Cultural, molecular and pathogenic variability of Mycosphaerella pinodes and Phoma medicaginis var. pinodella isolates from dried pea (Pisum sativum) in France

The characteristics of 50 isolates of Mycosphaerella pinodes and 17 isolates of Phoma medicaginis var. pinodella , originating from several regions of France where ascochyta blight is prevalent, were investigated using cultural, physiological, molecular and pathogenicity analyses. M. pinodes was distinguished from P. medicaginis var. pinodella on the basis of presence of pseudothecia, a higher proportion of larger, bicellular conidia, compared with the smaller, predominantly unicellular conidia of P. medicaginis var. pinodella , and a slower linear growth rate on agar under a 12-h light regime. RAPD analysis clearly distinguished the two species, which had low intraspecific variability. Although both species gave identical symptoms, they could be distinguished by their incubation period and aggressiveness, respectively, shorter and higher for M. pinodes . Virulence tests gave no definitive evidence for the existence of pathotypes among the M. pinodes isolates. Two unidentified isolates had similar characters to both M. pinodes and P. medicaginis var. pinodella in some features but were distinguished from them by their RAPD patterns.     

Identification of Sources of Resistance to Phoma medicaginis Isolates in Medicago truncatula SARDI Core Collection Accessions, and Multigene Differentiation of Isolates

ABSTRACT Phoma medicaginis is a necrotrophic fungal pathogen, commonly found infecting the annual medic Medicago truncatula. To differentiate eight P. medicaginis isolates, five gene regions were examined: actin, beta-tubulin, calmodulin, translation elongation factor 1-alpha (EF-1alpha), and the internal transcribed spacer ribosomal DNA. Sequence comparisons showed that specimens isolated from M. truncatula in Western Australia formed a group that was consistently different from, but allied to, a P. medicaginis var. medicaginis type specimen. EF-1alpha contained a hyper-variable 55-bp repeat unit, which forms the basis of a rapid polymerase chain reaction-based method of reliably distinguishing isolates. Characterization of three isolates showed that all exhibited a narrow host range, causing disease only in M. sativa and M. truncatula among eight commonly cultivated legume species sampled. Infection of 86 M. truncatula single-seeded accessions showed a continuous distribution in disease phenotypes, with the majority of accessions susceptible. On a 1-to-5 disease reaction scale increasing in severity, individual fungal isolates showed means of 2.6 to 3.2, and scores ranged from 1 to 4.8 among accessions. The results presented here suggest that M. truncatula harbors specific and diverse sources of resistance to individual P. medicaginis genotypes.     

Development of monoclonal antibodies against the fungi of the 'Ascochyta complex'

Two monoclonal antibodies (MAbs) were produced against soluble antigens from the 'Ascochyta complex' fungi. Specificity of MAbs was tested by ELISA using antigen-coated wells. MAbs secreted by the monoclonal hybridoma cell line JIM 44 recognized epitopes present in the antigen preparations from Mycosphaerella pinodes and Phoma medicaginis var. pinodella , but not those present in preparations from Ascochyta pisi . At high tissue culture supernatant concentration, MAbs produced by the monoclonal line JIM 45 recognized epitopes from all three fungi, however, on dilution of MAb the antigens from A. pisi were recognized preferentially to those from M. pinodes and P. medicaginis var. pinodella . On the basis of heat and periodate treatment of the antigens from the three fungi it can be concluded that the epitope recognized by JIM 44 is carbohydrate in nature, whereas that recognized by JIM 45 is proteinaceous in nature, carried on a glycoprotein antigen. Antigen preparations from other fungi, including other pea pathogens, non-pathogens associated with pea and other fungi closely related to the 'Ascochyta complex', were not detected with either of the two MAbs. Antigen preparations from peas could be used to differentiate healthy and infected seeds in a dot-blot assay, therefore indicating the potential of using the MAbs in the development of a diagnostic test for infection of Pisum seeds by the 'Ascochyta complex' fungi.     

