2.28 t/ha. Acephate 0.01% and neem oil 1% spray were also found to be effective by reducing the disease incidence up to 39.6 and 36.4% with 2.23 and 2.18 t/ha of grain yield respectively. Enhanced activation of defense-related enzymes such as peroxidase (PO), polyphenol oxidase (PPO) and phenylalanine ammonia lyase (PAL) were noticed in AVP sprayed plants compared to control.     

Intercrops, Cicadulina spp., and maize streak virus disease

Intercrops of bean and finger millet were tested as a possible means of controlling maize streak virus disease (MSVD) in maize by disrupting the mating behaviour of the insect vectors of the maize streak virus, Cicadulina mbila and C. storeyi. A series of three trials were done. In the first, MSVD incidence 2 months after sowing was reduced to 14.9% and 17.4% in millet and bean intercrops compared to 29.5% in the pure maize stand. The number of male Cicadulina spp. caught on sticky pole traps was also significantly reduced relative to the control, but there was little effect on the catch of females. There was no significant yield penalty for the millet intercrop but maize yield was 49% lower in the bean intercrop treatment than in the pure stand. In the second trial, there were two millet and two bean intercrop treatments and a maize only control. Fewer male Cicadulina spp. were caught in the intercrop treatments relative to the control but MSVD incidence was reduced in one millet intercrop treatment only for which the associated maize yield penalty was 89%. In the final trial the bean intercrop was again tested but it had no effect on MSVD incidence. These experiments demonstrated that intercropping maize with bean or millet decreased vector activity and/or vector numbers. Vector catches were predominantly male, and catches of males but not females were reduced in the intercrop treatments compared with pure stands. However the lower vector catch was not consistently associated with a significant reduction in MSVD incidence, and when it was there was often an associated yield penalty in the maize due to the intercrop.     

A study of the mode of transmission of maize streak virus by Cicadulina mbila using an enzyme-linked immunosorbent assay

Two batches of Cicadulina mbila were given two distinct acquisition access periods (AAP) (3 h and 50 h) on maize plants infected with maize streak virus (MSV). Infectivity assays on susceptible maize were carried out 1, 3, 10, 17, 26 and 35 days after the AAP. Transmission efficiency was significantly higher for C. mbila subjected to the 50-h AAP. At the same time as the infectivity assays, the amount of MSV in each leafhopper was determined by an indirect double antibody sandwich (IDAS) ELISA. There were more ELISA-positive insects after the 50-h AAP than after the 3-h AAP. In the group given a 3-h AAP, only 7% of the insects tested between day 1 and 35 were found to be positive by ELISA. In contrast, after the 50-h AAP, the majority of C. mbila were positive, yet a decrease in ELISA-positive insects was noticed from day 17 onwards. Using a calibration curve obtained with purified virus, as little as 0.15 ng of MSV per insect could be measured by the IDAS-ELISA. A mean value of 0.36 ng of MSV per C. mbila was found 3 days after the 50–h acquisition, whereas 14 days later there was only 0.20 ng of virus per insect. For comparison, when leafhoppers were kept on infected maize, they displayed substantial accumulation of MSV up to an average of 3.83 ng of MSV per insect after 35 days of continuous acquisition. The amount of virus per insect detected in females was usually greater than the amount detected in males. Our results suggest that MSV does not multiply in C. mbila and contribute to the understanding of the persistence of transmission efficiency in the absence of virus multiplication.     

Vector and Host-Plant Relationships of the Leafhopper-Borne Maize Yellow Stripe Virus

Maize yellow stripe virus (MYSV), associated with tenuivirus-like filaments, is transmitted in a persistent manner by the leafhopper Cicadulina chinai. In this vector, MYSV acquisition and inoculation threshold times were 30 min each, latent period ranged from 4.5 to 8 days depending on temperature (14-25 °C), and retention periods were as long as 27 days. Up to 26 % of C, chinai collected from maize fields in Giza, Egypt, during September and October 1985 were naturally infective with MYSV. Two symptom-types (fine and coarse stripe) appeared on experimentally infected plants, usually on separate leaves of the same plant. However, these two symptom-types could not be isolated on separate plants through transmission by single C. chinai leafhoppers. MYSV was transmitted by nymphs and adults of C. chinai from maize to maize, wheat and barley, and from wheat to maize plants. Up to 6 % of the wheat plants examined in Naga Hamadi (Southern Egypt) in February 1986, were naturally infected. It is suggested that wheat, barley and possibly graminaceous weeds may serve as winter hosts or reservoirs for MYSV and its leafhopper vector in Egypt.     

