Abstract Anthracnose, caused by several Colletotrichum species, is a major restricting factor for guava production, but no comprehensive study of the causal agent has been conducted from the plant ...host centre of diversity. This work characterizes isolates from guava, mainly from the fruit, but also leaves and flowers, representing most Brazilian physiographies according to the partial sequences of the rDNA‐ITS, ApMAT , TUB2 , HIS3 and GAPDH gene regions. In addition, the pathogenicity and aggressiveness to fruits of two widely planted guava varieties (SLG and RM) are described. Guava‐derived Colletotrichum isolates were found in five complexes: gloeosporioides, acutatum, boninense, gigasporum and orchidearum. The gloeosporioides complex was the most prevalent (81%), followed by the acutatum complex (14%). A total of 16 Colletotrichum species were naturally associated with guava anthracnose: C . aeschynomenes , C . asianum , C . chrysophilum , C . fructicola , C . gigasporum , C . gloeosporioides , C . karsti , C . melonis , C . musae , C . nymphaeae , C . paranaense , C . siamense , C . sojae , C . syzygicola , C . theobromicola and C . tropicale . Apart from C . nymphaeae and C . gloeosporioides , all the remaining 14 taxa are reported for the first time in P . guajava . The most aggressive species belonged to the C . gloeosporioides complex. C . siamense was the most prevalent, especially in warmer regions, followed by C . chrysophilum , mostly in temperate environments. The most aggressive species were C . siamense , C . chrysophilum , C . fructicola and C . tropicale . Fruits of the variety SLG were consistently more resistant to anthracnose than the fruits of RM.
Rice blast management is heavily reliant on chemical control, and the rise of fungicide resistance is contingent on fitness costs. This study contrasts the responses of 322 blast isolates from ...southern (n = 224) and northern Brazil (n = 98) to fungicides in the Quinone outside inhibitor (QoI), Sterol demethylation inhibitor (DMI), and Melanin biosynthesis inhibitor (MBI) groups, and associated epidemiological traits. Resistance was more prevalent in northern Brazil, where conditions are most conducive to blast. Northern populations displayed higher EC50 for spore germination, faster mycelial growth rates, and higher rates of appressorium formation for fungicides in all chemical groups. Results were most striking for the QoI fungicide azoxystrobin. We examined possible fitness penalties and related in planta competitive abilities associated with the gain of azoxystrobin resistance. QoI-resistant (QoI-R) genotypes, carrying the G143A cytochrome b gene mutation, were found in 90% of the northern isolates. The EC50 values of these QoI-R isolates exceeded 10 μg L−1. However, QoI-R isolates had a reduced number of conidia, longer latent period, reduced infection efficiency and aggressiveness, compared to the sensitive isolates. These impairments were reflected in a decrease of competitive ability in planta in the absence of azoxystrobin. Nevertheless, under continuous fungicide applications, the proportion of resistant and sensitive isolates in mixed cultures in planta remained constant after four generations. The results are valuable for managing strategies as the increase of resistance may hamper the competitive abilities of QoI-R mutants in the absence of the respective active ingredient.
•Fungicide resistance is widespread in rice-derived isolates of P. oryzae from Brazil.•Fungicide resistance and the G143A mutation are more prevalent in northern Brazil.•Fitness penalties are associated with resistance to the QoI azoxystrobin.•Fitness costs of QoI-R isolates resulted in loss of competitiveness in planta.
The long-term dynamics of fungicide resistance of the rice blast fungus
was monitored by examining the reaction of the fungal field isolates, collected over a period of 26 years, to the active ...ingredients of commercially relevant fungicides. The in vitro sensitivity of all isolates was measured against quinone outside inhibitors (QoI), melanin biosynthesis inhibitors, and sterol demethylation inhibitor (DMI) fungicides, namely azoxystrobin (as a QoI), tricyclazole (as a melanin biosynthesis inhibitor), tebuconazole (as a DMI), and trifloxystrobin + tebuconazole (QoI + DMI). Over the 26-year collection period, a gradual rise in the EC
estimates for mycelial growth sensitivity was observed for all fungicides, but most strikingly for azoxystrobin. A rise in conidial germination and appressorium formation was also noted, most markedly for azoxystrobin. Consistently, the earlier isolates were much more sensitive to the active ingredients than the more contemporary isolates. The sequencing of the amplified
fragment distinguished two haplotypes, H1 and H2. Haplotype H1 (six isolates) contained the G to C transversion at codon 143 (resulting in change G143A), linked to the resistant phenotype QoI-R. Haplotype H2 (40 isolates), gathered the isolates sensitive to QoI. This work documents the gradual rise in the frequency of fungicide-resistant isolates in
rice populations on a long-term basis.
