Symptom severity of cassava mosaic disease in relation to concentration of African cassava mosaic virus in different cassava genotypes

The concentration of African cassava mosaic virus (ACMV) was assessed by enzyme-linked immunosorbent assay in relation to symptom severity among resistant, moderately resistant and susceptible cassava genotypes. Resistant genotype NR 8083 had significantly lower symptom severity scores ( P  < 0·05) than the susceptible genotype TMS 91934, but the two genotypes contained similar levels of virus concentration. The moderately resistant genotypes TMS 30572 and NR 8082 expressed significantly lower symptom severities ( P  < 0·05) than the susceptible genotypes TMS 91934 and TME 117, but they contained significantly higher virus concentrations ( P  < 0·05) than TMS 91934 and similar virus concentration as in TME 117. However, two other resistant genotypes, TME 1 and TME 8, had low symptom severity scores and virus concentrations. There was significant interaction ( P  ≤ 0·05) between cropping season and virus concentration in all the genotypes except TMS 30572. The resistant and moderately resistant genotypes that had high virus concentrations sustained storage root yield losses. The severity of symptoms expressed was not necessarily a reflection of the virus concentration in some of the genotypes. In addition to the use of symptom severity scores to group genotypes into resistant classes, it is recommended that virus concentration should also be considered. Genotypes displaying mild symptoms, but with high levels of virus accumulation, could be an important source of inoculum in the spread of ACMV by the whitefly vectors. This suggests that each genotype should be tested for virus accumulation prior to its release to the farmers.     

Molecular epidemiology of cassava mosaic disease in Madagascar

Cassava is the staple food for hundreds of millions of people in Africa but its cultivation is seriously constrained by cassava mosaic disease (CMD) in Madagascar, and in Africa in general. This study identified the cassava mosaic geminiviruses (CMGs) involved in CMD in Madagascar and their associated epidemiological characteristics from countrywide surveys. Molecular characterization of CMGs in Madagascar revealed an unprecedented diversity and co‐occurrence of six viruses: African cassava mosaic virus (ACMV), East African cassava mosaic Cameroon virus (EACMCV), East African cassava mosaic Kenya virus (EACMKV), East African cassava mosaic virus (EACMV), South African cassava mosaic virus (SACMV) and the recently described Cassava mosaic Madagascar virus (CMMGV). Distinct geographical distributions were observed for the six viruses. While ACMV was more prevalent in the central highlands, EACMV and EACMKV were prevalent in lowlands and coastal regions. Both EACMCV and SACMV occurred in almost all the localities visited. PCR diagnosis revealed that mixed infection (up to four co‐infected viruses) occurred in 21% of the samples and were associated with higher symptom severity scores. Pairwise comparisons of virus associations showed that EACMCV was found in mixed infections more often than expected while ACMV and SACMV were mostly found in single infections. A greater abundance of whiteflies was observed in lowland and coastal areas. Nevertheless, infected cuttings remain the primary source of CMD propagation (95%) in Madagascar.     

Variations and taxonomic status of begomoviruses causing severe epidemics of cassava mosaic disease in Kenya,Uganda, and Democratic Republic of the Congo

The molecular variability in the DNA-A of cassava-begomoviruses, East African cassava mosaic virus (EACMV), East African cassava mosaic virus – Uganda variant (EACMV-UG), African cassava mosaic virus (ACMV), and East African cassava mosaic Zanzibar virus (EACMZV) in Kenya, Uganda, and Democratic Republic of the Congo (DRC) was investigated. Most samples from western Kenya, Uganda, and eastern DRC contained EACMV-UG. Fewer than half of the samples had a mixed infection of ACMV and EACMV-UG, and a small percentage had only ACMV infections. EACMV and EACMZV were the only begomoviruses detected in samples from the Kenyan coast. The coat protein gene nucleotide (nt) and the deduced amino acid sequence analyses revealed a high degree of sequence identity within each virus type and that EACMV-UG was most related to ACMV. However, analysis of the retrieved complete DNA-A (2781–2801nt) sequences of selected virus types revealed that EACMV-UG DNA-A share more than 90% identity with EACMV and less than 80% with ACMV, confirming that the virus is a strain of EACMV.     

