Fusarium Wilt of Banana: Current Knowledge on Epidemiology and Research Needs Toward Sustainable Disease Management

Banana production is seriously threatened by Fusarium wilt (FW), a disease caused by the soil-borne fungus Fusarium oxysporum f. sp. cubense (Foc). In the mid-twentieth century FW, also known as “Panama disease”, wiped out the Gros Michel banana industry in Central America. The devastation caused by Foc race 1 was mitigated by a shift to resistant Cavendish cultivars, which are currently the source of 99% of banana exports. However, a new strain of Foc, the tropical race 4 (TR4), attacks Cavendish clones and a diverse range of other banana varieties. Foc TR4 has been restricted to East and parts of Southeast Asia for more than 20 years, but since 2010 the disease has spread westward into five additional countries in Southeast and South Asia (Vietnam, Laos, Myanmar, India, and Pakistan) and at the transcontinental level into the Middle East (Oman, Jordan, Lebanon, and Israel) and Africa (Mozambique). The spread of Foc TR4 is of great concern due to the limited knowledge about key aspects of disease epidemiology and the lack of effective management models, including resistant varieties and soil management approaches. In this review we summarize the current knowledge on the epidemiology of FW of banana, highlighting knowledge gaps in pathogen survival and dispersal, factors driving disease intensity, soil and plant microbiome and the dynamics of the disease. Comparisons with FW in other crops were also made to indicate possible differences and commonalities. Our current understanding of the role of main biotic and abiotic factors on disease intensity is reviewed, highlighting research needs and futures directions. Finally, a set of practices and their impact on disease intensity are discussed and proposed as an integrative management approach that could eventually be used by a range of users, including plant protection organizations, researchers, extension workers and growers

Weevil Borers Affect the Spatio-Temporal Dynamics of Banana Fusarium Wilt

Dispersal of propagules of a pathogen has remarkable effects on the development of epidemics. Previous studies suggested that insect pests play a role in the development of Fusarium wilt (FW) epidemics in banana fields. We provided complementary evidence for the involvement of two insect pests of banana, the weevil borer (Cosmopolites sordidus L., WB) and the false weevil borer (Metamasius hemipterus L., FWB), in the dispersal of Fusarium oxysporum f. sp. cubense (Foc) using a comparative epidemiology approach under field conditions. Two banana plots located in a field with historical records of FW epidemics were used; one was managed with Beauveria bassiana to reduce the population of weevils, and the other was left without B. bassiana applications. The number of WB and FWB was monitored biweekly and the FW incidence was quantified bimonthly during two years. The population of WB and the incidence (6.7%) of FW in the plot managed with B. bassiana were lower than in the plot left unmanaged (13%). The monomolecular model best fitted the FW disease progress data, and as expected, the average estimated disease progress rate was lower in the plot managed with the entomopathogenic fungus (r = 0.002) compared to the unmanaged plot (r = 0.006). Aggregation of FW was higher in the field with WB management. WB affected the spatial and temporal dynamics of FW epidemics under field conditions. Management of the insects may reduce yield loss due to FW

Caracterização de isolados de Phytophthora infestans do Distrito Federal e de Goiás

Foram caracterizados 123 isolados de Phytophthora infestans obtidos de 21 lavouras de tomateiro e oito de batateira, em municípios do Estado de Goiás e Cidades Satélites de Brasília, no período de abril de 2001 a setembro de 2003. Os isolados foram caracterizados para os marcadores grupo de compatibilidade (123 isolados); isoenzima glucose 6-fosfato-isomerase (Gpi) (34 isolados) e resistência aos fungicidas mefenoxam (77 isolados) e metalaxyl (32 isolados de batateira), usando o método de disco de folhas. Todos os 78 isolados de tomateiro foram classificados no grupo de compatibilidade A1, enquanto os 45 de batateira foram do grupo A2. Os fenótipos para Gpi dos isolados de tomateiro (19) e de batateira (15) foram 86/100, típico da linhagem clonal US-1, e 100/100, típico da linhagen clonal BR-1, respectivamente. Quanto à resistência a mefenoxam, constataram-se isolados de tomateiro resistentes (36%), intermediários (48%) e sensíveis (16%). A maioria dos isolados de batateira foi classificada como sensível (82%) e apenas 9% de intermediários e resistentes. Dos isolados de batateira avaliados para resistência ao metalaxyl, 25% foram resistentes, 62% intermediários e 13% sensíveis. A população de P. infestans no Distrito Federal e no Estado de Goiás é constituída de duas linhagens clonais, com especificidade por hospedeiro.From April 2001 to September 2003, 123 isolates of Phytophthora infestans were collected in 21 tomato and eight potato fields of Distrito Federal (DF) and Goiás State (GO), Brazil. Isolates were characterized for mating type (123 isolates), glucose 6-phosphate-isomerase alloenzyme (Gpi) (34 isolates), and for resistance to mefenoxam (77 isolates) and metalaxyl (32 potato isolates), using the leaf disc method. All the tomato isolates (78) were of the A1 mating type and all the potato isolates (45) were of the A2 mating type. Nineteen tomato and 15 potato isolates were tested for Gpi and were classified as 86/100 phenotype, typical of the US-1 clonal lineage, and 100/100 phenotype, typical of the BR-1 clonal lineage, respectively. Tomato isolates were 36% resistant to mefenoxam, 48% intermediate, and 16% sensitive. The majority of the potato isolates (82%) were sensitive, 9% were intermediate, and 9% were resistant. Potato isolates tested for metalaxyl resistance were 25% resistant, 62% intermediate, and 13% sensitive. The population of P. infestans in the DF and GO is clonal, with two host-specific clonal lineages

