Plant-Plant-Microbe Mechanisms Involved in Soil-Borne Disease Suppression on a Maize and Pepper Intercropping System

Background

Intercropping systems could increase crop diversity and avoid vulnerability to biotic stresses. Most studies have shown that intercropping can provide relief to crops against wind-dispersed pathogens. However, there was limited data on how the practice of intercropping help crops against soil-borne Phytophthora disease.

Principal Findings

Compared to pepper monoculture, a large scale intercropping study of maize grown between pepper rows reduced disease levels of the soil-borne pepper Phytophthora blight. These reduced disease levels of Phytophthora in the intercropping system were correlated with the ability of maize plants to form a “root wall” that restricted the movement of Phytophthora capsici across rows. Experimentally, it was found that maize roots attracted the zoospores of P. capsici and then inhibited their growth. When maize plants were grown in close proximity to each other, the roots produced and secreted larger quantities of 2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one (DIMBOA) and 6-methoxy-2-benzoxazolinone (MBOA). Furthermore, MBOA, benzothiazole (BZO), and 2-(methylthio)-benzothiazole (MBZO) were identified in root exudates of maize and showed antimicrobial activity against P. capsici.

Conclusions

Maize could form a “root wall” to restrict the spread of P. capsici across rows in maize and pepper intercropping systems. Antimicrobe compounds secreted by maize root were one of the factors that resulted in the inhibition of P. capsici. These results provide new insights into plant-plant-microbe mechanisms involved in intercropping systems.     

芽孢杆菌对黄瓜根腐病的防治效果

枯草芽孢杆菌B29从黄瓜根际土壤分离,经实验室平板抑菌活性和温室盆栽防效测定,发现该菌对黄瓜根腐病有较强的抑菌作用,温室防效达79．7％-83．3％,且对瓜苗有一定的促生长作用。     

Epidemiology of Maize Root Rot in South Africa

Maize roots were rapidly infected by soil inhabiting fungi as soon as they differentiated. The first tissues to develop were the seminal roots and mesocotyls. As their function became superseded by that of the adventitious roots, they rapidly became, completely rotted. Adventitious root rot developed slower and the roots did not become as severely rotted. Numerous fungi were isolated from the roots. The most frequently isolated were Helminthosporium pedicellatum and Fusarium moniliforme. Trichoderma sp. was the next most frequently isolated fungus; it was, however, significantly less frequently isolated than the above. Young and Kucharek (1977) found that the fungi isolated from maize roots occur in communities associated with certain plant growth stages. This was not clearly evident in this study. Numbers of root lesion nematodes (Pratylenchus species) correlated non-significantly with fungus frequencies and root rot.     

Rational Phosphorus Application Facilitates the Sustainability of the Wheat/Maize/Soybean Relay Strip Intercropping System

Wheat (Triticum aestivum L.)/maize (Zea mays L.)/soybean (Glycine max L.) relay strip intercropping (W/M/S) system is commonly used by the smallholders in the Southwest of China. However, little known is how to manage phosphorus (P) to enhance P use efficiency of the W/M/S system and to mitigate P leaching that is a major source of pollution. Field experiments were carried out in 2011, 2012, and 2013 to test the impact of five P application rates on yield and P use efficiency of the W/M/S system. The study measured grain yield, shoot P uptake, apparent P recovery efficiency (PRE) and soil P content. A linear-plateau model was used to determine the critical P rate that maximizes gains in the indexes of system productivity. The results show that increase in P application rates aggrandized shoot P uptake and crops yields at threshold rates of 70 and 71.5 kg P ha^-1 respectively. With P application rates increasing, the W/M/S system decreased the PRE from 35.9% to 12.3% averaged over the three years. A rational P application rate, 72 kg P ha^-1, or an appropriate soil Olsen-P level, 19.1 mg kg^-1, drives the W/M/S system to maximize total grain yield while minimizing P surplus, as a result of the PRE up to 28.0%. We conclude that rational P application is an important approach for relay intercropping to produce high yield while mitigating P pollution and the rational P application-based integrated P fertilizer management is vital for sustainable intensification of agriculture in the Southwest of China.     

