Breakdown of plant virus resistance: can we predict and extend the durability of virus resistance?

Cultivars with introgressed natural resistance genes have been widely used for plant disease control, especially in the control of virus diseases, for which no effective chemical control agent is available. However, we often encounter virus mutants that break down or overcome the resistance. In this review, recent studies will be discussed with respect to breakdown of plant virus resistance.     

Pea early-browning virus and tobacco rattle virus — two different,but serologically related viruses

Summary Early-browning virus (EBV) and tobacco rattle virus (RV) show similarities in many of their properties, although there are also some distinct differences (Bos & van der Want, 1962). Serological experiments carried out by the present author showed that EBV and RV are distantly related. Because of the difference in the normal lengths of their particles they have to be considered as separate viruses. It is suggested that they be regarded as members of one group.Samenvatting Vroege verbruining, een virusziekte bij erwten, is uitvoerig onderzocht doorBos & van der Want (1962). Uit hun onderzoek is gebleken dat de verwekker van deze ziekte, het vroege-verbruiningsvirus (EBV), veel overeenkomst vertoont met het ratelvirus van tabak (RV). Beide virussen hebben dezelfde deeltjesvorm, maar ze verschillen in lengte (EBV 105 en 210 m en RV 70 en 180 m). Beide gaan met de grond over en komen overeen wat betreft hun eigenschappenin vitro. Ook wat hun uitwerking op waardplanten betreft lijken ze, met uitzondering van enkele karakteristieke verschillen, veel op elkaar. Het duidelijke verschil in deeltjeslengte geeft aan, dat er verschillende virussen in het spel zijn. Het leek echter niet uitgesloten, dat ze toch in zekere mate verwant zouden zijn. Om deze mogelijkheid na te gaan werd uitvoerig serologisch onderzoek verricht. Met behulp van antisera met titers van ca. 1/128 kon geen verwantschap worden aangetoond (tabel 1). Daarom werden sterkere antisera bereid. Hiertoe werden EBV en RV vermeerderd opNicotiana rustica L. en gezuiverd volgens een enigszins gewijzigde methode vanWetter (1960).Door het toepassen van een combinatie van intraveneuze en intramusculaire injecties werden zeer sterke antisera verkregen. Met behulp daarvan werden beide virussen opnieuw getoetst. Als controle diende aardappel-X-virus (PVX), dat op dezelfde wijze was vermeerderd en gezuiverd als het EBV en het RV. De resultaten van deze proef zijn vermeld in tabel 2. Daar er nog een zwakke reactie met de controle optrad, werden de antisera verzadigd en werd de proef herhaald. De resultaten hiervan zijn weergegeven in tabel 3. Met de controle trad toen geen reactie meer op, terwijl EBV en RV nog duidelijk met elkaars antisera reageerden. Daar zowel een wederzijdse besmetting van EBV en RV als een gemeenschappelijke besmetting met een ander virus was uitgesloten, kunnen we concluderen, dat EBV en RV verwante virussen zijn. Naar analogie van wat door Duitse onderzoekers voor andere virussen is gevonden, kunnen we ook hier spreken van afzonderlijke virussen, die waarschijnlijk in eenzelfde groep ondergebracht kunnen worden.In het algemeen zijn in de tabellen slechts die antigeenverdunningen weergegeven, waarbij een maximale antiserumtiter werd gevonden. Wanneer geen bepaalde verdunning is aangegeven, betekent dit dat geen enkele verdunning reageerde.     

Distribution and phylogenetic analysis of the 3′UTR and coat protein gene of Iranian Beet black scorch virus

Journal of Plant Diseases and Protection - Beet black scorch virus (BBSV) was surveyed in major sugar beet cultivation areas in Iran in 2008–2013 growing seasons. A total of 148 out of 308...     

