Mixed infections of plant viruses are common in nature, and a number of important virus diseases of plants are the outcomes of interactions between causative agents. Multiple infections lead to a variety of intrahost virus-virus interactions, many of which may result in the generation of variants showing novel genetic features, and thus change the genetic structure of the viral population. Hence, virus-virus interactions in plants may be of crucial significance for the understanding of viral pathogenesis and evolution, and consequently for the development of efficient and stable control strategies. The interactions between plant viruses in mixed infections are generally categorized as synergistic or antagonistic. Moreover, mixtures of synergistic and antagonistic interactions, creating usually unpredictable biological and epidemiological consequences, are likely to occur in plants. The mechanisms of some of these are still unknown. This review aims to bring together the current knowledge on the most commonly occurring facilitative and antagonistic interactions between related or unrelated viruses infecting the same host plant. The best characterized implications of these interactions for virus-vector-host relationships are included. The terms 'synergism' and 'helper dependence' for facilitative virus-virus interactions, and 'cross-protection' and 'mutual exclusion' for antagonistic interactions, are applied in this article.

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Facilitative and antagonistic interactions between plant viruses in mixed infections.

Indian cuisine is renowned and celebrated throughout the world for its spicy treat to the tongue. The flavor and aroma of the food generated due to the use of spices creates an indelible experience. Among the commonly utilized spices to stimulate the taste buds in Indian food, whole or powdered chilli constitutes an inevitable position. Besides being a vital ingredient of of Indian food, chilli occupy an important position as an economic commodity, a major share in Indian economy. Chilli also has uncountable benefits to human health. Fresh green chilli fruits contain more Vitamin C than found in citrus fruits, while red chilli fruits have more Vitamin A content than as found in carrots. The active component of the spice, Capsaicin possesses the antioxidant, anti-mutagenic, anti-carcinogenic and immunosuppressive activities having ability to inhibit bacterial growth and platelet aggregation. Though introduced by the Portuguese in the Seventeenth century, India has been one of the major producers and exporters of this crop. During 2010-2011, India was the leading exporter and producer of chilli in the world, but recently due to a decline in chilli production, it stands at third position in terms of its production. The decline in chilli production has been attributed to the diseases linked with crop like anthracnose or fruit rot causing the major share of crop loss. The disease causes severe damage to both mature fruits in the field as well as during their storage under favorable conditions, which amplifies the loss in yield and overall production of the crop. This review gives an account of the loss in production and yield procured in chili cultivation due to anthracnose disease in Indian sub-continent, with emphasis given to the sustainable management strategies against the conventionally recommended control for the disease. Also, the review highlights the various pathogenic species of Colletotrichum spp, the causal agent of the disease, associated with the host crop in the country. The information in the review will prove of immense importance for the groups targeting the problem, for giving a collective information on various aspects of the epidemiology and management of the disease.

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Chilli Anthracnose: The Epidemiology and Management.

Triticum mosaic virus (TriMV), the type member of the newly proposed Poacevirus genus, and Wheat streak mosaic virus (WSMV), the type member of Tritimovirus genus of the family Potyviridae, infect wheat naturally in the Great Plains and are transmitted by wheat curl mites. In this study, we examined the ability of these viruses to infect selected cereal hosts, and found several differential hosts between TriMV and WSMV. Additionally, we examined the interaction between WSMV and TriMV in three wheat cultivars at two temperature regimens (19 and 20 to 26 degrees C), and quantified the virus concentration in single and double infections by real-time reverse-transcription polymerase chain reaction. Double infections in wheat cvs. Arapahoe and Tomahawk at both temperature regimens induced disease synergism with severe leaf deformation, bleaching, and stunting, with a 2.2- to 7.4-fold increase in accumulation of both viruses over single infections at 14 days postinoculation (dpi). However, at 28 dpi, in double infections at 20 to 26 degrees C, TriMV concentration was increased by 1.4- to 1.8-fold in Arapahoe and Tomahawk but WSMV concentration was decreased to 0.5-fold. WSMV or TriMV replicated poorly in Mace at 19 degrees C with no synergistic interaction whereas both viruses accumulated at moderate levels at 20 to 26 degrees C and induced mild to moderate disease synergism in doubly infected Mace compared with Arapahoe and Tomahawk. Co-infections in Mace at 20 to 26 degrees C caused increased TriMV accumulation at 14 and 28 dpi by 2.6- and 1.4-fold and WSMV accumulated at 0.5- and 1.6-fold over single infections, respectively. Our data suggest that WSMV and TriMV induced cultivar-specific disease synergism in Arapahoe, Tomahawk, and Mace, and these findings could have several implications for management of wheat viruses in the Great Plains.