Use of Rhizobia in the Control of Root Rot Diseases of Sunflower, Okra, Soybean and Mungbean

Biocontrol potential of Rhizobium and Bradyrbizobium against soilborne root infecting fungi was tested. In vitro tests Rhizobium meliloti inhibited growth of Macrophomina phaseolina, Rhizoctonia solani and Fusarium solani while Bradyrhizobium japonicum inhibited M. phaseolina and R. solani producing zones of inhibition. In field R. meliloti, R. leguminosarum and B. japonicum used either as seed dressing or as soil drench reduced infection of M. phaseolina, R. solani and Fusarium spp., in both leguminous (soybean, mungbean) and non-leguminous (sunflower and okra) plants.     

Row Ratios of Intercropping Maize and Soybean Can Affect Agronomic Efficiency of the System and Subsequent Wheat

Intercropping is regarded as an important agricultural practice to improve crop production and environmental quality in the regions with intensive agricultural production, e.g., northern China. To optimize agronomic advantage of maize (Zea mays L.) and soybean (Glycine max L.) intercropping system compared to monoculture of maize, two sequential experiments were conducted. Experiment 1 was to screening the optimal cropping system in summer that had the highest yields and economic benefits, and Experiment 2 was to identify the optimum row ratio of the intercrops selected from Experiment 1. Results of Experiment 1 showed that maize intercropping with soybean (maize || soybean) was the optimal cropping system in summer. Compared to conventional monoculture of maize, maize || soybean had significant advantage in yield, economy, land utilization ratio and reducing soil nitrate nitrogen (N) accumulation, as well as better residual effect on the subsequent wheat (Triticum aestivum L.) crop. Experiment 2 showed that intercropping systems reduced use of N fertilizer per unit land area and increased relative biomass of intercropped maize, due to promoted photosynthetic efficiency of border rows and N utilization during symbiotic period. Intercropping advantage began to emerge at tasseling stage after N topdressing for maize. Among all treatments with different row ratios, alternating four maize rows with six soybean rows (4M:6S) had the largest land equivalent ratio (1.30), total N accumulation in crops (258 kg ha^-1), and economic benefit (3,408 USD ha^-1). Compared to maize monoculture, 4M:6S had significantly lower nitrate-N accumulation in soil both after harvest of maize and after harvest of the subsequent wheat, but it did not decrease yield of wheat. The most important advantage of 4M:6S was to increase biomass of intercropped maize and soybean, which further led to the increase of total N accumulation by crops as well as economic benefit. In conclusion, alternating four maize rows with six soybean rows was the optimum row ratio in maize || soybean system, though this needs to be further confirmed by pluri-annual trials.     

Bacterial Vascular Necrosis and Root Rot Disease of Sugar Beet in Egypt

Five pectolytic bacteria were isolated from roots and petioles of sugar beet exhibiting soft rot and vascular necrosis symptoms that were collected from different fields in El-Minia Governorate, Egypt. Inoculation of the bacteria into sugar beet roots through wounded crowns or injured lateral roots reproduced soft rot and vascular necrosis symptoms. According to their biochemical and physiological characteristics, pathogenicity tests, fatty acid composition analysis, and electron microscopy, the bacteria were identified as Erwinia carotovora ssp. betavasculorum. This is the first report of the disease in Egypt.     

Nematode Control Related to Fusarium Wilt in Soybean and Root Rot and Zinc Deficiency in Corn

Nematode and disease problems of irrigated, double-cropped soybean and corn, and zinc deficiency of corn were investigated. Ethylene dibromide, phenamiphos, and aldicarb were equally effective for controlling nematodes and increasing yields of corn planted minimum-till and soybean planted in a moldboard plow prepared seedbed. The residual effects on yields of nematicides applied to the preceeding crop occurred during 3 years for soybean and 1 year for corn. Fusarium wilt symptoms of soybean that developed during 2 years of the study were less severe in all nematicide-treated plots than in control plots. Typical zinc deficiency symptoms on 30-day-old corn plants were observed during 1 year of the study in certain plots. Symptoms were not evident on plants grown on plots treated with ethylene dibromide, and only occasional plants had symptoms on plots treated with phenamiphos and aldicarb. The amount of yield response directly related to nematode control could not be determined because of the apparent interaction of nematodes on the expression of Fusarium wilt of soybean. Our study strongly indicates that the expression of Fusarium wilt of soybean and zinc deficiency in corn are influenced by nematodes and that nematicides will reduce their severity.     