Detection of maize streak virus antigens over time in different parts of maize plants of a sensitive and a so-called tolerant cultivar by ELISA

Maize streak virus (MSV) capsid antigens were detected over time in different parts of maize plants of a sensitive (INRA508) and a so-called tolerant (“tolerant”) cultivar (IRAT297) using a direct or an indirect double antibody sandwich ELISA. Based on three types of experiments, it was shown that the antigens were distributed in the plant according to the age of the tissues. When the virus was inoculated on a particular leaf of 18-day old plants with infective Cicadulina mbila, only the young leaves above the inoculated one were positive by ELISA but not the older ones below. The antigens could not be detected in the inoculated leaf. At day 3 after inoculation, the antigens were detected in the sheath and/or in the whorl of the third leaf above the inoculated one but not in the oldest part of the leaf, the unfolded lamina. Plants of the sensitive cultivar were inoculated at 9 days with C. mbila deposited in the whorl. At 23 h after inoculation, the antigens were detected in the sheath but not in the whorl which was found to be positive only at 32 h. On the basis of these results, a hypothesis of the mode of virus infection is proposed. Our results contribute to a better understanding of the relationship between the age of the plant at inoculation and yield loss as well as secondary infection. By transmission tests with C. mbila, it was shown that virus could only be acquired from leaves exhibiting symptoms. Virus concentrations were measured in plant samples by ELISA using a range of dilutions of purified virus. The virus concentrations were higher in the sensitive than in the “tolerant” cultivar, but no difference in antigen distribution was observed between the two cultivars. The “tolerant” cultivar appeared to be resistant to virus multiplication.     

六种寄主植物对二点叶蝉生长发育和繁殖的影响

在25℃恒温条件下,以盆栽玉米、小麦、高粱、水稻、谷子、大麦及虮子草为食料,研究了食物对二点叶蝉实验种群生长发育及繁殖的影响．结果表明,二点叶蝉在虮子草上不能完成世代发育．在其余6种寄主植物上各虫态的发育历期、存活率、雌成虫寿命及单雌平均产卵量存在显著差异．从卵到若虫期的发育历期在高粱上最短(24．1d),玉米上次之(24．2d),小麦上最长(25．5d);取食水稻时。若虫的存活率(40．8％)最低,成虫寿命最短(12．2d),单雌平均产卵量(12．3粒)也显著低于其它5种寄主植物．用生命表参数综合评价表明,6种寄主植物中,谷子和玉米最适合二点叶蝉生长发育及繁殖。其次分别为高粱、大麦、小麦、水稻。     

Priming of Production in Maize of Volatile Organic Defence Compounds by the Natural Plant Activator cis-Jasmone