Role of weed hosts and the western flower thrips, Frankliniella occidentalis, in epidemiology of Tomato spotted wilt virus in the Çukurova region of Turkey

Surveys of thrips and Tomato spotted wilt virus (TSWV) on weeds were conducted in the eastern Mediterranean region of Turkey during the years 2004–2006. Thrips species were collected by vigorously shaking weedy plants into a white container for 15 sec. Plant material collected during field surveys and plants which were used for mechanical inoculation of TSWV, were tested by DAS–ELISA. The weed species Ranunculus muricatus, Melilotus officinalis, Sinapis arvensis and Portulaca oleracea were used for the virus transmission experiments in an enclosed high plastic tunnel and in cage experiments. Western flower thrips, Frankliniella occidentlis (Pergande) (Thysanoptera: Thripidae), was the most common thrips, inhabiting 80 of the 82 weed species sampled. Adults of F. occidentalis and thrips larvae were significantly more abundant on S. arvensis than on the other weed species sampled (P<0.05). Adult and larval thrips showed peak densities on most weeds in April or May. Summer annual weeds were not good hosts for reproduction of the thrips. A total of 90 samples from 17 plant species belonging to 11 plant families were ELISA positive for TSWV. No TSWV was detected on 65 weed species belonging to 26 plant families. High numbers of plant samples infected by TSWV were obtained in P. oleracea (21 samples) and in R. muricatus (15 samples). In field surveys symptoms of TSWV were detected on only R. muricatus. Incidence of the TSWV on weeds ranged between 5% and 25%. Transmission rates of TSWV by F. occidentalis to the weeds ranged from 33% to 83% in the pepper plastic tunnel and cage experiments.     

Vertical transmission of granulosis virus of sugarcane shoot borer,Chilo infuscatellus Snell.†

Abstract

The granulosis virus (GV) of sugarcane shoot borer, Chilo infuscatellus Snelt., was found to be transmitted vertically when the fourth instar larvae were microfed with the virus at a sublethal dose of 1.1 x 10³ inclusion bodies/larva. The virus infection was found to alter the sex ratio in pupae in favour of male (1 : 5.70—♀ : ♂) from the normal 1 : 1 ratio. The per cent pupation and adult emergence were also significantly reduced. No adults emerged from abnormal pupae that lacked pigmentation on the ventral surface of the first abdominal segment. The GV was also found to be transmitted from the infected adult to the offspring through eggs and 50% of the larvae that hatched were found to die due to virosis.     

Comparison of Pathogenicity and Nucleotide Sequences of 3′-terminal Regions of Bean yellow mosaic virus Isolates from Gladiolus

Sixty-four isolates of Bean yellow mosaic virus (BYMV) from cultivated and naturalized gladioli were divided into two pathogenic groups, necrotic spot (NS) and chlorotic spot (CS) groups on Chenopodium quinoa. NS-type isolates (S-22N and E-24N), CS-type isolates (S-22C and E-92C), and broad bean isolates (Sb-50C and Sb-12C) differed in their pathogenicity on Antirrhinum majus, Nicotiana benthamiana, Phaseolus vulgaris, Spinacia oleracea and Vigna unguiculata. The four gladiolus isolates were different from BYMV-B, -P, -O and C1YVV-N in their pathogenicity on these plants, while the two broad bean isolates were similar to BYMV-B, originally from broad bean. The nucleotide (nt) sequences of the 3′-terminal region of the BYMV RNA genome of the two NS-type isolates, the two CS-type isolates, the two broad bean isolates and BYMV-B, -P and -O were determined. In a phylogenetic tree based on the CP amino acid (aa) sequences, the two NS-type isolates clustered together (identity 98.4% and 98.2% at the nt and aa level, respectively). The two CS-type isolates clustered with BYMV-O (93.2 to 99.3% nt identity and 95.6 to 98.5% aa identity). The two broad bean isolates clustered with BYMV-B (99.0 to 99.5% nt identity and 98.9 to 99.6% aa identity). BYMV-P clustered with BYMV-CS (identity 97.7% and 99.3% at the nt and aa level, respectively). The obtained sequences were compared with those of the 3′-terminal regions of seven published BYMV isolates. In a phylogenetic tree based on deduced aa sequences, BYMV isolates were divided into four clusters. Received 1 July 1999/ Accepted in revised form 22 May 2000     