Biological Control of Phytophthora Root Rots on Alfalfa and Soybean with Streptomyces

A collection of 53 antibiotic-producing Streptomyces isolated from soils from Minnesota, Nebraska, and Washington were evaluated for their ability to inhibit plant pathogenic Phytophthora medicaginis and Phytophthora sojae in vitro. Eight isolates having the greatest pathogen-inhibitory capabilities were subsequently tested for their ability to control Phytophthora root rots on alfalfa and soybean in sterilized vermiculite and naturally infested field soil. The Streptomyces isolates tested significantly reduced root rot severity in alfalfa and soybean caused by P. medicaginis and P. sojae, respectively (P < 0.05). On alfalfa, isolates varied in their effect on plant disease severity, percentage dead plants, and plant biomass in the presence of the pathogen. The same eight isolates of Streptomyces were also tested for inhibitory activities against each other and against three strains of Bradyrhizobium japonicum and two strains of Sinorhizobium meliloti isolated from soybean and alfalfa, respectively. Streptomyces isolates clustered into two major compatibility groups: isolates within the same group were noninhibitory toward one another in vitro. The compatibility groups corresponded with groupings obtained based upon inhibition of B. japonicum and S. meliloti strains.     

Isolation and Pathogenicity of Rhizosphere Fungi of Cocoyam in Relation to Cocoyam Root Rot Disease

Cocoyam is the second most important staple crop of Cameroon and root rot is a destructive disease of this plant. Pythium myriotylum (Pm), Fusarium solani (Fs), and Rhizoctonia solani (Rs) were isolated from the rhizosphere of root rot affected cocoyams and from the soil of a cocoyam experimental field plot temporarily devoid of same in Mamu, Cameroon. Pm was isolated from the above soil by the cocoyam leaf disc baits. Fs and Rs were also isolated from the same soils by the water dilution method and from the roots of diseased cocoyams but were always associated with mycelial growth of Pm. Pathogenicity of Pm and in combinations with Fs or Rs or Fs + Rs all developed cocoyam root rot disease (CRRD) symptoms on 3– and 7–month old cocoyam plantlets 2–7 days after inoculation. Symptoms included rotted roots and wilting with general chlorosis of inoculated plantlets. No symptoms of CRRD were noted on cocoyam plantlets inoculated with Fs, Rs, Fs + Rs, and distilled water. Results indicated that CRRD is not caused by several pathogens but only by Pm. Pm isolates from the soils and roots of diseased cocoyams and those maintained in the ROTREP laboratory have significantly bigger diameter of mycelial colony growth in 24 h–period at 31 °C on lima bean sucrose agar, V–8 juice sucrose agar, and potato sucrose agar than on potato dextrose agar and 2 % water agar. The cocoyam plantlets were raised axenically from tissue culture of explants in the laboratory.     

Focus Issue on Roots: Low Crown Root Number Enhances Nitrogen Acquisition from Low-Nitrogen Soils in Maize

