A Review: Late Wilt of Maize-The Pathogen, the Disease, Current Status, and Future Perspective

Antagonistic interactions between maize seeds microbiome species and the late wilt disease agent, Magnaporthiopsis maydis

Magnaporthiopsis maydis is a maize pathogen that causes severe damage to commercial corn fields in the late growth stages. Late wilt disease (LWD) has spread since its discovery in the 1960s in Egypt and is now reported in about 10 countries. The pathogen has a hidden endophytic lifecycle in resistant corn plants and secondary hosts such as green foxtail, watermelon lupin and cotton. At the same time, it could be an opportunist and hinder the host development under the right conditions. This study uncovered M. maydis interactions with newly identified maize endophytes. To this end, six fungi were isolated from the seeds of three sweet corn cultivars having varying susceptibility to LWD. These isolates were identified using colony morphology and microscopic characterization, universal internal transcribed spacer (ITS) molecular targeting and phylogenetic analysis. Most of them belonged to pathogenic species. Compared to three previously identified bioprotective microorganisms, the new species were tested for their ability to secrete metabolites that repress M. maydis in vitro and to antagonize it in a solid media confront test and a seedlings pathogenicity assay. The opportunistic fungal species Aspergillus flavus (ME1), Aspergillus terreus (PE3) and the reference biocontrol bacteria Bacillus subtilis (R2) achieved the highest M. maydis inhibition degree in the plates tests (74-100% inhibition). The seedlings' pathogenicity assay that predicts the seeds' microflora resistance to M. maydis highlighted the bio-shielding potential of most species (23% or more epicotyl elongation over the infected control). Fusarium sp. (ME2) was the leading species in this measure (43% enhancement), and B. subtilis gave the best protection in terms of seeds' germination (50%) and sprouts' biomass (34%). The results of this study could enhance our understanding of the pathobiome's role in the context of LWD and represent a first step in using the seeds' natural protective microflora to develop novel management strategies.

Streptomyces as a promising biological control agents for plant pathogens

Plant diseases caused by pathogenic microorganisms in agriculture present a considerable obstacle, resulting in approximately 30-40% crop damage. The use of conventional techniques to manage these microorganisms, i.e., applying chemical pesticides and antimicrobials, has been discovered to have adverse effects on human health and the environment. Furthermore, these methods have contributed to the emergence of resistance among phytopathogens. Consequently, it has become imperative to investigate natural alternatives to address this issue. The Streptomyces genus of gram-positive bacteria is a potentially viable natural alternative that has been extensively researched due to its capacity to generate diverse antimicrobial compounds, such as metabolites and organic compounds. Scientists globally use diverse approaches and methodologies to extract new bioactive compounds from these bacteria. The efficacy of bioactive compounds in mitigating various phytopathogens that pose a significant threat to crops and plants has been demonstrated. Hence, the Streptomyces genus exhibits potential as a biological control agent for combating plant pathogens. This review article aims to provide further insight into the Streptomyces genus as a source of antimicrobial compounds that can potentially be a biological control against plant pathogens. The investigation of various bioactive compounds synthesized by this genus can enhance our comprehension of their prospective utilization in agriculture.

Management of sunflower charcoal-rot and maize late-wilt diseases using the aqueous extract of vermicompost (vermitea) and environmental-safe biochar derivative (wood vinegar)

In Egypt, sunflower charcoal-rot caused by Macrophomina phaseolina and maize late-wilt caused by Magnaporthiopsis maydis are the most prevalent, and can lead to huge yield losses of both crops under epidemic conditions. In this study, the potential use of vermitea and wood vinegar for management of both diseases was investigated. Data revealed that, among the 17 bacterial strains obtained from vermitea, three strains named VCB-2, VCB-7 and VCB-11 were chosen for having the greatest in vitro inhibitory effect against M. phaseolina and M. maydis, with fungal inhibition values of 54.2; 61.7, 65.2; 74.0 and 57.1; 87.0% against both pathogens, respectively. These strains were identified as Bacillus amyloliquefaciens, Serratia marcescens and Bacillus velezensis, respectively. Wood vinegar significantly reduced the colony diameter of M. phaseolina and M. maydis in in vitro trials conducted on potato dextrose agar medium amended with the desired concentrations of 0.5, 1.0, 1.5, 2.0, and 2.5%. The efficiency increased with increasing wood vinegar concentration, and 2.0% was the most effective (100% suppression). Data from greenhouse experiments showed that the application of vermitea or wood vinegar tended to decrease the incidence (% dead plants) of sunflower charcoal-rot (by 61.1 and 66.7%) and maize late-wilt (by 70.6%). These treatments had positive impacts on the plant growth parameters, photosynthetic pigments and antioxidative enzymes of sunflower and maize plants. Data from field experiments showed that the application of vermitea or wood vinegar decreased the incidence of charcoal-rot (by 72.8 and 72.0%) and late-wilt (by 88.7 and 87.0%) as well as increased the production sunflower and maize plants.

