Tropical Plant Pathology

Samples of a wild grass showing white leaf symptoms were collected in 2015 in Veracruz, Mexico around sugarcane plantations. DNA amplification, sequencing and phylogenetic analysis revealed the presence of a ‘Candidatus Phytoplasma asteris’-related strain in the plants that were positive using 16S-based and cpn60 PCR assays. The strain determined to be associated with the grass white leaf disease was identified as a member of the 16SrI-B subgroup and a member of the cpn60 I-IIIB subgroup through in silico RFLP analysis. This is the first report of phytoplasma-infected grass plants associated with white leaf disease in Mexico. The implications of these findings are vital for the management of other plant hosts of the family Poaceae such as sugarcane and corn, and point to weedy grasses as a potential source of phytoplasma inoculum for nearby crop plants.

Bacterial fruit blotch (BFB), caused by Acidovorax citrulli, is the most important bacterial disease affecting melon plants in Brazil. The aim of the present study was to analyze the efficacy of yeasts for the biological control of BFB, applied by spraying to protect seedlings, foliage and fruits, and as a seed treatment. Additionally, the in vitro activity of the yeasts and plant growth promotion were evaluated. Among 60 strains, LMA1 (Rhodotorula aurantiaca), CC-2 (Pichia anomala) and LMS (Rhodotorula glutinis) were the most efficient for seedling protection. Among the three selected strains, only LMA1 and CC-2 maintained efficacy when tested on foliage and seeds, with reductions in the disease index and area under the disease progress curve of 58.6 and 47.2 %, respectively. Furthermore, LMA1 and LMS protected fruits. In vitro, neither of the strains inhibited bacterial growth, produced killer toxins, or showed competition for nutrients with the pathogen. None of the three yeasts promoted the growth of melon plants. Therefore, these yeasts have potential for BFB biocontrol in different disease stages and may be used in the integrated management of BFP.

The variability of Moniliophthora perniciosa, the causal agent of witches’ broom disease of cacao (Theobroma cacao), was assessed based on measures of (i) mycelial growth rates (MGR), (ii) area of starch degradation (ASD) and (iii) aggressiveness towards susceptible and resistant clones. Ninety-six isolates were obtained from vegetative and flower cushion brooms from five resistant clones and one susceptible clone at two experimental sites at CEPEC, BA, Brazil. Seven isolates varying in the production of basidiospores, MGR and ASD were inoculated into six cacao clones being one susceptible, (SIC17) and five resistant (CCN10, Sca 6, TSH 1188, TSH 565, and TSH 516) to the disease. Disease incidence (SYMP) and severity (TNB, total number of brooms and DI, disease index) were assessed. Isolates obtained from TSH565, TSH516 and SIC17 VB were the most aggressive towards the cacao genotypes. MGR was not associated with aggressiveness, but ASD was significantly associated, at moderate levels, with disease incidence (r = 0.44; p = 0.018), TNB (r = 0.44; p = 0.018), and DI (r = 0.47; p = 0.012). Our results suggest that ASD is a promising biochemical marker to select for aggressiveness within M. perniciosa based on DI and TNB. On the other hand, MGR is a poor predictor of aggressiveness.

Citrus is one of the most ancient fruit crops cultivated in the world. For many countries, citrus production is an important source of revenues. However, despite the richness of citrus production worldwide, fruit yields are constantly threatened by diseases that cause serious economic and social impacts to growers and consumers. One such example is citrus canker, a disease that affects all commercial citrus varieties and for which there is no cure. Currently, control measures for citrus canker are restricted to the application of copper-based compounds and the elimination of infected trees to prevent pathogen spreading in the field. However, new alternatives for the control of this disease are being developed, spanning through transgenic plants to innovative chemicals and biological control. New genome editing approaches and a repertoire of plant defense-related genes have been exploited to produce citrus lineages more tolerant to X. citri. In addition, a series of new molecules have been tested to not only inhibit X. citri growth but to also boost the plant immune system to suppress disease progression. In this review, we provide the state of art of all citrus canker control measures in use today, highlighting their utility and drawbacks and commenting on novel strategies to better control this important citrus disease.

Records of Golovinomyces species new to Thailand are described on the hosts Ageratum conyzoides, Bidens pilosa, Dahlia pinnata, D. × hortensis, Helianthus annuus, Lactuca indica, Laggera crispata, Sonchus oleraceus (Asteraceae), Lygisma inflexum (Asclepiadaceae), Myosotis scopioides (Boraginaceae), Coccinia indica, Coccinia grandis (Cucurbitaceae), Vigna umbellata (Fabaceae), Torenia fournieri (Linderniaceae), Plantago major (Plantaginaceae) and Verbena × hybrida (Verbenaceae). The identifications of the particular Golovinomyces species have been performed by means of morphological examinations supplemented by molecular sequence analyses. On the basis of molecular analyses, the powdery mildew on Ocimum tenuiflorum (Lamiaceae) proved to represent a species of its own, which is referred to as Golovinomyces ocimi comb. nov. The application of Oidium ocimi, the basionym of this combination, is determined by lecto- and epitypification. Lygisma inflexum, Laggera crispata and Vigna umbellata are new host records for Golovinomyces worldwide.