In developing nations, low soil nitrogen (N) availability is a primary limitation to crop production and food security, while in rich nations, intensive N fertilization is a primary economic, energy, and environmental cost to crop production. It has been proposed that genetic variation for root architectural and anatomical traits enhancing the exploitation of deep soil strata could be deployed to develop crops with greater N acquisition. Here, we provide evidence that maize (Zea mays) genotypes with few crown roots (crown root number [CN]) have greater N acquisition from low-N soils. Maize genotypes differed in their CN response to N limitation in greenhouse mesocosms and in the field. Low-CN genotypes had 45% greater rooting depth in low-N soils than high-CN genotypes. Deep injection of ¹⁵N-labeled nitrate showed that low-CN genotypes under low-N conditions acquired more N from deep soil strata than high-CN genotypes, resulting in greater photosynthesis and plant N content. Under low N, low-CN genotypes had greater biomass than high-CN genotypes at flowering (85% in the field study in the United States and 25% in South Africa). In the field in the United States, 1.8× variation in CN was associated with 1.8× variation in yield reduction by N limitation. Our results indicate that CN deserves consideration as a potential trait for genetic improvement of N acquisition from low-N soils.Maize (Zea mays) is one of the world’s most important crops and is a staple food in Latin America and Africa. Maize production requires a large amount of fertilizer, especially nitrogen (N). In the United States, N fertilizers represent the greatest economic and energy costs for maize production (Ribaudo et al., 2011). However, on-farm studies across the northcentral United States revealed that more than half of applied N is not taken up by maize plants and is vulnerable to losses from volatilization, denitrification, and leaching, which pollute air and water resources (Cassman et al., 2002). Conversely, in developing countries, suboptimal N availability is a primary limitation to crop yields and, therefore, food security (Azeez et al., 2006). Increasing yield in these areas is an urgent concern, since chemical fertilizers are not affordable (Worku et al., 2007). Cultivars with greater N acquisition from low-N soils could help alleviate food insecurity in poor nations as well as reduce environmental degradation from excessive fertilizer use in developed countries.The two major soil N forms available to plants are ammonium and nitrate. Nitrate is the main N form in most maize production environments (Miller and Cramer, 2004). Nitrate is highly mobile in soil, and the spatiotemporal availability of soil N is rather complex. In the simplest case, N fertilizers applied to the soil and/or N released from the mineralization of soil organic matter are rapidly converted to nitrate by soil microbes. After irrigation and precipitation events, nitrate moves with water to deeper soil strata. Leaching of nitrate from the root zone has been shown to be a significant cause of low recovery of N fertilizer in commercial agricultural systems (Raun and Johnson, 1999; Cassman et al., 2002). Differences in root depth influence the ability of plants to acquire N. Studies using ¹⁵N-labeled nitrate placed at different soil depths showed that only plants with deep rooting can acquire N sources from deep soil strata, which would otherwise have been lost through leaching (Kristensen and Thorup-Kristensen, 2004a, 2004b). Therefore, selection for root traits enhancing rapid deep soil exploration could be used as a strategy to improve crop N efficiency.The maize root system consists of embryonic and postembryonic components. The embryonic root system consists of two distinct root classes: a primary root and a variable number of seminal roots formed at the scutellar node. The postembryonic root system consists of roots that are formed at consecutive shoot nodes and lateral roots, which are initiated in the pericycle of all root classes. Shoot-borne or nodal roots that are formed belowground are called crown roots, whereas those that are formed aboveground are designated brace roots (Hochholdinger, 2009). While the primary root and seminal roots are essential for the establishment of seedlings after germination, nodal roots and particularly crown roots make up most of the maize root system and are primarily responsible for soil resource acquisition later in development (Hoppe et al., 1986).Lynch (2013) proposed an ideotype for superior N and water acquisition in maize called Steep, Cheap, and Deep (SCD), which integrates root architectural, anatomical, and physiological traits to increase rooting depth and, therefore, the capture of N in leaching environments. One such trait is crown root number (CN). CN is an aggregate trait consisting of the number of belowground nodal whorls and the number of roots per whorl. The crown root system dominates resource acquisition during vegetative growth after the first few weeks and remains important during reproductive development (Hochholdinger et al., 2004). CN in maize ranges from five to 50 under fertile conditions (Trachsel et al., 2011). At the low end of this range, crown roots may be too spatially dispersed to sufficiently explore the soil. There is also a risk of root loss to herbivores and pathogens. If roots are lost in low-N plants, there may be too few crown roots left to support the nutrient, water, and anchorage needs of the plant. At the high end, a large number of crown roots may compete with each other for water and nutrients as well as incur considerable metabolic costs for the plant (Fig. 1). The SCD ideotype proposes that there is an optimal CN for N capture in maize (Lynch, 2013). Under low-N conditions, resources for root growth and maintenance are limiting, and nitrate is a mobile resource that can be captured by a dispersed root system. The optimal CN should tend toward the low end of the phenotypic variation to make resources available for the development of longer, deeper roots rather than more crown roots. According to the SCD ideotype, in low-N soils, maize genotypes with fewer crown roots could explore soils at greater depth, resulting in greater N acquisition, growth, and yield than genotypes with many crown roots.Open in a separate windowFigure 1.Visualization of the maize root system of low- and high-CN genotypes at 40 d after germination. Crown roots are colored in blue, and seminal roots are in red. The CN is eight in the low-CN genotype and 46 in the high-CN genotype. (Image courtesy of Larry M. York.)The objective of this study was to test the hypotheses that (1) low-CN genotypes have greater rooting depth than high-CN genotypes in low-N soils; (2) low-CN genotypes are better at acquiring deep soil N than high-CN genotypes; and (3) low-CN genotypes have greater biomass and yield than high-CN genotypes in low-N conditions.     