Maize seed endophytes

Maize is a vital global crop, and each seed (kernel) hosts an ecosystem of microbes living inside it. However, we know very little about these endophytes and what their role is in plant production and physiology. In this Microreview, I summarize the major questions around maize seed endophytes, including what organisms are present, how they get there, whether and how they transmit across generations, and how they and the plant affect each other. Although several studies touch on each of these areas, ultimately there are far more questions than answers. Future priorities for research on maize seed endophytes should include understanding what adaptations allow microbes to be seed endophytes, how the host genetics and the environment affect these communities, and how maize seed endophytes ultimately contribute to the next generation of plants.

Cultivars Resistance Assay for Maize Late Wilt Disease

Magnaporthiopsis maydis late wilt disease (LWD) in corn is considered to be the most severe in Israel and Egypt and poses a significant threat in other countries. Research efforts extending over a period of five decades led to the development of chemical, biological, agrotechnical, physical (solar disinfection) and other means for controlling late wilt disease. Today, some applications can reduce damage even in severe cases. However, cultivating disease-resistant maize varieties is the primary means for reducing the disease's impact. The current work uses a rapid (six days) laboratory seedling pathogenicity test and a full-season open encloser semi-field conditioned pots assay (101 days) to classify maize varieties according to their LWD resistance. To better evaluate differences between the cultivars, a real-time based molecular assay was applied to track the pathogen's presence in the plants' tissues, and visible light aerial imaging was used in parallel. The findings show that in cases of extreme sensitivity or tolerance (for example, in the highly susceptible Megaton cultivar (cv.) or the resistant Hatai cv.), a similarity in the results exists between the different methods. Thus, a reliable estimate of the varieties' sensitivity can be obtained in a seed assay without the need for a test carried out throughout an entire growing season. At the same time, in most situations of partial or reduced LWD sensitivity/resistance, there is no match between the various tests, and only the entire growing season can provide the most reliable results. Tracking the amount of M. maydis DNA in the plants' bodies is a precise, sensitive scientific tool of great importance for studying the development of the disease and the factors affecting it. Yet, no complete overlap exists between the fungal DNA amount and symptom severity. Such a correlation exists in high sensitivity or resistance cases but not in intermediate situations. Still, the valuation of the pathogen's establishment in asymptomatic corn hybrids can indicate the degree of LWD immunity and the chance of susceptibility development.

Aggressive strains of the late wilt fungus of corn exist in Israel in mixed populations and can specialize in disrupting growth or plant health

Maize late wilt disease caused by the fungus Magnaporthiopsis maydis significantly damages crops in Israel and in other countries. Resistant maize cultivars are the preferred method for disease restraining. However, the pathogen populations of Spain and Egypt have varying aggressiveness, and virulent strains can overcome host resistance. In 2001 and from 2016 to -2019, 17 M. maydis strains were isolated from infected maize fields in Israel. The isolates' effects on seed germination, plant development, and disease symptoms severity were evaluated. The isolates from Israel display a diverse degree of aggressiveness that is not linked to their geographic distribution. The virulent strains are found in mixed populations, whereas less virulent M. maydis isolates exist. Aggressive strains harmed the development of plants and ears and caused severe wilting and death. In contrast, plants inoculated with less virulent strains exhibited only mild dehydration signs, and crop yield was similar to that of the non-infected control. Interestingly, different host cultivars can evoke specific virulence of M. maydis strains. Moreover, some pathogen strains significantly repress plant development, while the impact of other strains was evidenced by wilting symptoms. The current research further increases our understanding of the pathogen and our ability to control it.

Distribution and Biodiversity of Seed-Borne Pathogenic and Toxigenic Fungi of Maize in Egypt and Their Correlations with Weather Variables

Studies of the biodiversity of plant pathogenic and toxigenic fungi are attracting great attention to improve the predictability of their epidemics and the development of their control programs. Two hundred maize grain samples were gathered from 25 maize-growing governorates in Egypt and 189 samples were processed for the isolation and identification of seed-borne fungal microbiome. Twenty-six fungal genera comprising 42 species were identified according to their morphological characteristics and ITS DNA sequence analysis. Occurrence and biodiversity indicators of these fungal species were calculated. Ustilago maydis, Alternaria alternata, Aspergillus flavus, A. niger, Penicillium spp., Cladosporium spp. and Fusarium verticillioides were the highly frequent (>90% for each), recording the highest relative abundance (˃50%). Al-Menia governorate showed the highest species diversity and richness, followed by Sohag, Al-Nobaria and New Valley governorates. Correlations of 18 fungal species with temperature, relative humidity, precipitation, wind speed, and solar radiation were analyzed using canonical correspondence analysis. Results showed that relative humidity, temperature, and wind speed, respectively, were the most impactful weather variables. However, the occurrence and distribution of these fungi were not clearly grouped into the distinctive climatic regions in which maize crops are grown. Monitoring the occurrence and distribution of the fungal pathogens of maize grains in Egypt will play an important role in predicting their outbreaks and developing appropriate future management strategies. The findings in this study may be useful to other maize-growing countries that have similar climatic conditions.