cis-Jasmone (CJ) is a natural plant product that activates defence against herbivores in model and crop plants. In this study, we investigated whether CJ could prime defence in maize, Zea mays, against the leafhopper, Cicadulina storeyi, responsible for the transmission of maize streak virus (MSV). Priming occurs when a pre-treatment, in this case CJ, increases the potency and speed of a defence response upon subsequent attack on the plant. Here, we tested insect responses to plant volatile organic compounds (VOCs) using a Y-tube olfactometer bioassay. Our initial experiments showed that, in this system, there was no significant response of the herbivore to CJ itself and no difference in response to VOCs collected from unexposed plants compared to CJ exposed plants, both without insects. VOCs were then collected from C. storeyi-infested maize seedlings with and without CJ pre-treatment. The bioassay revealed a significant preference by this pest for VOCs from infested seedlings without the CJ pre-treatment. A timed series of VOC collections and bioassays showed that the effect was strongest in the first 22 h of insect infestation, i.e. before the insects had themselves induced a change in VOC emission. Chemical analysis showed that treatment of maize seedlings with CJ, followed by exposure to C. storeyi, led to a significant increase in emission of the defensive sesquiterpenes (E)-(1R,9S)-caryophyllene, (E)-α-bergamotene, (E)-β-farnesene and (E)-4,8-dimethyl-1,3,7-nonatriene, known to act as herbivore repellents. The chemical analysis explains the behavioural effects observed in the olfactometer, as the CJ treatment caused plants to emit a blend of VOCs comprising more of the repellent components in the first 22 h of insect infestation than control plants. The speed and potency of VOC emission was increased by the CJ pre-treatment. This is the first indication that CJ can prime plants for enhanced production of defensive VOCs antagonist towards herbivores.     

A single amino acid change in the coat protein of Maize streak virus abolishes systemic infection, but not interaction with viral DNA or movement protein

Functional coat protein (CP) is important for host plant infection by monopartite geminiviruses. We identified a proline-cysteine-lysine (PCK) motif at amino acids 180–182 of the maize streak virus (MSV) CP that is conserved in most of the cereal–infecting Mastreviruses. Substitution of the lysine (K) with a valine (V) in the CP of MSV to produce mutant MSVCP182V abolished systemic infection in maize plants, although the mutant replicated around the inoculation site and, unlike other MSV CP mutants, enabled single-stranded (ss) DNA accumulation in suspension cells. The stability of the mutant protein, CP182V, in infected cells was confirmed by immunoblotting, but virions could not be detected. Like the wild-type (wt) CP, CP182V localized to the nucleus when expressed in insect and tobacco cells, and the Escherichia coli-expressed protein bound both ss and double-stranded DNA and interacted with movement protein in vitro. Taken together, these data suggest that mutation of amino acid 182 affects virion formation of MSV, either by affecting encapsidation per se or by affecting particle stability, and that virions are necessary for the long-distance movement of MSV in maize plants.     

Leafhopper transmission of a virus causing maize wallaby ear disease

A virus causing maize wallaby ear disease was transmitted experimentally by Cicadulina bimaculata to fourteen species of monocotyledonous plants. It was also transmitted by Nesoclutha pallida, and by grafting. The symptoms obtained resemble closely those reported for maize leaf gall disease in the Philippines and maize rough dwarf virus in Italy and Israel. About 85% of C. bimaculata caught in the field carried maize wallaby ear virus (MWEV), and many of their progeny were viruliferous even when not allowed access to infected plants. The proportion of infective individuals in clones bred for nine generations from selected non-transmitting adults decreased from 85% in the first nymphs to less than 1%; such individuals were difficult to rear, as their fecundity and longevity decreased greatly. N. pallida transmitted MWEV after injection with partially purified extracts of infected plants. Spherical particles c. 85 nm in diameter were found in the salivary glands of viruliferous C. bimaculata, but not in those of non-transmitting individuals. The particles occurred in tubules in the cytoplasm and each had a densely stained core c. 50 nm in diameter. Particles similar in size to the core were found in extracts of infected but not uninfected maize, and in extracts of viruliferous but not in non-viruliferous C. bimaculata and N. pallida.     

Amplification and expression of the β-glucuronidase gene in maize plants by vectors based on maize streak virus

Maize streak virus (MSV) is a geminivirus infecting monocotyledonous plants. Its genome consists of one molecule of circular, single-stranded DNA of 2.7 kb. The viral DNA can be efficiently introduced into maize plants by agroinfection which results in systemic infection. To explore the potential of MSV as a replicative gene vector, a reporter gene coding for β-glucuronidase (GUS) was inserted into the non-coding region of the viral genome. The resulting construct (MSV—GUS) of about 5.9 kb was still able to replicate in cells of maize plants although it was unable to induce viral symptoms. This replication led to a five to 10-fold increase in the mean number of GUS-positive spots per plant as compared with infections with the GUS gene without the MSV replicon. MSV—D—GUS, which differed from MSV—GUS by the deletion of genes V1 and V2 encoding a putative movement protein and the coat protein, respectively, also replicated and produced even more GUS-positive spots. In both MSV—GUS- and MSV—D—GUS-infected plants, the GUS-positive spots were located mainly on the veins of leaves whose primodia had already developed at the time of inoculation and never on the leaves developing later. Thus, neither viral construct was able to move systemically, most probably because the DNAs were too large to be packaged.     