Field trials conducted on a yellow-red latossol (pH 6.0), replicated in 2010 and 2011, sought to examine the effect of silicon, phosphite minerals, synthetic fungicides and genetic resistance for ...wheat blast management (
Magnaporthe grisea
) in Central Brazil. Disease intensity was measured on cvs. BRS 264 and BR18 subjected to the following Si treatments: pre-plant furrow application of Ca & Mg silicate (300 kg ha
-1
); post-plant scattered application of Ca & Mg silicate on top of the soil (1 ton ha
-1
); multiple foliar SiO
2
applications (30 g
l
-
1
); and non-treated control. Blast incidence and severity were scored. Further experiments were conducted on cv. BR-264, for examination of the effect of potassium phosphite and synthetic fungicides on wheat blast intensity, with the following treatments: K
2
HPO
3
(1ml
l
-
1
); epoxinazole + pyraclostrobin (700 ml ha
-1
); tebuconazole (600 ml ha
-1
); tebuconazole + trifloxystrobin (750 ml ha
-1
); and non-treated control. In 2010, disease intensity was lower than in 2011. In the silicate experiments, disease was significantly lower when plants were treated with foliar or furrow silicate. Si applications significantly reduced disease in BRS-264. While BR-18 consistently demonstrated lower disease levels, cv. BRS-264 generally responded more markedly to silicon applications. In the phosphite/fungicide experiment of 2010, all treatments reduced disease when compared with the control, and in 2011 phosphite efficiency was not significantly different from some fungicide treatments. Synthetic fungicides demonstrated an average blast control of 55% by severity values. Yields were increased in the phosphite-treated plots (by 9–80%), in the Si treatments (by 26–92%), and more so, and more consistently, with synthetic fungicides (by 90–121%). Combined results of all field studies, carried out under environmental conditions highly conducive to disease, indicated that control of wheat blast necessitates the joint integration of several alternatives for efficient disease management.
Brazilian soybean is mainly grown in the Cerrado, the South American Savannah. We report epidemiological studies of soybean target spot (STS), caused by
Corynespora cassiicola
in two Cerrado regions ...in the Federal District (DF) and the state of Bahia (BA), relating the environmental parameters with disease intensities. Here we: i) characterize the field response of soybean cultivars to STS and compute their respective latent periods; ii) relate STS severity to yield; and iii) verify the cross-reaction of
C. cassiicola
isolates of soybean and cotton to each of these hosts. Ten commercial soybean cultivars were cultivated in DF and BA fields. Additionally, the cv. reactions were studied in greenhouse assays in the spring/summer and autumn/winter seasons. Precipitation and relative humidity were higher in DF than in BA, whereas the temperature was cooler with fewer extremely hot days. Greater STS intensity was observed in DF, reaching 48% incidence and 19% severity. Fungicide applications reduced disease only in the DF trial. Cultivars were separated into four groups: highly susceptible, susceptible, moderately resistant, and resistant. Higher disease severities were recorded in the spring/summer than in the autumn/winter. Length of latent periods correlated with the cv. response to STS and climate conditions. Environmental differences were the probable causes for the lower disease levels recorded in the BA fields and in the autumn/winter trial. A negative relationship was established between severity and yield. In the cross-inoculation studies, isolates of soybean and cotton were able to infect both hosts, but aggressiveness correlated with the host of origin.
Brazil and Argentina have a combined soybean area of 53.6 million hectares, which accounts for over half of the total global production. The soybean crop in South America extends from latitude 8-10° ...S to 32-36° S. Such a vast, almost contiguous area imposes a serious sanitary risk to the crop. Currently, the prevalence of anthracnose is increasing, with recurring reports of severe epidemics and expressive yield losses. Soybean anthracnose is mainly associated with
, although other
species have also been reported as causal agents of this disease. Knowledge about the morphological, cultural, and molecular variability of
in South America is crucial for disease management. Here, we present data on the molecular, morphological, biological, cultural, and pathogenicity of
isolates collected in Brazil and Argentina. Light microscopy and randomly-amplified polymorphic DNA (RAPD) analysis were used for estimating the variability of isolates.
displayed three types of conidiogenesis, viz. conidial formation from conidiogenous cells on hyphal extremities, in conidiomas in acervuli, and directly from fertile setae (a mechanism yet-unreported for
). RAPD profiling was effective in revealing the genetic diversity among
isolates. The intra-group similarity was greater among the Argentinian isolates when compared to the Brazilian group. Furthermore, the results indicated a strong correlation between geographical origin and molecular grouping, with the exclusive or semi-exclusive assembling of Brazilian and Argentinian isolates in distinct clades. Finally, a preliminary account of the reaction of soybean accessions to
is also included.
Disease management options for organic tomato growers are limited. However, the dry winter in the Brazilian Mid-West is unfavourable to most foliar diseases, and is considered ideal for organic ...agriculture. We report on powdery mildew (PM) disease progress in organic tomato, under different irrigation configurations. The progress of late blight is also addressed. Staked, fresh-market tomatoes were grown in 2009 and 2010, under drip (DRP); furrow (FUR); microsprinkler (MIC); and overhead (OVH) systems. DRP, MIC and OVH were managed to maintain either high or moderate levels of soil moisture. FUR was managed to maintain moderate soil moisture only. In 2011, three treatments were added to study the extent of PM control with microsprinklers: MIC above plant canopy (MICAP); a DRP-MIC combination, above plant canopy, alternately used (DRP-MICA); and a DRP-MIC combination, where microsprinkler was applied as a PM control measure only (DRP-MICC). Irrigation systems that maximized leaf wetting provided the best PM control, reduced maximum disease severity (Ymax); severity at halfway point of the epidemic time course (Y50); area under disease progress curve (AUDPC); and disease progress rate (r). Overall, the intensity of late blight was much lower than PM, and no blight was detected in 2010. The DRP and FUR systems consistently hampered foliar diseases development in 2009/2011. PM is significantly reduced by irrigation management while late blight is a lesser disease in the Brazilian Mid-West winter season. Therefore, irrigation is a tool to control diseases in organic tomato, and likely, other cropping systems.