Notices

Abstract

Seed‐bed types, fertilizers, cassava stem portions, planting time and yam cultivars were studied for the incidence of termites in cassava and yam fields. Planting cassava on ridges reduced the incidence of termites and significantly (P<0.05) increased tuber yield. The bottom stem portion of cassava stick was the least attacked by termites. Planting cuttings when rains became steady reduced termite incidence. NPKMg (12:12:17:2) suppressed termites in yam and improved yield significantly. Delay in harvesting yam tubers increased termite infestation. Yield loss was highest for Dioscorea rotundata cv. Nwopoko, with losses of more than 5 metric tonnes/ha when harvest was done after December in termite infested soil. Chinese yam showed the greatest resistance to termites.     

Field experiments in Cameroon on cassava mosaic virus disease and the reversion phenomenon in susceptible and resistant cassava cultivars

Cassava is mainly propagated through stem-cuttings. There is evidence that some cuttings propagated from plants infected by cassava mosaic geminiviruses may produce virus-free plants. Field experiments were conducted under contrasting ecological conditions at two sites in southern Cameroon to study the reversion phenomenon, in particular the extent to which the virus causing cassava mosaic disease (CMD) was absent from cuttings collected from CMD-affected plants. Cuttings were taken from the basal, middle and upper portions of the stems of diseased plants of susceptible and partially resistant cassava cultivars. The proportions of disease-free shoots developing from the different batches of cuttings were recorded to indicate the extent of reversion. This was significantly less frequent in cv. Red Local (susceptible) (5-43%) than in the susceptible cv. Bambui Local (65%) and cv. Improved (70%), which is partially resistant to infection. Reversion was significantly higher with cuttings collected from the youngest portions of the stem compared with older portions and with short cuttings (5, 10 cm) compared with those 20 or 25 cm long. There was a significant negative correlation between symptom severity in different cultivars and the extent of reversion. This was greater at the lowland Mile 17 site, near Buea (ca 400 masl) than at the mid-altitude site at Dschang (ca 1300 masl) where conditions were cooler, suggesting an influence of temperature. In order to assess the effect of CMD in cassava the Disease Index (DI) was developed. The result of DI analyses showed that the DI could be used to identify cultivars on the basis of their ability to revert from CMD and the severity of CMD on the non-reverted plants.     

Host and virus effects on reversion in cassava affected by cassava brown streak disease

The phenomenon of virus‐infected plants naturally recovering health is known as ‘reversion’, and is a type of resistance mechanism exploited in some crop plants for disease control. Various parameters were investigated that affect reversion from cassava brown streak disease (CBSD) in three cassava varieties (Albert, Kaleso and Kiroba) that differ in levels of resistance to the disease. Cassava plants were inoculated by grafting with two virus species (Ugandan cassava brown streak virus, UCBSV and Cassava brown streak virus, CBSV) that cause CBSD, and the plants grown from them were subsequently assessed for reversion. The rate of reversion depended on the cassava variety, virus species, and the length and position of the stem cuttings used. A significantly high proportion of progenies were virus‐free (reverted) for the resistant variety Kaleso (64·1% for UCBSV and 54·9% of CBSV), compared to the tolerant variety Kiroba (56·7 and 45·5%) and the susceptible control Albert (38·9 and 35·1%). The highest number of virus‐free plants was generated from short 10 cm long cuttings (e.g. 60·1% for Kaleso for CBSV) compared to 20 cm long stem cuttings (e.g. 21·4% for Albert). Cuttings taken from upper stems of diseased plants produced most virus‐free progenies compared to middle and lower parts. More than 50% virus‐free plants were obtained in the resistant and tolerant varieties. This is a highly valuable finding and could be exploited for developing strategies to control the current CBSD epidemic in eastern and central Africa.     