Maize streak virus: an old and complex 'emerging' pathogen

Maize streak virus (MSV; Genus Mastrevirus, Family Geminiviridae) occurs throughout Africa, where it causes what is probably the most serious viral crop disease on the continent. It is obligately transmitted by as many as six leafhopper species in the Genus Cicadulina, but mainly by C. mbila Naudé and C. storeyi. In addition to maize, it can infect over 80 other species in the Family Poaceae. Whereas 11 strains of MSV are currently known, only the MSV‐A strain is known to cause economically significant streak disease in maize. Severe maize streak disease (MSD) manifests as pronounced, continuous parallel chlorotic streaks on leaves, with severe stunting of the affected plant and, usuallly, a failure to produce complete cobs or seed. Natural resistance to MSV in maize, and/or maize infections caused by non‐maize‐adapted MSV strains, can result in narrow, interrupted streaks and no obvious yield losses. MSV epidemiology is primarily governed by environmental influences on its vector species, resulting in erratic epidemics every 3–10 years. Even in epidemic years, disease incidences can vary from a few infected plants per field, with little associated yield loss, to 100% infection rates and complete yield loss. Taxonomy: The only virus species known to cause MSD is MSV, the type member of the Genus Mastrevirus in the Family Geminiviridae. In addition to the MSV‐A strain, which causes the most severe form of streak disease in maize, 10 other MSV strains (MSV‐B to MSV‐K) are known to infect barley, wheat, oats, rye, sugarcane, millet and many wild, mostly annual, grass species. Seven other mastrevirus species, many with host and geographical ranges partially overlapping those of MSV, appear to infect primarily perennial grasses. Physical properties: MSV and all related grass mastreviruses have single‐component, circular, single‐stranded DNA genomes of approximately 2700 bases, encapsidated in 22 × 38‐nm geminate particles comprising two incomplete T = 1 icosahedra, with 22 pentameric capsomers composed of a single 32‐kDa capsid protein. Particles are generally stable in buffers of pH 4–8. Disease symptoms: In infected maize plants, streak disease initially manifests as minute, pale, circular spots on the lowest exposed portion of the youngest leaves. The only leaves that develop symptoms are those formed after infection, with older leaves remaining healthy. As the disease progresses, newer leaves emerge containing streaks up to several millimetres in length along the leaf veins, with primary veins being less affected than secondary or tertiary veins. The streaks are often fused laterally, appearing as narrow, broken, chlorotic stripes, which may extend over the entire length of severely affected leaves. Lesion colour generally varies from white to yellow, with some virus strains causing red pigmentation on maize leaves and abnormal shoot and flower bunching in grasses. Reduced photosynthesis and increased respiration usually lead to a reduction in leaf length and plant height; thus, maize plants infected at an early stage become severely stunted, producing undersized, misshapen cobs or giving no yield at all. Yield loss in susceptible maize is directly related to the time of infection: infected seedlings produce no yield or are killed, whereas plants infected at later times are proportionately less affected. Disease control: Disease avoidance can be practised by only planting maize during the early season when viral inoculum loads are lowest. Leafhopper vectors can also be controlled with insecticides such as carbofuran. However, the development and use of streak‐resistant cultivars is probably the most effective and economically viable means of preventing streak epidemics. Naturally occurring tolerance to MSV (meaning that, although plants become systemically infected, they do not suffer serious yield losses) has been found, which has primarily been attributed to a single gene, msv‐1. However, other MSV resistance genes also exist and improved resistance has been achieved by concentrating these within individual maize genotypes. Whereas true MSV immunity (meaning that plants cannot be symptomatically infected by the virus) has been achieved in lines that include multiple small‐effect resistance genes together with msv‐1, it has proven difficult to transfer this immunity into commercial maize genotypes. An alternative resistance strategy using genetic engineering is currently being investigated in South Africa. Useful websites: 〈 http://www.mcb.uct.ac.za/MSV/mastrevirus.htm 〉; 〈 http://www.danforthcenter.org/iltab/geminiviridae/geminiaccess/mastrevirus/Mastrevirus.htm 〉.     