Bacterial spot caused by Xanthomonas hortorum pv. gardneri may produce significant losses in tomato crops. Chemical control of the disease is not efficient; therefore, alternative control measures should be assessed and low-cost products should be developed. The objective of this work was to evaluate preventive and curative applications of X. hortorum pv. gardneri suspensions inactivated using ultraviolet radiation, sterilization, and aging, to control tomato bacterial spot. The inactivated bacterial suspensions (IBSs) were obtained by exposure to ultraviolet radiation (IBS-UV), autoclave sterilization (IBS-A), aging in culture medium (IBS-CM), and aging in suspension on the laboratory bench (IBS-S). In the first assay, in the preventive application, the plants were sprayed with the IBS products, copper, acibenzolar-S-methyl (ASM), and water (control); 2 days after spraying, they were inoculated with the pathogenic bacterial suspension (108 and 109 CFU/mL). In the curative application, the plants were inoculated with the pathogenic bacterial suspension, and 2 days later were sprayed with the products. After adjustments, in the second assay the plants were only preventively sprayed with IBS-UV (108 CFU/mL) and IBS-A (108 and 109 CFU/mL), copper, ASM, and water, and 2 days later inoculated with the pathogenic bacterial suspension (107 and 108 CFU/mL). Disease severity was assessed and the area under the disease progress curve was calculated. IBS-UV, IBS-A, and ASM significantly reduced disease severity in the preventive application, in both assays. No significant difference was detected between these products in the curative application, in the first assay. IBS-UV and IBS-A effectively controlled tomato bacterial spot in the preventive application. IBS-A is a promising technique to control plant disease.

Fungicide resistance is an alarming challenge for the Brazilian tropical agricultural systems, with major implications for food safety, human and animal health, as well as for the environment. This review explores strategies to address fungicide resistance within the Brazilian agroecosystem context. We examined historical and current scenarios of fungicide resistance in the Brazilian agroecosystems and the approaches to delay the emergence and mitigate the selection of resistant variants. Our review indicates that the prevalence of resistance in field populations of key plant pathogens in Brazil was due to failures in the implementation of preventive measures. To address this issue, alternative evolutionary-smart strategies against fungicide resistance are proposed, emphasizing institutional actions and public policies. Crucial steps involve strengthening national networks for large-scale foliar and seed fungicide efficacy testing and resistance monitoring, as well as imposing tighter restrictions on the labeling of high-risk single-active formulations. Additionally, the integration of non-chemical disease management strategies and the establishment of a centralized database and information system on fungicide resistance in Brazil are identified as essential for effective resistance monitoring and informed decision-making. To enhance fungicide resistance management, the adoption of a warning system (e.g., based on aerobiology- or on weather-monitoring) for predicting disease epidemics and minimizing fungicide applications is recommended. Increased funding, collaboration, mandatory reporting, and capacity building are required to overcome these challenges. In addition, promoting integrated disease management approaches is vital. By implementing these tailored strategies, Brazil can actively contribute to safeguarding its food safety, protecting human and animal health, and preserving the delicate balance of its unique agroecosystem. The adoption of evolutionary-smart strategies against fungicide resistance will prolong fungicide efficacy, reduce economic costs, and minimize environmental impacts, ensuring sustainable and resilient agriculture in Brazil.

During the falls of 2015 and 2016, rust symptoms were observed on grapevine leaves in a backyard garden located in Tlaxiaco, Oaxaca, Mexico. Based on morphology, analysis of the variable D1/D2 region of the large subunit rDNA, and the fulfilment of Koch’s postulates on grapevine leaves, the causal agent was identified as Phakopsora meliosmae-myrianthae. This is the first report of Asian grapevine leaf rust disease caused by Phakopsora meliosmae-myrianthae in Mexico.

Rice (Oryza sativa) is one of the cereal crops that has great importance worldwide, especially in various regions of Asia. There are several viruses that are considered the most serious threat to rice productivity. Despite the worldwide identification of many viruses that cause dramatic reductions in rice yield, there might be many other viral agents that infect rice but have not been identified yet. In this study, we aimed to determine whether maize dwarf mosaic virus (MDMV) could infect rice plants under the field conditions of the Kurdistan region of Iraq. The leaves and seeds of rice plants that were likely to be infected with the virus were collected randomly in each survey at different locations in the rice fields in Akre, Bedole, Rovia, Sheladze, Deralok, and Zakho during different periods of July and September for each growing season of 2020 and 2021. The collected samples were tested for the incidence of MDMV using RT-PCR. The results showed that the collected samples from the fields located in Akre, Bedole, and Rovia were infected by MDMV. The amplified fragments were cloned, sequenced, and compared with those available at the GenBank. The seeds of the infected rice plants were MDMV-free. The incidence of MDMV in symptomatic rice plants was confirmed by RT-PCR, mechanical transmission, and aphid transmission. To the best of our knowledge, this is the first report that demonstrates that MDMV has the capability to infect rice plants naturally and, therefore, rice is considered a new natural host of MDMV.