Influence of variety on the within-plant distribution of cassava green spider mite (Acari: Tetranychidae), and leaf anatomical characteristics and chemical components in relation to varietal resistance

The within-plant distribution of the cassava green spider mite, Mononychellus tanajoa Bondar, and the anatomical characteristics and the chemical components relating to varietal resistance of cassava, were studied using 11 cassava genotypes with varying levels of resistance for two dry seasons and one wet season. The results show that M. tanajoa aggregates on the top leaves of cassava at low levels of resistance as compared with a more even within-plant distribution at higher levels of resistance. Thus, for accurate sampling of mite populations, it might be important to consider young and old leaves, not just the young leaves, in fields containing resistant and susceptible cassava genotypes. Correlations between the anatomical characteristics and mite population density and damage scores were not consistent across months, either within or over seasons. This suggests that leaf anatomical characteristics may not be important in the varietal resistance of cassava to M. tanajoa. During the dry season, mite population density was positively correlated with leaf nitrogen, potassium and phosphorus and negatively correlated with leaf calcium and fat. Only calcium was negatively associated with mite damage at the peak of the dry season (January 1993 and 1994). Breeding cassava genotypes with high levels of leaf calcium and fat, and low levels of leaf nitrogen, potassium and phosphorus, may improve the level of resistance to M. tanajoa.     

Epidemiological assessment of cassava mosaic disease in Burkina Faso

Surveys were conducted in 2016 and 2017 across the main cassava-growing regions of Burkina Faso to assess the status of cassava mosaic disease (CMD) and to determine the virus strains causing the disease, using field observation and phylogenetic analysis. CMD incidence varied between regions and across years but was lowest in Hauts-Bassins (6.0%, 2016 and 5.4%, 2017) and highest in Centre-Sud (18.5%, 2016) and in Boucle du Mouhoun (51.7%, 2017). The lowest CMD severity was found in Est region (2.0) for both years and the highest in Sud-Ouest region (3.3, 2016) and Centre-Sud region (2.8, 2017). The CMD infection was primarily associated with contaminated cuttings in all regions except in Hauts-Bassins, where whitefly-borne infection was higher than cuttings-borne infection in 2016. PCR screening of 687 samples coupled with sequence analysis revealed the presence of African cassava mosaic-like (ACMV-like) viruses and East African cassava mosaic-like (EACMV-like) viruses as single infections at 79.5% and 1.1%, respectively. Co-infections of ACMV-like and EACMV-like viruses were detected in 19.4% of the tested samples. In addition, 86.7% of the samples positive for EACMV-like virus were found to be positive for East African cassava mosaic Cameroon virus (EACMCMV). Phylogenetic analysis revealed the segregation of cassava mosaic geminiviruses (CMGs) from Burkina Faso into three clades specific to ACMV, African cassava mosaic Burkina Faso virus (ACMBFV), and EACMCMV, confirming the presence of these viruses. The results of this study show that EACMCMV occurrence may be more prevalent in Burkina Faso than previously thought.     

Distribution and accumulation of cassava brown streak viruses within infected cassava (Manihot esculenta) plants

Cassava brown streak disease (CBSD), caused by Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV), ranks among the top seven biological threats to global food security. The disease poses a significant threat to cassava production in East and Central Africa (ECA). In Uganda, overall CBSD incidence increased by c. 20% since it re‐emerged in 2004, and the disease persistently reduces cassava yields and storage root qualities. The spread of CBSD has been studied spatially in fields in different agroecologies. However, within‐host distribution and accumulation of CBSV and UCBSV in naturally infected cassava plants is unknown. Therefore, within‐host CBSV and UCBSV distribution was studied to correlate CBSD symptoms with virus titre in organs of infected cassava. Leaf, stem and storage root samples, with and without symptoms, were collected from 10 genotypes of field‐grown cassava. Presence of CBSV and UCBSV was detected by RT‐PCR and virus levels determined by qRT‐PCR. CBSV was present in 100% of CBSD samples with symptoms, with 45·3% positive for presence of both CBSV and UCBSV. Tolerant cassava genotypes were infected with CBSV alone and accumulated higher titre in roots than in aerial organs. Susceptible genotypes were co‐infected with CBSV and UCBSV and exhibited variation in virus titre in each organ. Across genotypes, virus titre was lowest in the youngest leaves and highest in mature non‐senescing leaves. This information provides insight into the relationship between CBSV, UCBSV and their cassava host, and is valuable for CBSD resistance breeding, epidemiology studies and CBSD control.     