Involvement of phenolic compounds in the resistance of grapevine callus to downy mildew (Plasmopara viticola)

Callus tissue of different grapevines (Vitis spp.) was inoculated withPlasmopara viticola. Short, highly-branched hyphae with necrosis, and long hyphae with heavy sporulation were observed on resistant and susceptible callus respectively. Thin-layer chromatography and spectrophotometric analysis showed that resistant callus contained greater quantities of gallocatechin derivatives than susceptible callus. Regression analysis between the field disease rating of each variety and its gallocatechin derivatives content indicated 92.2% correlation. Histochemical studies showed that, after infection withP. viticola, flavonoids appeared in the superficial cell walls of the callus, to a lesser degree on susceptible callus than on resistant callus. At a late stage of infection, the superficial cells of resistant callus were suberized, which did not occur in susceptible callus. This study showed that the preformed gallocatechin derivatives, the induced flavonoids and suberized superficial cells might play a role in the resistance of grapevine callus tissue to this fungus.Abbreviations CallusI Callus ofV. riparia var. Gloire de Montpellier - CallusV Callus ofV. vinifera var. Grenache - TLC Thin Layer Chromatography - var. variety - GAD Gallic acid derivatives - GD gallocatechin derivatives - RC resistant callus - SC susceptible callus     

Quantifying Aphanomyces euteiches in Alfalfa with a Fluorescent Polymerase Chain Reaction Assay

ABSTRACT A polymerase chain reaction (PCR) assay using a set of specific primers and a dual-labeled probe (TaqMan) was developed to quantify the amount of Aphanomyces euteiches DNA in alfalfa plants exhibiting varying levels of disease severity. The study included isolates of race 1 and race 2 of A. euteiches. The assay also discriminated between alfalfa populations for resistance based on analysis of DNA extracted from bulked plant samples. Analysis of individual plants and bulked plant samples of standard check populations with both pathogen isolates resulted in Spearman rank correlations between pathogen DNA content and disease severity index ratings that were greater than 0.75 and highly significant (P < 0.0005). In experiments with a race 1 isolate, the amount of pathogen DNA present in the resistant check WAPH-1 was significantly less than in the susceptible check Saranac. In experiments with a race 2 isolate, the amount of pathogen DNA in the resistant check WAPH-5 was significantly less than in either of the susceptible checks, Saranac and WAPH-1. Discrimination between commercial cultivars based on quantitative PCR analysis of bulked plant samples was similar to classification based on visual assessment of disease severity.     

Differences in the responses of melon accessions to fusarium root and stem rot and their colonization by Fusarium oxysporum f. sp. radicis‐cucumerinum

Fusarium root and stem rot caused by the fungus Fusarium oxysporum f. sp. radicis‐cucumerinum is a major disease in greenhouse cucumbers. Over the past decade, the disease has been documented in melon greenhouses in Greece, and recently it has been sporadically recorded in greenhouse melons in Israel. Variations in disease response were found among 41 melon accessions artificially inoculated with the pathogen: 10 accessions were highly susceptible (90–100% mortality), 23 exhibited an intermediate response (20–86%) and eight were resistant (0–4%). Two melon accessions – HEM (highly resistant) and TAD (partially resistant) – were crossed with the susceptible accession DUL. The responses of the three accessions and F₁ crosses between the resistant and susceptible parents were evaluated. HEM contributed higher resistance to the F₁ hybrid than TAD. Roots of susceptible and resistant accessions were 100 and 79% colonized, respectively, following artificial inoculation. However, only susceptible plants showed colonization of the upper plant tissues. Microscopic evaluation of cross sections taken from the crown region of the susceptible DUL revealed profuse fungal growth in the intercellular spaces of the parenchyma and in xylem vessels. In the resistant cultivar HEM, very little fungal growth was detected in the intercellular spaces of the parenchyma, and none in the xylem or any other vascular tissue. Finding resistant accessions may create an opportunity to study the genetics of resistance inheritance and to develop molecular markers that will facilitate breeding resistant melon cultivars.     

