Comparison of screening methods to evaluate the response of St. Augustine grass to Magnaporthe oryzae

The objective of this study was to develop a reliable and high throughput screening method to evaluate the response of St. Augustine grass (Stenotaphrum secundatum) genotypes to the grey leaf spot (GLS) caused by Magnaporthe oryzae infection. Whole plant, detached stolon and detached leaf assays under growth chamber conditions were compared to field conditions on eight commercial and nine advanced breeding lines of St. Augustine grass. Disease was assessed using two variables, lesion size (LS) and overall plant disease severity (SEV). LS and SEV were highly correlated for field and growth chamber screening methods using the whole plant assay (LS r² = 0·79; SEV r² = 0·83; P ≤ 0·001), the detached stolon assay (LS r² = 0·75; SEV r² = 0·72; P ≤ 0·001), and the detached leaf assay (LS r² = 0·46; SEV r² = 0·60; P ≤ 0·001). Genotypic variation for resistance in 17 St. Augustine grass genotypes was identified using all screening methods for LS (P < 0·05) and SEV (P < 0·05). The rank‐sum method was used to classify St. Augustine grass genotypes into highly resistant (HR), resistant (R), moderately resistant (MR), moderately susceptible (MS), susceptible (S) and highly susceptible (HS) classes based on the rank‐sum values of LS and SEV. Two introduced African polyploids used as parents, and two F₁ interploid progeny obtained using an in vitro embryo rescue technique, were classified as highly resistant (HR), or resistant (R), across all screening methods.     

Pathogenic and molecular comparison of Puccinia kuehnii isolates and reactions of sugarcane varieties to orange rust

Sugarcane orange rust, a disease caused by Puccinia kuehnii, was first reported in Brazil in 2009. There are no studies comparing the Brazilian P. kuehnii collections and the reaction of important sugarcane varieties under controlled conditions. This work compared the reaction of seven sugarcane varieties inoculated with six different P. kuehnii isolates from Brazilian sugarcane areas and verified the pathogenic and genetic variability of these isolates. The incubation (I) and latency (L) disease periods, disease severity (SEV), total number of lesions (TNL), total number of sporulating lesions (TNSL), and percentage of sporulating lesions (%SL) were evaluated. Furthermore, ITS1 and IGS ribosomal sequences of all P. kuehnii isolates used in this study were compared with pathogen sequences from 13 different countries. The disease incubation ranged from 7 to 10 days and the latency ranged from 10 to 21 days. SEV and TNL showed large variations and few significant differences between the reaction of the varieties to P. kuehnii, in contrast with the variables TNSL and %SL. The P. kuehnii isolates did not compose different virulent races, but the isolate from one site (Araras) was a more aggressive race. The ITS1 and IGS ribosomal sequences of six P. kuehnii isolates were identical with each other and to most P. kuehnii American sequences deposited at GenBank. The studied sequences of P. kuehnii isolates differed from the sequences from Asia, Tahiti and Oceania.     

Spatiotemporal analyses of rhizopus rot progress in peach fruit inoculated with Rhizopus stolonifer

Rhizopus rot, caused by Rhizopus stolonifer, is one of the main postharvest diseases in stone fruits, but there is little known about the processes of disease development during transport and postharvest storage. The objective of this study was to characterize temporal progress and spatial distribution of the disease in peach fruit. Rhizopus rot development was evaluated using two different fruit arrangements. Only one fruit of each arrangement was inoculated with a R. stolonifer spore suspension. Disease incidence and severity were assessed daily for all the fruit. Nonlinear models were fitted to the quantity of fruit and to the area of fruit that became infected over time and distance in relation to the source of inoculum. Disease‐free fruit placed next to the artificially inoculated peaches showed disease symptoms due to pathogen dissemination by mycelial stolons. The disease incidence and severity progress rates varied from 0.33 to 0.53 day^?1 and from 0.30 to 0.49 day^?1, respectively. The spatial spread of the disease followed a dispersive wave pattern with increasing speed over time, but decreasing speed with disease severity. For disease severity y = 0.5, the velocity at day 3 varied from 0.14 to 0.32 fruit diameter day^?1, while it ranged from 0.38 to 1.46 fruit diameter day^?1 at day 12.     