Evidence that maize wallaby ear disease is caused by an insect toxin

Colonies of the leafhopper Cicadulina bimaculata were established from single male and female insects raised from surface sterilised eggs and shown to be free of leafhopper A virus (LAV). Insects from these colonies were as capable of inducing maize wallaby ear disease (MWED) in maize seedlings as those with LAV indicating that the virus is not involved in the etiology of MWED. Maize seedlings colonised by C. bimaculata in glasshouse trials developed initial MWED symptoms within 6–8 days of infestation. The symptoms intensified thereafter and many plants died after more than 16 days' exposure, even after the insects were killed with insecticide. However, when freed from the insects before symptoms became very severe, plants recovered and assumed normal growth. These observations support the view that MWED is caused by an insect toxin.     

A gene for resistance to the maize streak virus in the African CIMMYT maize inbred line CML202

Resistance to maize streak virus (MSV) is an essential trait of improved maize varieties in sub-Saharan Africa. We mapped quantitative trait loci (QTL) for resistance to MSV in a population of 196 F_2:3 lines derived from a cross between the maize inbred lines CML202 (resistant) from CIMMYT-Zimbabwe and Lo951 (susceptible) from Italy. Field tests were planted at two locations in Zimbabwe, inoculated with viruliferous leaf hoppers (Cicadulina mbila), and scored twice (21 and 83 days after infesting, DAI) on a 1–5 scale. The mean final streak intensity (score 2) of the parent lines was 2.2 (CML202) and 4.8 (Lo951). Genotype × location interaction was large for score 1 but negligible for score 2. Consequently, the heritability was higher for score 2 (0.93) than for score 1 (0.62). By composite interval mapping across locations, using a linkage map with 110 RFLP loci, four significant (LOD 3.0) QTL were identified for score 1 on chromosomes (C) 1, 2, 3, and 4, respectively. All four were contributed by CML202. For score 2, only the QTL on C 1 was significant (LOD =37), explaining 59% of the phenotypic and 64% of the genotypic variance. The QTL's partially dominant gene action was consistent with the nearly intermediate resistance of the F₁ generation (relative heterosis for resistance 12%). The presence of one major QTL is consistent with the bimodal frequency distribution of the mapping population showing a clear 3:1 segregation. This gene seems to be allelic or identical to Msv1, a major resistance gene which was previously identified in the same genomic region in Tzi4, an inbred line from IITA. Inbred CML202 had lower final disease ratings than Tzi4. The greater resistance of CML202 may be due to allelic differences at the msv1 locus or due to the minor QTL on C 2, 3, and 4 which were not detected in Tzi4.z y Trigo (International Maize and Wheat Improvement Center); IITA, International Institute of Tropical Agriculture; IRAT, Institute de Recherches Agronomiques Tropicales et des Cultures Vivrières; KARI, Kenya Agricultural Research Institute; MSV, maize streak virus; QTL, quantitative trait locus/loci     

Excision of a transposable element from a viral vector introduced into maize plants by agroinfection

The geminivirus maize streak virus (MSV) was used as a vector to introduce the maize transposable element Dissociation (Ds) and to study its excision in maize plants. MSV carrying Ds1 in its genome was introduced into maize plants by agroinfection. Excision of the Ds1 element from the MSV genome was detected only when functions from the transposable element Activator (Ac) were supplied in trans, either endogenously by the recipient maize plant or by co-transformation with Agrobacterium carrying a genomic Ac clone. The excision of Ds1 could easily be visualized by the appearance of viral symptoms induced by the revertant virus. The junction sequences left on the MSV genome after excision revealed 'footprints' typical of transposition as described for maize. From these results, we conclude that transposition functions in our system and that the use of the MSV replicon provides a rapid and simple tool for the investigation of the excision of transposable elements in maize plants.     