香蕉假茎细胞对枯萎病菌不同小种及其粗毒素的病理反应

 以香蕉枯萎病菌(Fusarium oxysporum f.sp.cubense)1号小种和4号小种及其粗毒素分别接种香牙蕉和粉蕉的组培苗及离体假茎后,用组织切片法观察香蕉假茎细胞的病理反应,以探明香蕉枯萎病菌不同小种及其粗毒素的致病作用。结果表明,枯萎病菌不同小种人工接种仅能感染相应的香蕉种类,但不同香蕉种类的离体假茎细胞用不同小种接种及其粗毒素处理,均产生褐变等病理反应,且病变程度不存在小种间的差异。表明枯萎病菌不同小种对香蕉不同种类的致病力差异可能与存在其它致病因子或专化性识别的因子有关。同时证实了病菌不同小种的毒素对蕉类不存在着选择毒性     

Dehydrodimers of Ferulic Acid in Maize Grain Pericarp and Aleurone: Resistance Factors to Fusarium graminearum

ABSTRACT The relationship between the primary cell wall phenolic acids, dehydrodimers of ferulic acid, and maize grain resistance to Fusarium graminearum, the causal agent of gibberella ear rot, was investigated. Concentrations of dehydrodimers of ferulic acid were determined in the pericarp and aleurone tissues of five inbreds and two hybrids of varying susceptibility and in a segregating population from a cross between a resistant and susceptible inbred. Significant negative correlations were found between disease severity and diferulic acid content. Even stronger correlations were observed between diferulic acid and the fungal steroid ergosterol, which is an indicator of fungal biomass in infected plant tissue. These results were consistent over two consecutive field seasons, which differed significantly for temperature and rainfall during pollination, the most susceptible stage of ear development. No correlation was found between the levels of these phenolics and deoxynivalenol levels. This is the first report of in vivo evidence that the dehydrodimers of ferulic acid content in pericarp and aleurone tissues may play a role in genotypic resistance of maize to gibberella ear rot.     

Real-time PCR as a promising tool to monitor growth of Venturia spp. in scab-susceptible and -resistant apple leaves

Apple scab, the most important disease of apple worldwide, is caused by Venturia inaequalis. Currently, evaluation of fungal pathogenicity and host resistance is based on a symptomatic disease rating. However, this method does not provide an accurate measurement of the degree of infection and cannot detect early fungal development in symptomless leaves. In this study, a Venturia-specific real-time PCR assay was developed using primers designed around the specific internal transcribed spacer 2 (ITS2) region of the 5.8S rRNA gene. Using SYBR? Green I technology, the assay can accurately quantify Venturia DNA over a concentration range of at least five orders of magnitude. Detection sensitivities were in the order of 100?fg. The method was used to quantify Venturia genomic DNA levels in leaves of three apple cultivars with different levels and types of scab resistance and artificially infected with V. inaequalis. The assay clearly discriminated between Venturia levels in monogenic resistant (‘Topaz’), polygenic resistant (‘Discovery’), and susceptible (‘Golden Delicious’) cultivars, and proved especially useful to quantify pathogen levels during the initial latent stage of infection. The real-time PCR data of ‘Golden Delicious’ were consistent with the observed evolution of the degree of sporulation during a time-course experiment. Although measurements were influenced by the presence of co-extracted PCR-inhibitors, the impact of these compounds was independent of the apple cultivar or the initial amount of fungal DNA present. In conclusion, real-time PCR amplification of the ITS2-5.8S rDNA of Venturia spp. is a faster, more objective and more sensitive method to monitor fungal growth and to evaluate host resistance than phenotypic disease rating scores.     

接种大豆花叶病毒条件下大豆上位叶离体培养物的生长与形态发生

 本研究从接种大豆花叶病毒(SMV)江苏株系Sa、Sg的大豆植株上取上位叶为外植体,研究不同株系对愈伤组织产生及芽与根分化的影响。结果发现,愈伤组织的诱导率及其形态特点在接种与未接种间无明显差异;接种Sa的其芽分化率低于未接种的,但接种Sg的其芽分化率反略高于未接种Sg的;接种Sa、Sg后,根的发生能力减弱,但对抗病品种的影响小于感病品种。     

Assessing resistance to Crinipellis perniciosa using cocoa callus

Callus cultures from cocoa clones reported to have different susceptibilities to Brazilian isolates of Crinipellis perniciosa , causal fungus of witches' broom disease, including Catongo (susceptible). Scavina-6, CAB 64 and CAB 67 (resistant) were inoculated with basidiospores of C. perniciosa and the subsequent development of mycelia was assessed. Generally, on Catongo callus, mycelium similar to that found in green brooms (parasitic type), developed abundantly but on callus from Scavina-6 and the two CAB clones, mycelial growth was relatively poor and tended to be similar to that in dry brooms and on agar media (saprotrophic type). There was often considerable variation in fungal development on individual callus cultures from the same clone, but there was an overall consistency between successive experiments in ranking the callus material in terms of fungal development. However, growth of isolates from Colombia and Trinidad was similar on callus from Scavina-6 which in the field is known to reac differentially to these two isolates. Experiments with isolates from Colombia indicated that inoculum from basidiocarps produced on brooms which were kept for long periods in cabinets to induce fruiting had a decreased ability to colonize callus.     