Distinguishing sporulating and nonsporulating lesions as a method for evaluating the resistance of sugarcane genotypes to orange rust

Sugarcane breeding programmes rank the resistance of genotypes to Puccinia kuehnii, causal agent of orange rust, according to levels of disease severity. However, during the screening stages, this method of assessment can lead to precipitous elimination of genotypes with promising agronomic traits but showing mild symptoms of rust such as flecks or lesions that do not produce spores. This study aimed to propose a new method to classify the resistance of sugarcane genotypes to orange rust by counting sporulating lesions. Five sugarcane varieties with different levels of resistance to P. kuehnii were inoculated with two pathogen populations under controlled conditions. The disease severity (SEV), total number of lesions (TNL), and total number of sporulating lesions (TNSL) were evaluated in a 20 cm leaf fragment from the most diseased leaf. The TNL and TNSL evaluations were performed at 11, 16 and 21 days after inoculation (DAI) and SEV at 21 DAI. The thresholds of 80% and 8% of sporulating lesions (SL) separated susceptible from the intermediate varieties and intermediate from the resistant ones, respectively. It is proposed that the method of counting sporulating lesions be used in screening genotypes for resistance to P. kuehnii in sugarcane breeding programmes.     

Soybean cultivar performance in the presence of soybean Asian rust,in relation to chemical control programs

Soybean rust is caused by an obligate parasite (Phakopsora pachyrhizi) which has spread in Brazil in each new season since 2001 and, despite the efforts to control the disease, losses have occurred every year. Its control demands several tactics amongst which chemical control with fungicides is the main method and remains indispensable. Control strategies such as the use of cultivars with partial resistance are desirable, but are not yet commercially available. The present study analyzed the existing differences in the reactions of short, medium and long cycle soybean cultivars against Asian rust and their responses to fungicide sprays. The experiment was conducted at Uberlandia-MG, Brazil, under field conditions from December 2007 to May 2008, in the Syngenta Seeds Experimental Station. The high pressure of the disease in the experiment simulated the natural pressure that the disease often reaches in Brazil. The studied variables were: visual severity (percentage of infected leaf area), percentage defoliation and productivity (kg ha⁻¹). Disease severity was expressed as AUDPC (area under disease progress curve). Variance analysis and comparison of means by the Tukey test (5% significance) were done for all variables studied. Significant differences were observed between cultivar effects and chemical control programs. The results obtained here indicate that the cultivars M-Soy 8199RR and Emgopa 315RR were less susceptible to disease, and that a control program termed “monitoring” (in which the appearance of new pustules of the pathogen were monitored to make the decision at each fungicide spray) was the most effective.     

Overwintering and epidemiology of Puccinia dracunculina,the causal agent of rust in open tarragon fields

Rust, caused by Puccinia dracunculina, is the main foliar disease of open‐field tarragon (Artemisia dracunculus) crops in Israel. As not much is known about the biology or epidemiology of this pathogen, the long‐term objective of the current study was to accumulate the knowledge needed to develop an effective, environmentally friendly means of adequately managing the disease. Puccinia dracunculina is an autoecious brachy‐form pathogen, but it is not known whether the life cycle is completed under field conditions in Israel. Field observations and greenhouse studies revealed that although the telial stage is produced, the pathogen overwinters in the uredinial stage. In vitro experiments were used to quantify the temperature and wetness requirements for urediniospore germination and to calculate the daily duration of conducive weather (DDCW); DDCW was defined as the number of hours during which temperature ranged between 15 and 25°C and RH was >90%. Cumulative DDCW values (CDDCW) were a good predictor of disease under natural conditions in two growing seasons. Disease onset occurred when CDDCW values reached a level of 10 and the relationship between log CDDCW values and season‐long severity values (in logit) was highly significant, explaining 90·6% of the variation.     