Cigar‐end disease of banana in the southern region of Oman (Dhofar)

Abstract

Cigar‐end disease of banana, Verticillium theobromae has become apparent in the southern region of Oman, and is prevalent under warm and humid conditions. Benomyl and thiabendazole were found to effectively control V. theobromae in artificially induced cigar‐end disease.     

Management of banana leafspot (Sigatoka) disease in the Windward Islands∗

Abstract

The rugged terrain and large number of smallholdings are important factors in the control of yellow Sigatoka (Mycosphaerella musicola Mulder) in the Windward Islands. A network of reference points has been established in each island where the disease level is monitored by the simple method of Stover and Dickson (1970) and a new method that assesses early infections on the young leaves. This information is used to decide spray applications. The use of Piche evaporimeter data to forecast spray applications has been investigated and is considered useful. Several fungicides have been tested and tridemorph, biloxazol and imazalil found to give satisfactory control with a view to introducing a rotation programme to avoid the continued sole use of benzimidazole formulations and possible occurrence of tolerance.     

Effect of Cotton leaf curl virus infected plants on the biology of the whitefly,Bemisia tabaci (Hemiptera: Aleyrodidae): Vector–virus mutualism

Cotton leaf curl virus (CLCuV) (Geminiviridae : Begomovirus), the causative agent of leaf curl disease in cotton plants (Gossypium hirsutum), is exclusively transmitted by whitefly species Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae). CLCuV transmission occurs in Sriganganagar (Rajasthan), an area endemic with cotton leaf curl disease. The relationships between plant viruses, their herbivore vectors and host plants can be beneficial, neutral, or antagonistic, depending on the species involved. To further understand these relationships, fecundity and life history parameters of an indigenous non- b (Asia II genetic group) biotype whitefly, B. tabaci, were compared on 10, 25, and 40 days post-inoculation (DPI), in CLCuV-infected and healthy cotton plants to determine the effect of virus on its vector. The development time of the immature stages of whiteflies was significantly reduced on CLCuV-infected plants. The development time of the immature stages did not change with severity of symptoms at 25 and 40 DPI (45- and 60-d-old plants). Cotton leaf curl virus infection increased percent egg viability of B. tabaci. Whiteflies deposited significantly fewer eggs on virus-infected plants than on healthy plants. Whiteflies had better egg viability on younger plants than older plants, whereas plant age did not affect the fecundity. Male and female whiteflies had shorter longevity on CLCuV-infected plants than on healthy plants.     

Drought as a modulator of plant–virus–vector interactions: Effects on symptom expression,plant immunity and vector behaviour

Plant–virus interactions are affected by environmental conditions that determine plant vulnerability to pathogens and the population dynamics of insect vectors. We hypothesize that drought enhances horizontal transmission by dampening the basal immunity of plants, which triggers symptom expression and vector manipulation. The potato yellow vein virus (PYVV) causes potato yellow vein disease (PYVD), a re-emerging epidemic of potato crops in South America, and is transmitted horizontally by the greenhouse whitefly (GWF), Trialeurodes vaporariorum (Hemiptera: Aleyrodidae), or vertically through infected seed tubers. We investigated the role of drought and temperature as modulators of PYVD symptom expression, plant immune response, and vector survival, development and host preference. We found that drought induced symptom expression, suppressed the salicylic acid pathway and increased PYVV replication. GWF survival was reduced on PYVV-infected hosts and development was slowest when they fed on plants with PYVD symptoms, which also triggered adults’ attraction to PYVV-infected plants. However, adults previously fed on infected plants showed the opposite effect, being more attracted to PYVV-free plants. We propose a theoretical model that explains the role of drought in modulating potato–PYVV–GWF interactions and provides new insights into plant–virus–vector coevolution.     