Toxicity of Glucosinolate Degradation Products from Brassica napus Seed Meal Toward Aphanomyces euteiches f. sp. pisi

ABSTRACT Brassica tissues are potentially useful in the control of Aphanomyces root rot of peas (Pisum sativum), but identity of the responsible compounds and specific impacts of those compounds on the pathogen's infection potential remain uncertain. Brassica napus seed meals and water extracts from these meals were used to determine the effect of glucosinolate hydrolysis products on Aphanomyces euteiches f. sp. pisi. B. napus meal ('Dwarf Essex') containing glucosinolates and intact myrosinase, the enzyme responsible for glucosinolate hydrolysis, completely inhibited infection by A. euteiches f. sp. pisi oospores. Water extracts from this meal, likewise, severely inhibited infection by oospores, as well as mycelial growth. Extracts from autoclaved 'Dwarf Essex' meal, in which myrosinase was denatured, and a low glucosinolate B. napus variety ('Stonewall') produced little disease reduction and had less impact on mycelial growth. Gas chromatographic analysis of Brassica tissues and water extracts confirmed that glucosinolates remained in autoclaved 'Dwarf Essex' meal and that 'Stonewall' meal contained low glucosinolate concentrations. 5-Vinyloxazolidine-2-thione was identified by mass spectrometry as a dominant glucosinolate hydrolysis product in aqueous extracts of the inhibitory meal. Bioassays conducted with aqueous solutions of this compound reduced mycelial growth, but not to the extent of those from intact 'Dwarf Essex' meal. Water-soluble compounds produced from the hydrolysis of glucosinolates in B. napus tissues reduced A. euteiches oospore infection and inhibited mycelial growth, thus, demonstrating potential utility of Brassica species in the control of A. euteiches.     

Impact of carrot resistance on development of the <Emphasis Type="Italic">Alternaria</Emphasis> leaf blight pathogen (<Emphasis Type="Italic">Alternaria dauci</Emphasis>)

The interaction between Alternaria dauci and two carrot cultivars differing in their resistance to leaf blight was investigated by microscopy. The fungal development between 1 and 15 days post-inoculation was quite similar in the susceptible cv. Presto and the partially resistant cv. Texto: After conidial germination, leaf adhesion of the pathogen was achieved with mucilaginous filaments; hyphae penetrated the leaves directly with/without the formation of appressoria-like structures or via stomata; the fungus spread by epiphytic hyphae with hyphopodia and subcuticular mycelia. Intense necrotic plant cell reactions occurred under the fungal structures. At 21 days post-inoculation, typical features of fungal development were noted for each cultivar: growing hyphae emerged from stomata in cv. Presto, whereas conidiophores without conidia were observed in cv. Texto. Leaf tissues of both cultivars were strongly damaged and vesicle-like structures (assumed to be plant phenolics) were abundantly present between mesophyll cells. A real-time PCR method was developed for in planta quantification of A. dauci. Between 1 and 15 days post-inoculation, the fungal biomass was equivalent in the two cultivars and was about fourfold higher in cv. Presto than cv. Texto at 21 and 25 days post-inoculation. Taken together, our results indicated that A. dauci was able to colonize both cultivars in a similar manner during the first steps of the interaction, then fungal development in the partially resistant cultivar was restricted due to putative plant defence reactions. The results of this study enhance the overall understanding of infection processes in the A. dauci-carrot pathosystem.     