Cultural and Aggressiveness Variability of Cercospora coffeicola

Although brown eye spot of coffee, caused by Cercospora coffeicola, is important for coffee production in Brazil, there is a general lack of knowledge regarding the disease. In this study, we evaluated the variability of both the cultural and aggressiveness traits of 60 isolates from coffee plants grown under conventional and organic systems in three regions of Minas Gerais State, Brazil. Variability among the isolates was detected with regard to all of the traits and was unrelated to an effect of either the region or cropping system. Mycelial growth, cercosporin production and sporulation were assessed in the laboratory. Of the 60 isolates, 27 did not sporulate at 25°C; the mycelial growth of all of the isolates and cercosporin production by 18 of the isolates linearly increased as the temperature rose from 18 to 26°C. We inoculated six selected isolates on plants of two coffee cultivars (ÔCatuaı ́ Vermelho IAC44Õ and ÔCatucaı ́ Vermelho 785-15Õ) and evaluated the incubation period (IP), latent period (LP) and disease severity. All three of these traits were affected by temperature postinoculation and KCl amendment. The significant correlations were as follows: IP and LP in both cultivars; severity and leaf fall in both cultivars; and cercosporin production in vitro and severity values in ÔCatucaı ́ Vermelho 785-15Õ. In conclusion, we found that (i) C. coffeicola is highly variable for both cultural and aggressiveness traits; (ii) laboratory and glasshouse experiments were suitable to assess the pathogen variability; (iii) research protocols should account for the effect of environmental factors, such as temperature and KCl, on the traits evaluated; and (iv) these protocols should include the assessment of the IP instead of the LP, as both are correlated, and the IP is easier to evaluate

Métodos de isolamento de bactérias do filoplano de tomateiro visando populações específicas e implicações como agentes de biocontrole

Em um processo de seleção de bactérias do filoplano de tomateiro (Lycopersicon esculentum) com potencial para o controle de doenças da parte aérea da cultura, diferentes métodos de isolamento foram utilizados visando obter isolados da população total, da população da superfície foliar e isolados que habitam locais protegidos do filoplano e/ou que toleram fatores de estresse. Foi testada a capacidade de 300 isolados em controlar in vivo, as doenças causadas por Alternaria solani, Pseudomonas syringae pv. tomato e Phytophthora infestans. Os testes foram repetidos para cada um dos antagonistas selecionados, estudando-se, também, a capacidade de controlar a mancha-bacteriana, causada por Xanthomonas vesicatoria. Os resultados demonstraram haver predomínio de antagonistas provenientes de folíolos do terço superior da planta de população total ou da superfície. Entretanto, o único antagonista selecionado, isolado de folíolos do terço inferior, foi obtido de locais protegidos do filoplano e/ou capaz de tolerar fatores de estresse.In a process of selecting tomato (Lycopersicon esculentum) phylloplane bacteria with potential for controlling aerial diseases of this crop, different isolation methods were used to obtain the isolates from total leaf surface and also from protected sites of the phylloplane. Three hundred isolates were tested in vivo for their ability to control diseases caused by Alternaria solani, Pseudomonas syringae pv. tomato and Phytophthora infestans. The results demonstrated a predominance of antagonists in the total population originating on the surface of the leaflets harvested from the upper leaves of the plants. Only one selected antagonistic isolate obtained from the lower leaves was recovered from the protected sites of the phylloplane and/or was stress tolerant