Functional analysis of the G-protein α subunits FGA1 and FGA3 in the banana pathogen Fusarium oxysporum f. sp. cubense

The asexual fungus Fusarium oxysporum f. sp. cubense (Foc) is the causal agent of fusarium wilt in bananas (Musa spp.). This fungus poses a threat to banana production throughout the world. Here, two Foc genes, fga1 and fga3, were functionally characterized. These genes encode proteins homologous to the G-protein α subunits GPA1 from Saccharomyces cerevisiae and MAGC from Magnaporthe grisea, respectively. The deletion of fga1 leads to a phenotypic defect in colony morphology and reductions in vegetative growth, conidiation and pathogenicity against the banana plant (Musa spp. cv. Brazil), which was not observed for the Δfga3 deletion mutant. Intriguingly, both Δfga1 and Δfga3 deletion mutants showed declines in intracellular cyclic AMP levels and increases in heat resistance, suggesting that FGA1 regulates growth, development, pathogenicity, and heat resistance, whereas FGA3 modulates heat resistance, potentially through the cAMP-dependent protein kinase A pathway. These findings offer insights into the roles of the G-protein α subunits in the development and pathogenicity of the fungus Foc.     

Modelling black Sigatoka epidemics with seasonal dispersal of Mycosphaerella fijiensis ascospores over a banana plantation in the Ribeira Valley,São Paulo,Brazil

Knowing the patterns of Black Sigatoka development is essential to propose adequate disease management practices and evaluate their effects, which can be achieved through temporal analysis by integrating the evolving interactions of the pathosystem components, expressed by data on cumulative incidence and severity, and summarizing these data in a disease progress curve. Airborne spores are essential components for the progression of an epidemic in the context of a specific pathosystem. In this perspective, the spore count is an essential approach to assess and model an epidemic. This study evaluated the temporal dynamics of Black Sigatoka in a banana plantation in the Ribeira Valley, state of São Paulo, Brazil, while simultaneously performing a year-long evaluation of fungal spore aerobiology. The disease was intense during the rainy season, but the leaf emergence rate was high enough for quickly inverting the severity peak (between 169 and 197 days of evaluation). After that, the disease severity raised until reach the higher rates (around the score 7 out of 8). The disease progress curve of Black Sigatoka showed peaks of extreme severity, one in the rainy and another in the dry season, with high levels of ascospores in the air. The ascospore concentration and the severity of the disease correlated significantly on the same day of sampling and 15 days after ascospore sampling, corresponding to the average latency period of the disease in the region. The patterns of the disease progress curve in both peaks fitted the monomolecular model, with higher rates of disease increase in the rainy season.

     

Sequence diversity in the coat protein and 3″-untranslated region of Chinese yam necrotic mosaic virus RNA

Sixteen isolates of Chinese yam necrotic mosaic virus (ChYNMV) were collected from nine sites in Japan and one site in Korea, and 1098 nucleotides of the 3-terminal of the genome were sequenced. Identity of the coat protein gene was 95.5%–99.7% among the isolates. Substitution in the deduced amino acids of the coat protein ranged from 0 to 7, mainly in the N-terminal region. The 3-untranslated region consisted of 231 nucleotides, which had 96.5%–100% nucleotide identity among the isolates. Sequence diversity was considerably less in ChYNMV than in Yam mosaic virus or Japanese yam mosaic virus.     