Influence of brown stink bug feeding,planting date and sampling time on common smut infection of maize

Phytopathogen infections are frequently influenced by both biotic and abiotic factors in a crop field. The effect of brown stink bug, Euschistus servus (Hemiptera: Pentatomidae), feeding and planting date and sampling time on common smut (Ustilago maydis) infection percentage of maize plants was examined in 2005 and 2006, and 2010 and 2011, respectively. Brown stink bug adult feeding on maize hybrid “DKC6971” at flowering in 2005 and 2006 did not influence smut infection percentage when examined using 3 treatments (i.e., 0 adult, 5 adults, and 5 adults mixed with the smut spores). The smut infection percentages were <3% (n = 12) in the 3 treatments. The smut infection percentage among the 4 weekly samplings was the same, so was natural aflatoxin contamination at harvest among the treatments. The 2nd experiment showed that planting date did not affect the smut infection percentage in either 2010 or 2011. But, the smut infection percentage from the postflowering sampling was greater than preflowering sampling in both years. The smut infection percentage varied among the germplasm lines in 2010, but not in 2011. This study demonstrated that brown stink bug feeding at flowering had no effect on smut infection in maize, and the best time for smut evaluation would be after flowering. The temperature and precipitation might have also influenced the percentage of smut‐infected maize plants during the 4 years when the experiments were conducted. The similarity between kernel‐colonizing U. maydis and Aspergillus flavus infections and genotype × environment interaction were also discussed.     

Development of streak virus-resistant maize populations through improved challenge and selection methods

Tolerance to maize streak virus (MSV) was found and rapidly incorporated into high yielding maize populations for the tropics. Methods were developed for vector propagation and rapid accurate screening of many accessions for virus tolerance in large screenhouses. Tolerance was found in only two accessions and at low frequencies. Further refinements enabled field evaluation for virus tolerance to be combined with high agronomic performance. The tolerance found is simply inherited and was fixed rapidly in breeding. Non-strain specific tolerance was sought by collecting vectors and different indigenous host grasses from a wide area. The tolerance developed was sufficiently high hardly to affect yield of infected plants. It provided epidemiological or field resistance by reducing disease incidence to insignificance under natural conditions. This tolerance and field resistance has proved effective in several countries of East, West and southern Africa. Varieties derived from this work are now being promoted in Nigeria, and they have potential application elsewhere in the lowland tropics.     

Biological characteristics of grass geminiviruses from eastern Australia

Two serotypes of chloris striate mosaic virus (CSMV), paspalum striate mosaic virus (PSMV) and geminiviruses infecting Bromus catharticus and Digitaria didactyla were investigated. Their field occurrence and experimental hosts are listed. Serial transmission data for CSMV by single Nesoclutha pallida show a minimum latent period of 12–14 h, and regular transmission with occasional breaks for up to 50 days. Cicadulina bimaculata did not transmit any isolates after plant feeding acquisition, but transmitted CSMV inefficiently after insect injection. The vector of PSMV was found to be a specific biotype of N. pallida which bred only on Paspalum spp. The rate of transmission of CSMV with the Chloris biotype of N. pallida and of PSMV with the Paspalum biotype reached c. 50% with single insects, but only when freshly-infected source plants were used. Geminate particles were found in thin sections of leaf tissue infected with all four viruses, and partially purified preparations were made of three of these. In gel diffusion tests, the virus from Microlaena stipoides produced a spur reaction with CSMV, when reacted with CSMV antiserum. The B. catharticus and D. didactyla isolates failed to react serologically with CSMV, maize streak or Vanuatu digitaria streak viruses.