Interactions between a crinivirus, an ipomovirus and a potyvirus in coinfected sweetpotato plants

Novel and severe symptoms of chlorosis, rugosity, leaf strapping and dark green islands, designated as sweetpotato severe mosaic disease (SPSMD), were caused by dual infection of Sweet potato mild mottle virus (SPMMV; Ipomovirus ) and Sweet potato chlorotic stunt virus (SPCSV; Crinivirus ) in three East African sweetpotato cultivars (Tanzania, Dimbuka and New Kawogo). The storage root yield was reduced by ∼80%, as compared with healthy plants under screenhouse conditions in Uganda. Plants infected with SPMMV or SPCSV alone showed nonsignificant or 50% yield reduction, respectively. SPCSV reduced resistance to SPMMV in sweetpotato, similar to the situation with resistance to Sweet potato feathery mottle virus (SPFMV; Potyvirus ) that breaks down following infection with SPCSV, followed by development of sweet potato virus disease (SPVD). In single virus infections with SPMMV and SPFMV or their coinfection, cvs Tanzania and Dimbuka were initially systemically infected, displayed symptoms and contained readily detectable virus titres, but new leaves were symptomless with very low virus titres, indicating recovery from disease. In contrast, cv. New Kawogo remained symptomless and contained low SPMMV and SPFMV titres following graft inoculation. These moderate and high levels of resistance to SPMMV and SPFMV, respectively, were lost and cultivars succumbed to a severe disease following coinfection with SPCSV. The synergistic interactions increased titres of SPMMV and SPFMV RNA by ∼1000-fold as quantified by real-time PCR, whereas SPCSV titres were reduced twofold, indicating an antagonistic interaction. Coinfection with SPMMV and SPFMV caused no detectable changes in virus titres or symptom severity.     

Regulation of defense-related enzymes associated with bacterial spot resistance in Tomato

Twenty tomato (Solanum lycopersicon) cultivars were screened for resistance against bacterial spot disease incited byXanthomonas axonopodis pv.vesicatoria under field conditions with and without pathogen infection. Screening was done by artificially inoculating aX. axonopodis pv.vesicatoria suspension to 4-week-old tomato seedlings and observing them for typical symptoms of the disease. Seedlings were categorized into highly resistant, resistant, susceptible and highly susceptible cultivars on the basis of disease incidence. Tomato cultivars were screened for defense-related enzymes, total phenols and lignin contents. The temporal patterns of all these enzymes were estimated with a moderately susceptible tomato cultivar. Native PAGE analysis of both peroxidase (POX) and polyphenol oxidase (PPO) was carried out for the time course of enzyme activities and also by selecting three different tomato cultivars, following infection with the pathogen. Based on the inducible amounts of these enzymes upon pathogen infection, the tomato cultivars were correlated with the disease incidence under field conditions. A significant (P≤0.05) correlation was observed between the degree of host resistance and the enzyme levels. In highly resistant tomato cultivars the enzyme levels, total phenols and lignin contents were increased in comparison with highly susceptible tomato cultivars. Isoform analysis of POX and PPO enzymes indicated a clear difference between resistant and susceptible tomato cultivars in the number of isoforms and also in the intensity of each isoform in the presence of pathogen infection. The possible regulation of defense-related enzymes in imparting host resistance is discussed. http://www.phytoparasitica.org posting March 11, 2008.     