•Powdery mildew much more severe than late blight in organic tomato in the dry season.•Irrigation systems that maximize leaf wetting optimize tomato powdery mildew control.•Irrigation configuration is an efficient tool to control diseases in organic tomato.
The use of resistant cultivars is one of the most efficient strategies for reducing the amount of pesticides in agricultural crops. Several species of the Solanaceae and Cucurbitaceae families are ...susceptible to diseases caused by
Phytophthora capsici
isolates. Even though sources of resistance have been identified in other host plants, reports of resistance to
P. capsici
in cultivated and wild tomato species genus
Solanum
(section
Lycopersicon
) germplasm are yet scarce. The main objective of the present work was to characterize the reaction of 244
Solanum
(
Lycopersicon
) accessions to
P. capsici
under controlled greenhouse experiments. In the first set of experiments, tomato seedlings were inoculated at the collar area with a 3 mL of a spore suspension (2
×
10
4
zoospores mL
−1
). Disease incidence (dead plants/total plants) was assessed 14 days after inoculation. The accessions were separated in five well-defined reaction groups: highly resistant (HR), resistant (R), moderately susceptible (MR), susceptible (S), and highly susceptible (HS). The reaction of a group of the ten accessions with the highest levels of resistance was then evaluated against a collection of five
P. capsici
isolates. Differential reaction was observed among accessions and isolates. The resistant response was host species-dependent with susceptibility being more often found in
S. peruvianum
accessions, whereas sources of resistance were identified more frequently among accessions of the cultivated tomato (
S. lycopersicum
).
Two standard area diagrams (SADs) to assess the severity of powdery mildew (Leveillula taurica) on tomato leaves and leaflets were developed and evaluated. The SADs are composed of two sets of images ...of leaves and leaflets with six distinct percentages of diseased areas (1, 5, 10, 20, 40 and 60%). Diagrams were evaluated by 16 raters divided into two groups, based on their former experience in assessing plant disease intensities, using the two sets of images, first without SADs and then using the proposed SADs. Lin's concordance correlation analysis of estimated vs. actual disease severities showed that precision and accuracy were clearly improved when raters were aided by the proposed diagrams, for both leaflets and leaves. Based on coefficients of determinations (R2) and on the intra-class correlations (ρ), the estimates of severity were consistently more reliable when using SADs. Furthermore, when the scales were adopted, the performance of the inexperienced group improved to the same level of that of the experienced group. The diagrams improved subjects' ability to accurately, precisely and reliably estimate tomato powdery mildew severity, and as such can be used to assess severity for studies in epidemiology, quantitative host resistance and in the evaluation of management practices in this important pathosystem.
•More accurate field assessments of tomato powdery mildew are now possible.•Diagrams for tomato powdery mildew on leaves and leaflets are made available.•Scientifically-tested diagrams for disease assessment improve raters' performance.
Despite the wide adoption of mefenoxam (Ridomil Gold EC) for vegetables in North Carolina, the incidence of Phytophthora blight on pepper (Capsicum annuum) and squash (Cucurbita pepo) is high. ...Seventy-five isolates of Phytophthora capsici were collected in five pepper and one squash field in order to assess mefenoxam sensitivity. The relative fitness of resistant and sensitive isolates was contrasted in vitro by their respective rates of colony growth and their ability to produce sporangia in unamended V8 juice agar medium. In in vivo experiments, the aggressiveness of isolates on pepper was evaluated. The frequency of resistant isolates in North Carolina populations was 63%, considerably higher than resistance levels in areas where mefenoxam is not widely adopted. Resistant isolates grew on amended media at rates >80 to 90% and >100% of the nonamended control at 100 μg ml-1 and 5 μg ml-1, respectively. Sensitive isolates did not growth at 5 or 100 μg ml-1. All isolates from three fields, including two pepper and a squash field, were resistant to mefenoxam. Populations from other fields were composed of either mixes of sensitive and resistant isolates or only sensitive isolates. Response to mefenoxam remained stable during the course of in vitro and in planta experiments. Occurrence of a mefenoxam-resistant population of P. capsici on squash is reported here for the first time in North Carolina. When measured by rate of colony growth, sporulation in vitro, or aggressiveness in planta, fitness of resistant isolates was not reduced. Mefenoxam-resistant isolates from squash were as aggressive on pepper as sensitive or resistant pepper isolates. These results suggest that mefenoxam-resistant populations of P. capsici are as virulent and fit as sensitive populations.
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