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Wheat cultivar-specific disease synergism and alteration of virus accumulation during co-infection with Wheat streak mosaic virus and Triticum mosaic virus.

Natural mixed infections of plant viruses are frequent, often leading to unpredictable variations in symptoms, infectivity, accumulation and/or vector transmissibility. Cauliflower mosaic caulimovirus (CaMV) has often been found in mixed infections with turnip mosaic potyvirus (TuMV) in plants of the genus Brassica. This study addressed the effect of mixed infection on infectivity, pathogenicity and accumulation of CaMV and TuMV in Arabidopsis thaliana plants inoculated mechanically with cDNA infectious clones. In singly infected plants, TuMV accumulation was approximately 8-fold higher than that of CaMV. In co-infected plants, there was 77 % more TuMV accumulation compared with single infections, whilst the accumulation of CaMV was 56 % lower. This outcome describes a biological game in which TuMV always plays the winner strategy, leading to the competitive exclusion of CaMV. However, the infectivity of each virus was not affected by the presence of the other, and no symptom synergism was observed.

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Application of game theory to the interaction between plant viruses during mixed infections

We describe a novel system of exploiting the biolistic process to generate stable transgenic cowpea (Vigna unguiculata) plants. The system is based on combining the use of the herbicide imazapyr to select transformed meristematic cells after physical introduction of the mutated ahas gene (coding for a mutated acetohydroxyacid synthase, under control of the ahas 5′ regulatory sequence) and a simple tissue culture protocol. The gus gene (under control of the act2 promoter) was used as a reporter gene. The transformation frequency (defined as the total number of putative transgenic plants divided by the total number of embryonic axes bombarded) was 0.90%. Southern analyses showed the presence of both ahas and gus expression cassettes in all primary transgenic plants, and demonstrated one to three integrated copies of the transgenes into the genome. The progenies (first and second generations) of all self-fertilized transgenic lines revealed the presence of the transgenes (gus and ahas) co-segregated in a Mendelian fashion. Western blot analysis revealed that the GUS protein expressed in the transgenic plants had the same mass and isoelectric point as the bacterial native protein. This is the first report of biolistic-mediated cowpea transformation in which fertile transgenic plants transferred the foreign genes to next generations following Mendelian laws.

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Biolistic-mediated genetic transformation of cowpea (Vigna unguiculata) and stable Mendelian inheritance of transgenes

Natural multiple viral infections of cultivated cowpeas have been reported in Nigeria. In this study, three Nigerian commercial cowpea cultivars ("Olo 11", "Oloyin" and "White") and two lines from the IITA (IT86D- 719 and TVU 76) were mechanically inoculated with Cowpea aphid-borne mosaic virus (CABMV), Bean southern mosaic virus (SBMV) and Cowpea mottle virus (CMeV) singly, as well as in all possible combinations at 10, 20 and 30 days after planting (DAP). Samples of leaves or stems were collected at 10, 20 and 30 days after inoculation (DAI) and analyzed for relative virus concentration by Enzyme-Linked Immunosrbent Assay. All the cultivars and lines {CVS/L} were susceptible to the viruses but the commercial CVS showed more severe symptoms and had relatively higher viral concentration. In single virus infections, CABMV which induced the most severe symptoms had absorbance values (at 405 nm) of 0.11 to 0.46 while SBMV and CMeV which induced moderate symptoms had virus titre of 0.74 to 1.99 and 0.11 to 0.90 respectively. Plants inoculated 10 DAP had significantly higher virus concentration than those inoculated 30 DAP. In mixed infections involving CABMV (10 DAP) apical necrosis and death were observed in commercial cultivars "Olo 11" and "White". Enhancement of CMeV titers were observed in plants infected with CMeV + CABMV. Multiple viral infections of cowpeas may result in complete yield loss, hence, the availability of seeds of cultivars with a high level of multiple virus resistance is recommended as a means of control.