Dual begomovirus infections and high Bemisia tabaci populations: two factors driving the spread of a cassava mosaic disease pandemic

A cassava mosaic disease (CMD) pandemic currently affects much of East and Central Africa. To understand the factors driving the pandemic's continued spread, complementary data sets were collected from cassava plots, planted with healthy cuttings, at eight sites along a north–south transect in southern Uganda, through the pandemic's leading edge. Data were collected on virus incidence, symptom severity, populations of the whitefly vector, Bemisia tabaci , their infectivity and ability to transmit different viruses. In 1996, 6 months after planting, CMD incidences were highest at sites 1 and 2, then decreased progressively until site 6, and remained low at sites 7 and 8. The largest B. tabaci populations also occurred at northernmost sites, 1–3. In 1997, CMD incidence increased significantly at sites 5–8 and this was associated with significant increases in the B. tabaci populations. The pandemic's spread was also associated with significant increases in the percentage of dual infections of East African cassava mosaic virus -Uganda and African cassava mosaic virus , which caused the severest symptoms and the greatest reduction in leaf area. Whitefly adults collected from within the pandemic area were infective, whereas those collected ahead of the pandemic were not. The transmission rate of African cassava mosaic virus from plants with dual infections was significantly less than that of East African cassava mosaic virus -Uganda, which may explain the latter's predominance within the pandemic. These results show that the arrival of East African cassava mosaic virus -Uganda into areas affected previously only by African cassava mosaic virus , has resulted in novel virus/vector/host–plant interactions that drive the pandemic's continued spread.     

Plant yield response to artificial infestation of the false chinch bug,Nysius raphanus, confined on spring canola

A 2-year study was conducted to evaluate the relationship of plant yield responses to artificial infestations of the false chinch bug (FCB)Nysius raphanus (Howard) on spring canola in Colorado, USA. Yield losses were greater when infestation occurred at the early flowering stage (EFS) than at the early pod stage (EPS), in both 2001 and 2002. In the first trial in 2001, with infestations of 10, 20 and 40 FCB/head, the respective average yield losses over all cultivars were 43%, 68% and 69%, respectively, compared with the non-infested control at the EFS, but 11%, 26% and 23% at the EPS. In the second trial in 2001, with infestations of 10, 20 and 40 FCB/head, the respective average yield losses over all cultivars were 26%, 58% and 55% at the early EFS, but 35%, 20% and 35% at the EPS. Yield reductions from FCB infestation were lower in 2002 than in 2001. In the first trial in 2002, with infestations of 10, 20 and 40 FCB/head, the respective average yield losses of all cultivars combined were 31%, 51% and 68% at the EFS, and 13%, 32% and 18% at the EPS. However, in the second trial in 2002, with the same numbers of FCB per head, no yield reductions were observed at either EFS or EPS. The number of FCB causing 10% yield loss in the four trials ranged from 6.1 to 39.4 FCB/head (avg. 14.8) following infestation at the EFS and 15.4–109.8 (avg. 41.8) following infestation at the EPS. Cultivar responses to FCB may also influence FCB yield reductions. However, in these studies all eight tested cultivars sustained yield loss in at least one trial at some FCB infestation level. Variation between trials was substantial but a significant level of resistance to FCB injury did not occur among the tested cultivars. http://www.phytoparasitica.org posting Sept. 18, 2006.     