Vascular colonization patterns in susceptible and resistant tomato cultivars inoculated with Fusarium oxysporum f.sp. lycopersici races 0 and 1

The vascular colonization pattern of Fusarium oxysporum f.sp. lycopersici races 0 and 1 in tomato was studied in five susceptible and five resistant cultivar–fungus combinations during a 26-day period after inoculation by root immersion. Propagules spread discontinuously along the stems in all five cultivars 1 day after inoculation, irrespective of cultivar resistance. Five days later the fungus was limited to the stem bases in all cultivars. Between the fifth and 12th days, stem colonization by the fungus stopped in all cultivar–race combinations. Thereafter, the situation remained stable in resistant combinations, with inoculum distributed discontinuously, and no disease symptoms were apparent. By contrast, in the susceptible combinations a gradual upward colonization of the stems was seen such that fungal distribution was no longer discontinuous and disease symptoms appeared. These results suggest that a fungal 'incubation' period in the base of the vascular system is required before a secondary invasion of tissues occurs in susceptible genotypes. The slope of the regression line fitted between the height reached by the fungus up the stem ( y ) and the time after inoculation ( x ) provides a measure of the horizontal (polygenic) resistance in tomato cultivars     

玉米感染肿囊腐霉后寄主-病原物互作的超微结构研究

 本文利用透射电镜,首次对玉米不同抗性寄主与肿囊腐霉(Pythium inflatum)相互作用中的寄主反应及菌丝在寄主内的发展进行了系统研究。结果表明:玉米苗期根部接种后,孢子迅速萌发成菌丝在根表蔓延,随即穿透根表皮,进入表皮细胞、皮层甚至感病寄主的维管束组织。与此同时,寄主相应的反应迅速,寄主反应采取如下方式:细胞壁的沉积物及乳突在真菌的入侵处形成,各种无定形物质或纤丝构成的质网包围入侵菌丝。沉积在寄主细胞壁上并在受侵染细胞内聚集的化合物可能是酚类物质,这些嗜锇酸的电子致密物是用来机械地加强细胞壁的强度及产生抗真菌的环境。寄主反应的早晚及程度的强弱决定着菌丝在寄主体内发展繁殖的程度及寄主抗性的强弱。抗病玉米自交系的反应程度及速度显著强于感病玉米。     

Latency- and Defense-Related Ultrastructural Characteristics of Apple Fruit Tissues Infected with Botryosphaeria dothidea

ABSTRACT Apple fruit tissues infected with Botryosphaeria dothidea were examined by transmission electron microscopy using susceptible cv. Fuji and resistant cv. Jonathan. Immature (green) and mature (red) fruits of cv. Fuji with restricted or expanding lesions were also examined to reveal subcellular characteristics related with latent and restricted disease development. In infected susceptible mature fruits, cytoplasmic degeneration and organelle disruption commonly occurred, accompanying cell wall dissolution around invading hyphae. Cell wall dissolution around invading hyphae in subepidermis was rare in immature, red halo-symptomed cv. Fuji and resistant cv. Jonathan fruits. In infected immature fruits of cv. Fuji, presumably at the latent state of disease development, cellular degeneration was less severe, and invading hyphae contained prominent microbody-lipid globule complexes or the deposition of thin electron-dense outer layer around cell wall of intercellular hyphae. Both mature fruits with red halos and resistant apple fruits formed cell wall protuberances at the outside of cell walls. In addition, electron-dense extramural layers were formed in the resistant apple fruits. Aberrant hyphal structures such as intrahyphal hyphae were found only in resistant fruit tissues, indicating the physiologically altered fungal growth. These ultrastructural changes of host tissues and fungal hyphae may reflect the pathogenesis of apple white rot under varying conditions of apple fruits.     

Host genotype- and temperature-dependent colonizationof In vitro carnation callus cultures byFusarium oxysporum f.sp.dianthi race 2

Differentin vivo resistance/susceptibility levels of 14 carnation cultivars toFusarium oxysporum f.sp.dianthi race 2, the causal agent of Fusarium wilt disease of carnation, were also expressed in anin vitro system and assayed as the degree of fungal colonization of callus cultures at 20° C. Temperature influenced thein vitro expression of carnation resistance. An incubation temperature of 27° C increased the colonization of calli derived from both the susceptible (‘Corrida’ and ‘Ambra’) and the resistant (‘Pulcino’ and ‘Pallas’) cultivars. At 15°C, the colonization of calli derived from Pulcino and Pallas diminished significantly more than for Ambra and Corrida. Inhibition of fungal growth on resistant calli was correlated to retardation in hyphal development. Both scanning electron microscopy and light microscopy observations showed that hyphae did not penetrate into carnation cells.     