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Cowpea viruses: Effect of single and mixed infections on symptomatology and virus concentration

Viral diseases constitute obstacles to pepper production in the world. In Nigeria, pepper plants are primarily affected by pepper veinal mottle virus (PVMV), Cucumber mosaic virus (CMV), Pepper leaf curl Virus (TLCV), Tobacco mosaic virus (TMV), Pepper mottle virus (PMV) and a host of other viruses. The experiment was carried out with a diagnostic survey on the experimental field of the National Horticultural Research Institute, Ibadan, Nigeria and on pepper farms in six local government areas within Ibadan Oyo State, Nigeria, forty samples were collected from each of the farms. Diseased samples were obtained from the field and taken to the laboratory for indexing. In ELISA test some of the samples from the pepper farms showed positive reaction to single infection with PVMV (36.79%), CMV (22.14%) while some others showed positive reaction to mixed infection of the two viruses (10%) but some also negative reaction to PVMV and CMV antisera (31.07).

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Occurrence and distribution of pepper veinal mottle virus and cucumber mosaic virus in pepper in Ibadan, Nigeria

Sweet pepper is attacked by a number of viruses which cause serious yield losses for many growers. The influence of mulching on virus disease incidence in sweet pepper was studied under irrigation in NIHORT. Healthy seedlings were transplanted to the field and arranged in a randomized complete block design (RCBD). Different mulch used included air-dried leaves of Azadirachta indica (Neem), Tithonia diversifolia (Tithonia), Neem + Tithonia (w/w) and plastic mulch (white, black and yellow) while the control had no mulch. Incidence and severity of virus-like diseases on growth and yield were evaluated. Result indicated that plants grown using white plastic mulch had the lowest disease incidence of 34.43%. Plants grown with Tithonia, Neem, and Neem + Tithonia mulch had high disease incidence of 100%, 88%, and 90.60% respectively. Average virus disease severity ranged from 2.30 and 3.33 on a scale of 1-5 scoring system. Symptoms observed on symptomatic plants include mosaic, mottle, wrinkle and leaf reduction. Plots treated with black plastic mulch produced the lowest fruit weight, while the white and yellow plastic mulch induced highest fruit weight found. The results obtained showed that white and yellow plastic mulches are effective management options for viruses of sweet pepper.

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Influence of mulching on virus disease incidence, growth and yield of sweet pepper (Capsicum annuum)

Evaluation of three plant powders as protectants of bambara groundnut against Callosobruchus maculatus F. (Coleoptera: Chrysomelidae) infestation and damage

Abstract. Insect infestations are major constraints to vegetable production in Nigeria causing low yields and poor quality of produce. A study was conducted at the experimental farm of the National Horticultural Research Institute (NIHORT) Ibadan, Nigeria, to compare the efficacy of ethanol seed extracts of Azadirachta indica A.Juss (neem), Annona muricata L(soursop) and cypermethrin against field insect pests of Amaranthus hybridus L. under rain fed conditions. Four major insect species were observed causing damage to A. hybridus during the study and they include: cotton bollworm (Heliothis armigera Hubner), okra leaf roller (Sylepta derogata Fabricius), lagriid beetle (Lagria villosa Fabricius) and variegated grasshopper (Zonocerus variegatus Linnaeus). Ethanol seed extracts of A. indica were as effective as cypermethrin and more effective than A. muricata seed extracts in protecting A. hybridus against insect damage. Plots treated with A. indica extracts and cypermethrin significantly (p<0.05) reduced leaf damage, observed insect density and enhanced leaf yield compared to plots treated with A. muricata and control. There was no significant difference (p>0.05) between A. indica and cypermethrin treated plots on observed insect density, leaf damage and leaf yield. A. indica has proved to be efficient as cypermethrin and can be successfully used for the control of field insect pests of A. hybridus.

Kehinde T. Kareem

Papers

Cowpea viruses: Effect of single and mixed infections on symptomatology and virus concentration

Occurrence and distribution of pepper veinal mottle virus and cucumber mosaic virus in pepper in Ibadan, Nigeria

Influence of mulching on virus disease incidence, growth and yield of sweet pepper (Capsicum annuum)

Evaluation of three plant powders as protectants of bambara groundnut against Callosobruchus maculatus F. (Coleoptera: Chrysomelidae) infestation and damage

Comparative efficacy of two plant extracts and cypermethrin against field insect pests of Amaranthus hybridus L.