Plectosporium tabacinum Root Rot Disease of White Lupine (Lupinus termis Forsk.) and its Biological Control by Streptomyces Species

Plectosporium tabacinum root rot disease of white lupine is reported for the first time in Cairo Governorate provinces. Symptoms of the disease appeared on young seedlings as stunting, yellowing and wilting of the foliage. Rotted roots were first became light brown and the lesions progressively extended over the whole root system and became dark brown. Plants died when the whole root system had become infected. For successful biological control of the disease 70 rhizosphere actinomycetes were isolated by the use of normal standard isolation techniques. Three of these isolates were found to be strongly antagonistic against P. tabacinum in vitro . They also significantly reduced the incidence of white lupine root rot disease in soil infested with P. tabacinum in greenhouse trials. The three actinomycete isolates were identified as Streptomyces cyanoviridis , Streptomyces murinus and Streptomyces griseoplanus .     

Reproductive Allometry in Soybean, Maize and Sunflower

We compared the relationship between grain yield per plant (Y_P)and shoot biomass per plant (S_P) in three annual crops withcontrasting reproductive strategies: sunflower, a determinatespecies with a single inflorescence; maize, a determinate specieswith a limited capacity to adjust the number of ears in responseto resource availability; and indeterminate soybean, a specieswith a large capacity to adjust the number of inflorescences.Our working hypotheses were: H₁—the relationship betweenY_PandS_P is linear; H₂—the intercept of the model is zero,i.e. there is not a threshold plant mass for reproduction. Awide range of Y_Pand S_Pwas generated by manipulation of plantdensity;S_Pvaried between 0.3 and 196 g per plant in soybean,between 6 and 873 g per plant in sunflower and between 23 and697 g per plant in maize. Within these broad ranges of plantsize, both hypotheses were rejected in five out of six experiments,i.e. the relationship between Y_Pand S_Pdeparted from linearityand there was a threshold for S_Pbelow which no grain set occurred.TheS_P threshold for grain set varied widely among species; itwas close to 2 g per plant for soybean, 27 g per plant for sunflowerand 43–71 g per plant for maize. Because of this sizethreshold and non-linearity, harvest index (HI = Y_PS_P^-1) wasstable for mid-size plants, diminished slightly for large plants,and diminished sharply for smaller plants in all three crops.Harvest index stability was highest in soybean, intermediatein sunflower and lowest in maize. Differential stability ofreproductive partitioning partially derived from contrastingpatterns of meristem allocation. Copyright 2000 Annals of BotanyCompany Helianthus annuus L., Zea mays L., Glycine max(L.) Merrill, grain yield, harvest index, plant density, reproductive allocation, meristem allocation, plasticity     

The Genetic Structure of Phellinus noxius and Dissemination Pattern of Brown Root Rot Disease in Taiwan

Since the 1990s, brown root rot caused by Phellinus noxius (Corner) Cunningham has become a major tree disease in Taiwan. This fungal pathogen can infect more than 200 hardwood and softwood tree species, causing gradual to fast decline of the trees. For effective control, we must determine how the pathogen is disseminated and how the new infection center of brown root rot is established. We performed Illumina sequencing and de novo assembly of a single basidiospore isolate Daxi42 and obtained a draft genome of ~40 Mb. By comparing the 12,217 simple sequence repeat (SSR) regions in Daxi42 with the low-coverage Illumina sequencing data for four additional P. noxius isolates, we identified 154 SSR regions with potential polymorphisms. A set of 13 polymorphic SSR markers were then developed and used to analyze 329 P. noxius isolates collected from 73 tree species from urban/agricultural areas in 14 cities/counties all around Taiwan from 1989 to 2012. The results revealed a high proportion (~98%) of distinct multilocus genotypes (MLGs) and that none of the 329 isolates were genome-wide homozygous, which supports a possible predominant outcrossing reproductive mode in P. noxius. The diverse MLGs exist as discrete patches, so brown root rot was most likely caused by multiple clones rather than a single predominant strain. The isolates collected from diseased trees near each other tend to have similar genotype(s), which indicates that P. noxius may spread to adjacent trees via root-to-root contact. Analyses based on Bayesian clustering, F _ST statistics, analysis of molecular variance, and isolation by distance all suggest a low degree of population differentiation and little to no barrier to gene flow throughout the P. noxius population in Taiwan. We discuss the involvement of basidiospore dispersal in disease dissemination.