Selection of Populations of Puccinia recondita f. sp. tritici for Shortened Latent Period on a Partially Resistant Wheat Cultivar

ABSTRACT Wild-type fungal population 851-WT was selected for shortened latent period on cv. CI 13227 for five uredinial generations to study the adaptation of Puccinia recondita f. sp. tritici to partially resistant wheat cultivars. Differences among wild-type and selected populations for traits contributing to parasitic fitness (i.e., latent period, infection frequency, and uredinium area and growth rate) were assessed in monocyclic infection experiments on susceptible cv. Monon and partially resistant cvs. Suwon 85, Sw 72469-6, L-574-1, and CI 13227. Differences were greatest among fungal populations on cv. CI 13227. The mean latent period of selected population 851-C5 was 2 days shorter (~20%) than that of wild-type population 851-WT. In addition, uredinia of population 851-C5 expanded 40% faster and produced ~75% more urediniospores. On cv. L-574-1, the selected population was also more fit than the wild-type progenitor for initial uredinium area and growth rate and cumulative urediniospore production. In contrast to wild-type and selected populations on cvs. CI 13227 and L-574-1, selected population 851-C5 on cv. Monon produced slower expanding uredinia with fewer urediniospores than did population 851-WT on Monon. These results show that variation in the latent period of P. recondita f. sp. tritici populations is partially under genetic control and wild-type P. recondita f. sp. tritici populations contain members reproductively more fit on partially resistant wheat cultivars but not necessarily on susceptible cultivars. Such members are capable of partially overcoming quantitative host resistance.     

Modelling effects of vector acquisition threshold on disease progression in a perennial crop following deployment of a partially resistant variety

Deployment of resistant varieties is a key strategy to mitigating economic losses due to arthropod‐transmitted plant pathogens of perennial crops. In many cases, the best available resistant traits for introgression confer only partial resistance. Plants displaying partial resistance have lower pathogen titres than susceptible counterparts, but remain hosts for the pathogen. As partially resistant varieties maintain yield after infection, infected plants are unlikely to be rogued (i.e. removed). Accordingly, there is a risk that partially resistant plants could serve as a source of inoculum for pathogen spread to susceptible plants. Here, a mathematical model that tracked spread of an arthropod‐transmitted pathogen in a plant population consisting of susceptible and partially resistant plants was used to identify a threshold acquisition rate from partially resistant plants that resulted in limited spread of the pathogen from partially resistant plants to susceptible plants. The acquisition threshold from partially resistant plants varied with parameters influenced by disease management decisions such as number of vectors per plant, vector turnover, replacement of susceptible plants, and proportion of plants that were partially resistant. In model simulations, effects of deploying a partially resistant variety on disease incidence in a susceptible variety depended on the extent to which pathogen spread among susceptible plants was suppressed and acquisition rates from partially resistant plants. Collectively, the results indicate that risk of partially resistant plants serving as inoculum sources could be assessed prior to deployment, thereby enabling design of complementary disease management tactics to minimize economic losses in susceptible varieties following deployment.     

Fusarium wilt‐resistant lines of Brazil banana (Musa spp., AAA) obtained by EMS‐induced mutation in a micro‐cross‐section cultural system

This study combined the micro‐cross‐section cultural system with in vitro mutagenesis induced by ethyl methanesulphonate (EMS) to screen for fusarium wilt‐resistant lines of Brazil banana (Musa spp., AAA). The results indicated that the optimum EMS concentration and duration for the treatment of micro‐cross‐sections cut from the pseudostem of tissue‐cultured plantlet were 300 mm and 60 min, respectively. Under the optimal treatment, an average of 2·2 regenerated shoots were produced from each explant. One hundred regenerated plantlets were used for screening for fusarium wilt‐resistant lines by the early screening technique. The initial disease symptom – yellowing in lower leaves of susceptible plantlets – was observed 2 weeks after inoculation. After 2 months, only six plants survived – the putative fusarium wilt‐resistant lines. The fusarium wilt pathogen Fusarium oxysporum f. sp. cubense race 4, was identified in the preliminary test field by a SCAR marker technique. Of the six putative resistant lines, five survived the preliminary field test. The regenerated plantlets from these five fusarium wilt‐resistant lines were subjected to early screening again, where they showed markedly reduced disease incidences compared with regenerated plantlets of Brazil banana (control). It was concluded that EMS‐induced mutation of banana through the micro‐cross‐section cultural system is potentially useful for banana improvement.