Effect of eosin Y on the formation of local lesions and on the accumulation of callose in ‘Samsun NN’ tobacco and French bean leaves inoculated with tobacco mosaic virus

When leaf-halves of Samsun NN tobacco or bean plants were floated on a solution of 10–15 M eosin Y after inoculation with tobacco mosaic virus (TMV) and kept at 20° C, local lesion formation was markedly diminished. There was also a decrease in the size of the lesions.Depending on the temperature very strong fluorescence due to callose formation was seen around the lesions in eosin Y-treated leaf-halves of Samsun NN tobacco and bean plants. It lasted from 3–5 days after inoculation, whereas fluorescence around lesions in the water-treated control leaves disappeared within 2–3 days after inoculation.When leaf discs of Samsun tobacco, a systemic host for TMV, were floated on a solution of eosin Y after inoculation with TMV and kept at 20°C for 5 days, TMV multiplication was not prevented. Callose deposition could be detected, neither in eosin Y-treated nor in water-treated control leaves.The relation between the inhibition of local lesion formation and the accumulation of callose in eosin Y-treated leaves is discussed.Samenvatting Het is bekend dat er een verband bestaat tussen afzetting van callose en de vorming van lokale lesies bij verschillende virus-waardplantcombinaties. Tevens staat vast dat eosine ophoping van callose op zeefplaten veroorzaakt. Onderzocht werd nu of eosine Y van invloed is op de vorming van lokale lesies en tevens ophoping van callose veroorzaakt in bladeren van Samsun NN-tabak en boon, geïnoculeerd met tabaksmozaïekvirus (TMV).Het bleek dat bladhelften van tabak die, na inoculatie met TMV, bij 20°C gedreven hadden op een oplossing van 10 of 15 M eosine Y, minder en kleinere lesies hadden dan de controlehelften die op water hadden gedreven (Tabel 1; Fig. 1). Bij 25°C werden deze effecten niet waargenomen (Fig. 2). Een concentratie van 15 M veroorzaakte wat beschadiging van de bladeren (verbruining van een deel van de zijnerven).Bij bladhelften van de boon was geen beschadiging te zien bij een concentratie van 15 M. Bij bonebladeren geïnoculeerd met TMV bleek behandeling met eosine Y zowel bij 20°C als bij 25°C en zelfs bij 30°C minder en kleinere lesies tot gevolg te hebben (Tabel 2; Fig. 3 en 4).Als gezonde bladhelften van Samsun NN-planten te drijven waren gelegd op water verscheen er callose in de buurt van het wondvlak (Fig. 5). Deze ophoping van callose was sterker als de bladhelften hadden gedreven op eosine Y in concentraties van 10 of 15 M (Fig. 6). In de zijnerven, die als gevolg van de behandeling met 15 M bruin waren geworden, was eveneens een sterke fluorescentie als gevolg van de aanwezigheid van callose te zien (Fig. 7 en 8).Als bladhelften van Samsun NN-planten na inoculatie met TMV te drijven waren gelegd op oplossingen van 10 of 15 M bij een temperatuur van 20°C bleek er niet alleen een reductie in aantal en afmeting van de lesies opgetreden te zijn, maar ook sterke fluorescentie als gevolg van aanzienlijke callose-afzettingen rondom de in hun uitbreiding geremde lesies (Fig. 9). De sterke fluorescentie verdween zelfs niet 3–5 dagen na inoculatie, hoewel bij bladeren die op water hadden gedreven dit al na 2–3 dagen het geval was (Fig. 10). Hadden de bladhelften na inoculatie echter bij 25°C op eosine Y gedreven dan was de fluorescentie rondom de lesies dezelfde als in de bladhelften die op water hadden gedreven.Bij bladhelften van gezonde bonen was niet veel ophoping van wondcallose waarneembaar en evenmin stimuleerde eosine Y de vorming ervan. Wel bracht deze stof een ophoping van callose teweeg op de zeefplaten van bladnerven (Fig. 11). Eveneens was er in de met eosine Y behandelde bladhelften fluorescentie in delen van de zijnerven (Fig. 12).Bij bonebladeren die waren geïnoculeerd met TMV was een sterke afzetting van callose te zien rondom de lesies die zich niet meer uitbreidden als gevolg van behandeling met eosine Y bij temperaturen tussen 20°C en 30°C (Fig. 13). In de controlehelften die op water hadden gedreven verdween de fluorescentie binnen 2 dagen na inoculatie (Fig. 14).Hadden bladschijfjes van Samsun-tabak na inoculatie met TMV gedreven op een oplossing van eosine Y dan bleek de virusvermeerdering in deze schijfjes niet geremd te zijn (Tabel 3).We kunnen concluderen dat er een correlatie bestaat tussen de hoeveelheid afgezette callose in de bladeren en de uiteindelijke grootte van de lokale lesies. De vraag blijft echter nog wel bestaan of callose-afzetting inderdaad de verspreiding van virus naar naburige cellen verhindert.     