Cassava common mosaic virus infection causes alterations in chloroplast ultrastructure,function, and carbohydrate metabolism of cassava plants

Cassava common mosaic virus (CsCMV) is a potexvirus that causes systemic infections in cassava plants, leading to chlorotic mosaic and producing significant yield losses. To date, the physiological alterations and the mechanism underlying biotic stress during the cassava–CsCMV compatible interaction remains unknown. In this study, we found that CsCMV infection adversely modified chloroplast structure and had functional effects on chloroplasts in source leaves during the course of viral infection. Extrusion of the chloroplast membrane with amoeboid-shaped appearance and disorganized grana stacks were observed in infected mesophyll cells. These alterations were associated with up to 35% reduction of relative chlorophyll content, and a decline of CO₂ fixation (13.5% and 24.2% at 90 and 210 days after planting, respectively). The effects of CsCMV infection on the performance index on absorption basis dropped up to 37%. The analysis of chlorophyll a fluorescence showed a progressive loss of both oxygen evolving complex activity and “connectivity” within the tripartite system (core antenna-LHCII-reaction centre). Here, we report the latter phenomenon for the first time in a viral infection. The oxidative stress process was observed in CsCMV-infected plants (20.8% reduction of antioxidant capacity with respect to noninfected plants). Other effects of the pathogen included reduction of starch and maltose content in source leaves, and a significant increase (24.7%) of the sucrose:starch ratio, which indicates an altered pattern of carbon allocation. Our results suggest that CsCMV induces chloroplast distortion associated with progressive chloroplast function loss and diversion of carbon flux in source leaf tissue, leading to the loss of cassava tuber yield.     

A new approach to evaluate relative resistance and tolerance of tomato cultivars to begomoviruses causing the tomato yellow leaf curl disease in Spain

A simple procedure to evaluate relative resistance and tolerance of tomato cultivars to the begomoviruses causing tomato yellow leaf curl (TYLC) disease in Spain was developed. To estimate the resistance and tolerance levels of a cultivar, several formulae were developed based on the ratio of infected plants, virus titre (estimated by tissue–print hybridization) and symptom intensity. The formulae were applied to five commercial tomato cultivars (Amoretto, Birloque, Royesta, Tovigreen and Ulises) naturally infected by TYLC viruses. The analyses showed that Ulises, Birloque and Tovigreen exhibited a moderate resistance, and Ulises was also highly tolerant. There was a positive correlation between symptom intensity and virus titre in infected plants, suggesting that the hybridization technique could also be used as an early estimator of tolerance. Finally, molecular hybridization and nucleotide sequence analyses of the begomovirus intergenic region showed that the local TYLC virus population consisted of a single species, Tomato yellow leaf curl virus (TYLCV, formerly TYLCV-Israel), with low genetic variation (nucleotide identity between isolates higher than 97%).     

Surveys of cassava plantations in Colombia for virus and virus-like diseases

The cassava common mosaic virus (CsCMV) and the frogskin (FSD) disease agent have been reported to reduce cassava yields significantly in South America. However, little information is available on the distribution and incidence of these and other cassava virus diseases in Colombia. Cassava plants collected in three principal cassava production zones of Colombia were tested for the presence of CsCMV, cassava × virus (CsXV), and the Caribbean mosaic disease (CMD) and FSD agents. Some plants were also tested for the presence of double-stranded RNA (dsRNA). CsCMV was not detected in any of the 870 plants from 86 plantations. CsXV was detected in 51% of the 150 plants collected in the Cauca Department in south-central Colombia. The virus was present on all 15 cassava plantations surveyed. The CMD agent was detected in 17% of the 138 plants sampled in the Department of Magdalena in northern Colombia. FSD root symptoms were observed on 25 and 3% of plants examined in the Departments of Cauca and Magdalena, respectively. None of the 570 plants collected in areas west of the Rio Magdalena in the Departments of Atläntico, Bolivar, Córdoba and Sucre were found to be infected with any of these disease agents. However, some plants sampled in this region were found to contain multiple dsRNA species of unknown origin.     