Crop Rotation and Minimal Tillage Selectively Affect Maize Growth Promotion under Late Wilt Disease Stress

In recent years, worldwide scientific efforts towards controlling maize late wilt disease (LWD) have focused on eco-friendly approaches that minimize the environmental impact and health risks. This disease is considered to be the most severe threat to maize fields in Israel and Egypt, and a major growth restraint in India, Spain, and Portugal. Today’s most commonly used method for LWD control involving resistant maize genotypes is under constant risk from aggressive pathogen lines. Thus, this study’s objectives were to evaluate the effect of crop rotation and avoiding tillage on restraining the disease. Such an agrotechnical approach will support the continuity of soil mycorrhiza networks, which antagonize the disease’s causal agent, Magnaporthiopsis maydis. The method gained positive results in previous studies, but many knowledge gaps still need to be addressed. To this end, a dual-season study was conducted using the LWD hyper-susceptible maize hybrid, Megaton cv. The trials were performed in a greenhouse and in the field over full dual-growth seasons (wheat or clover as the winter crop followed by maize as the summer crop). In the greenhouse under LWD stress, the results clearly demonstrate the beneficial effect of maize crop rotation with clover and wheat on plant weight (1.4-fold), height (1.1–1.2-fold) and cob yield (1.8–2.4-fold), especially in the no-till soil. The clover-maize growth sequence excels in reducing disease impact (1.7-fold) and pathogen spread in the host tissues (3-fold). Even though the wheat-maize crop cycle was less effective, it still had better results than the commercial mycorrhizal preparation treatment and the uncultivated non-infected soil. The results were slightly different in the field. The clover-maize rotation also achieved the best growth promotion and disease restraint results (2.6-fold increase in healthy plants), but the maize rotation with wheat showed only minor efficiency. Interestingly, pre-cultivating the soil with clover had better results in no-till soil in both experiments. In contrast, the same procedure with wheat had a better impact when tillage was applied. It may be concluded that crop rotation and soil cultivation can be essential in reducing LWD, but other factors may affect this approach’s benefits in commercial field growth.

A Case Study in Saudi Arabia: Biodiversity of Maize Seed-Borne Pathogenic Fungi in Relation to Biochemical, Physiological, and Molecular Characteristics

Microbiodiversity is usually correlated with environmental conditions. This investigation is a case study to cover the lack of knowledge on the correlation of biochemical, physiological, and molecular attributes with the distribution of seed-borne pathogenic fungi of maize under the environmental conditions of the Kingdom of Saudi Arabia to help forecast any destructive epidemics. Forty-one fungal species belonging to 24 genera were detected using standard moist blotter (SMB), deep freezing blotter (DFB), and agar plate (AP) techniques. SMB was superior in detecting the maximum numbers (36 species) of seed-borne mycoflora. The pathogenicity assay revealed that, among 18 seed-borne fungal pathogens used, 12 isolates caused high percentages of rotted seeds and seedling mortality symptoms, which were identified molecularly using an internal transcribed spacer sequence. Two Curvularia spp. and Sarocladium zeae were reported for the first time in KSA. The strains showed various enzymatic activities and amino acid profiles under different environmental setups. Temperature and humidity were the environmental variables influencing the fungal pathogenicity. The highest pathogenicity was correlated with the presence and concentration of threonine, alanine, glutamic, aspartic acids, and protein. The study concluded with the discovery of four new phytopathogens in KSA and, further, evidenced a marked correlation among the investigated variables. Nevertheless, more studies are encouraged to include additional physiological properties of the phytopathogens, such as toxigenic activity, as well as extend the fungal biodiversity study to other plants.

Control Strategies to Cope with Late Wilt of Maize

Control of maize late wilt disease (LWD) has been at the forefront of research efforts since the discovery of the disease in the 1960s. The disease has become a major economic restraint in highly affected areas such as Egypt and Israel, and is of constant concern in other counties. LWD causes dehydration and collapsing at a late stage of maize cultivation, starting from the male flowering phase. The disease causal agent, Magnaporthiopsis maydis, is a seed- and soil-borne phytoparasitic fungus, penetrating the roots at sprouting, colonizing the vascular system without external symptoms, and spreading upwards in the xylem, eventually blocking the water supply to the plant’s upperparts. Nowadays, the disease’s control relies mostly on identifying and developing resistant maize cultivars. Still, host resistance can be limited because M. maydis undergoes pathogenic variations, and virulent strains can eventually overcome the host immunity. This alarming status is driving researchers to continue to seek other control methods. The current review will summarize the various strategies tested over the years to minimize the disease damage. These options include agricultural (crop rotation, cover crop, no-till, flooding the land before sowing, and balanced soil fertility), physical (solar heating), allelochemical, biological, and chemical interventions. Some of these methods have shown promising success, while others have contributed to our understanding of the disease development and the environmental and host-related factors that have shaped its outcome. The most updated global knowledge about LWD control will be presented, and knowledge gaps and future aims will be discussed.