Effects of Cry1Ac toxin of Bacillus thuringiensis and nuclear polyhedrosis virus of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) on larval mortality and pupation

In the laboratory, the percentage mortality and pupation of Helicoverpa armigera (Hübner) were investigated when larvae were exposed to Cry1Ac of Bacillus thuringiensis Berliner, nuclear polyhedrosis virus of H. armigera (HaNPV) or Cry1Ac and HaNPV together. The results revealed that interactions between Cry1Ac and HaNPV varied with bioassay method and concentration of the suspension. When larvae were infected using a suspension containing both HaNPV and Cry1Ac, most combinations of Cry1Ac (62.5, 125 and 250 microg mL(-1)) and HaNPV (1.2 x 10(6), 6.0 x 10(6) and 3.0 x 10(7) PIB mL(-1)) showed an antagonistic effect. In the bioassay procedure where larvae were force fed diet containing Cry1Ac 48 h after being infected by HaNPV, interaction between Cry1Ac (0.5, 1, 2, 4 and 8 microg mL(-1)) and HaNPV (6.0 x 10(6) and 3.0 x 10(7) PIB mL(-1)) showed an additive effect, while combinations of Cry1Ac (0.5, 1, 2 and 4 microg mL(-1)) and HaNPV (1.2 x 10(6) PIB mL(-1)) showed an antagonistic effect. In the bioassay procedure where larvae being infected by HaNPV were fed on Cry1Ac diet from neonate to death or pupation, the results suggested that Cry1Ac and HaNPV showed an additive interaction. The percentage mortality was lower in the treatment of larvae infected by transgenic Bt cotton leaf discs containing HaNPV suspension than in the treatment of larvae by conventional cotton leaf discs containing HaNPV, while the pupation rate was higher. The combination of Bt cotton and HaNPV showed antagonism. The present results showed that a combination of Cry1Ac and HaNPV usually resulted in mortality levels greater than in the case of Cry1Ac but not greater than with the virus alone.     

Nucleotide Sequence of the 3′ -terminal Region of Carnation vein mottle virus RNA

The 2326 nucleotides of the 3′-terminal region of Carnation vein mottle virus (CVMV) RNA, which included part of the nuclear inclusion b gene, the complete coat protein (CP) gene and the entire 3′-noncoding region (3′-NCR) were determined. The region encoding the CP gene is 843 nucleotides long and the deduced protein consists of 280 amino acids. A search of the EMBL and PIR databases showed that the amino acid sequence of CVMV CP most resembled that of Plum pox virus with a similarity of 67.9%. The 3′-NCR of CVMV RNA is 541 nucleotides long, second longest in the genus Potyvirus. These results indicate that CVMV is closely related to Plum pox virus but is a distinct species in the genus Potyvirus. Received 8 October 1999/ Accepted in revised form 9 January 2000     

Reverse transcription-duplex-polymerase chain reaction for simultaneous detection of Citrus tristeza virus and ‘Candidatus Liberibacter’ from citrus plants

Journal of Plant Diseases and Protection - Tristeza and Huanglongbing are the most important diseases of citrus caused by Citrus tristeza virus (CTV) and ‘Candidatus Liberibacter’...