Unterschiedliche Formen der Resistenz gegen bodenbürtige Viren des Weizens

In Germany the furovirus Soil-borne cereal mosaic virus (SBCMV) and the bymovirus Wheat spindle streak mosaic virus (WSSMV) occur often together particularly in several rye production areas. Soil-borne wheat mosaic virus (SBWMV), a wheat infecting furovirus, has so far been found only in one field near Heidelberg. Each of these viruses is transmitted by Polymyxa graminis. The cultivation of resistant varieties is the only promising measure to prevent yield losses caused by soil-borne viruses. Resistance of wheat against the bymovirus WSSMV is comparable to the immunity of barley to the bymoviruses Barley yellow mosaic virus and Barley mild mosaic virus. In case of immunity no virus multiplication is observed in resistant cultivars. In contrast, all wheat cultivars are hosts of the furoviruses. All cultivars – including the resistant ones – can be infected following mechanical inoculation with SBWMV and SBCMV. Resistance to furoviruses is based on reduced levels of virus multiplication in roots and on inhibition of virus movement from roots to leaves. Because of the inhibited virus movement from roots to aerial parts of plants this type of resistance is referred to as translocation resistance. In spite of the different resistance mechanisms the absence of virus symptoms on the leaves is a common selection criterion for both immunity and translocation resistance. Therefore, the symptom free development of plants on uniformly infested fields is the best criterion for selecting wheat lines with resistance to soil-borne viruses. The limited suitability of other selection methods is discussed.     

117个玉米品种对禾谷镰孢根腐病的抗性鉴定及根腐病对苗势与产量的影响

为评价东北地区玉米主推品种对禾谷镰孢根腐病的抗性水平,探究根腐病发生与苗势、产量损失间的关系,采用人工接种方法鉴定东北地区117个玉米主推品种对禾谷镰孢根腐病的抗性水平,应用室内盆栽试验分析自交系LN810在施用氮、磷、钾及复合肥后对禾谷镰孢根腐病发生和苗势的影响,在田间对先玉335接种禾谷镰孢菌Fusarium graminearum后探究根腐病发生程度对其苗势及产量的影响。结果显示,117个玉米品种对禾谷镰孢根腐病的抗性差异明显,鉴定出高抗品种14个,抗病品种44个,中抗品种47个,感病品种12个,其中表现中抗以上的品种占89.74%,且中早熟材料均表现为抗性。与未施肥相比,施用磷钾肥后玉米禾谷镰孢根腐病发生率最低,为22.03%。按照禾谷镰孢根腐病发生程度从轻到重将先玉335群体划分为一、二、三类苗,级别越高苗势越弱,产量测定发现二、三类苗的平均产量较一类苗的平均产量分别下降了39.97%和76.39%。表明东北地区主推玉米品种大部分对禾谷镰孢根腐病表现出抗性,但仍有部分品种存在较大感病风险,且该病害的发生程度与幼苗长势和产量呈负相关,施用磷钾肥可降低该病害的发生率。     

Quantitative approach for the early detection of selection for virulence of Meloidogyne incognita on resistant tomato in plastic greenhouses

Resistant tomato cultivars are an important tool to control Meloidogyne spp., which cause the highest yield losses attributed to plant‐parasitic nematodes. However, the repeated cultivation of Mi resistant cultivars can select virulent populations. In the present study, the susceptible tomato cv. Durinta and the resistant cv. Monika were cultivated from March to July in a plastic greenhouse for 3 years to determine the maximum multiplication rate, maximum nematode density, equilibrium density, relative susceptibility and population growth rate of M. incognita; these were used as proxy indicators of virulence and yield losses. The values of population dynamics and growth rate on the resistant tomato increased year by year and were higher when it was repeatedly cultivated in the same plot compared to when it was alternated with the susceptible cultivar and the level of resistance decreased from very to moderately resistant. The relationship between the nematode density at transplanting (P_i) and the relative yield of tomato fitted to the Seinhorst damage model for susceptible, but not resistant, cultivars. The tolerance limit and the relative minimum yield were 2–4 J2 per 250 cm³ of soil and 0.44–0.48, respectively. The tomato yield did not differ between cultivars at low P_i, but it did at higher P_i values, at which the resistant yielded 50% more than the susceptible. This study demonstrates the utility of population dynamics parameters for the early detection of selection for virulence in Meloidogyne spp., and that three consecutive years were not sufficient to select for a completely virulent population.