Sphaceloma poinsettiae as a potential biological control agent for wild poinsettia (Euphorbia heterophylla)

Sphaceloma poinsettiae, a scab-causing fungus that attacks wild poinsettia (Euphorbia heterophylla) was evaluated as a mycoherbicide. Plants representing nine different E. heterophylla accessions were tested and variable resistance levels to 10 different isolates of S. poinsettiae were observed. One isolate was selected based on its high aggressiveness; it caused scab that led to stem-girdling in the majority of plants belonging to most wild poinsettia accessions. Plants inoculated with conidial suspension developed low disease severity; suspensions of mycelial fragments caused higher disease severity. Viability of mycelium declined rapidly either in storage under room temperature or at 4 °C. After 25 days, 60% of mycelial fragments kept in distilled water at 4 °C or in 35% sterile sucrose solution germinated. Cultures of S. poinsettiae were capable of growing at temperatures ranging from 10 to 30 °C (maximum growth at 25 °C) whereas the mycelial fragments germinated at 20–30 °C. The highest mycelial yield occurred in potato-dextrose broth after culturing for 12 days at 168 rpm at room temperature. A host-range test involving 37 plant species in addition to wild poinsettia indicated that S. poinsettiae is highly specific to E. heterophylla. The viability of S. poinsettiae as a mycoherbicide depends on the development of suitable mass production, formulation, and inoculum preservation methods. On several occasions, this fungus has been observed to cause devastating epidemics in the field on this host; therefore, it should be considered a potential biocontrol agent of wild poinsettia

Biological control of coffee rust by antagonistic bacteria under field conditions in Brazil

Rust (Hemileia vastatrix) is the most important coffee disease in Brazil. Organic coffee production has increased in the country and a research program aimed to develop alternatives to chemicals for disease control was required. Seven bacterial isolates, isolated from organic coffee plantings and selected in greenhouse tests, were evaluated under commercial organic crop conditions in 2005 (Experiment 1) and 2005/2006 (Experiment 2), in Machado, MG, Brazil. Ten treatments consisting of the seven bacterial isolates, copper hydroxide, calcium silicate and water were applied as three or four monthly sprays in Experiment 1 or 2, respectively. Rust severity and incidence were evaluated monthly. In Experiment 1, the sprays started in January when rust incidence was 23.8%, and none of the treatments reduced rust progress significantly. In Experiment 2, the sprays began in November 2005, when rust incidence was approximately 7.5%. There were significant differences (P < 0.0001) between treatments regarding maximum incidence and severity (as assessed in June, 2006), the rate of increase of the incidence between November 2005 and June 2006 and for the areas under disease progress curves for both rust incidence and severity. Lower values for these treatments were obtained in the plots treated with copper hydroxide or Bacillus sp. isolate B157, and intermediate values with the Pseudomonas sp. isolate P286. In a third experiment conducted in 2007 in Ervália, MG, isolates B157 and P286 were also evaluated; isolate B157 reduced rust intensity as effectively as copper hydroxide. Isolate B157 is considered a potential biocontrol agent for coffee rust for organic crop systems in Brazil

Biological control of eucalyptus bacterial wilt with rhizobacteria

The antagonistic potential of 298 rhizobacteria obtained from the rhizosphere and rhizoplane of tomato and eucalyptus plants was assessed for the control of bacterial wilt of eucalyptus caused by Ralstonia solanacearum. Several tests were performed using tomato plants as a screening system to select efficient rhizobacteria. Different methods for antagonist delivery and pathogen inoculation were evaluated: (1) seeds were microbiolized (soaked for 12 h in a suspension of the antagonist propagules) and germinated seedlings had their roots immersed in the pathogen inoculum suspension; (2) seedlings originated from microbiolized seeds were transplanted to soil infested with R. solanacearum and (3) roots of seedlings were immersed in a suspension of propagules of the antagonist and subsequently in a suspension of R. solanacearum. Nine isolates (UFV-11, 32, 40, 56, 62, 101, 170, 229, and 270) were selected as potential antagonists to R. solanacearum as they suppressed bacterial wilt in at least one of the methods assessed. The selected antagonists were evaluated against two isolates of R. solanacearum using in vitro and in vivo (inoculated eucalyptus) tests. Isolates UFV-56 (Bacillus thuringiensis), UFV-62 (Bacillus cereus) and a commercial formulation of several rhizobacteria (Rizolyptus®) suppressed bacterial wilt in eucalyptus protecting the plants during the early stages of development