Analysis of Colletotrichum musae populations from Brazil reveals the presence of isolates with high competitive ability and reduced sensitivity to postharvest fungicides

In this study, the sensitivity of 218 isolates of Colletotrichum musae to imazalil and thiabendazole was evaluated, as well the fitness and competitive ability of less sensitive isolates. There was a positive correlation between the sensitivity to the two fungicides, but the isolates were more sensitive to imazalil. The estimated effective concentration of the fungicide able to inhibit mycelial growth by 50% (EC₅₀) was used to select four isolates with the lowest and the highest values for both fungicides, which were considered as sensitive (S) and less sensitive (LS), respectively. The level of sensitivity was maintained after 10 successive transfers on fungicide-free medium. Both fungicides were effective in controlling the disease caused by S isolates of C. musae in detached banana fruit when recommended doses were used. However, only imazalil was able to control the disease caused by LS isolates. For both fungicides, analysis of fitness-related variables (mycelial growth, sporulation, germination, and virulence) showed no difference between the groups of S and LS isolates, but a large variation was observed within the group. The LS isolates to thiabendazole that showed a mutation (F200Y) in the β-tubulin gene did not have fitness penalties. Our results allow a better understanding of the sensitivity and fitness of isolates of C. musae from Brazil, and demonstrate the importance of periodic monitoring to determine the frequency of LS isolates in populations, aiming at more effective management of anthracnose in banana orchards in Brazil.     

Silicon suppresses tan spot development on wheat infected by <Emphasis Type="BoldItalic">Pyrenophora tritici-repentis</Emphasis>

Tan spot caused by Pyrenophora tritici-repentis is the main foliar diseases of wheat in Brazil. The effect of silicon (Si) on the components of resistance of a susceptible (Fundacep Horizonte) and a moderately resistant (Quartzo) wheat cultivar was studied in a controlled environment. Silicon was supplied as calcium silicate in the soil 30 days before sowing. At the booting stage, a conidial suspension of the fungus was sprayed onto the flag leaves of potted plants, which were incubated under moist conditions for 48 h. Afterwards, inoculated leaves were assessed for: incubation period (IP), infection efficiency (IE), area under lesion size curve (AULSC), lesion size (LS), severity (SEV) and area under severity curve (AUSC). Foliar Si concentrations were quantified at the end of the evaluations. Si supply to plants increased leaf Si concentration in 233% for Fundacep Horizonte (from 4.8 to 16.0 g kg^?1 of dry matter) and 211% for Quartzo (from 5.3 to 16.5 g kg^?1 of dry matter). In the Si + treatments, IP was longer by 24 and 17 h, IE declined by 53.5 and 65.5%, LS (at 264 h after inoculation) by 4.6 mm (from 9.5 to 4.9 mm) and 5.9 mm (from 8.2 to 2.3 mm), and SEV by 53% (from 54.4 to 18.8%) and 88% (from 47.7 to 5.5%) respectively, for the Fundacep Horizonte and Quartzo cultivars. The Si x cultivar interaction was not significant for AULSC and AUSC, and these variables were reduced by 55.8 and 80.8%, respectively, in plants supplied with Si. In conclusion, Si enhanced the resistance of wheat plants to tan spot development by affecting several resistance components, regardless of the resistance level of the cultivar. However, the greatest reduction in tan spot development by Si supply was observed when using a moderately resistant cultivar.     

Spatiotemporal characterization of citrus postbloom fruit drop in Brazil and its relationship to pathogen dispersal

Citrus postbloom fruit drop (PFD) is caused by Colletotrichum acutatum and C. gloeosporioides. These pathogens attack the flowers and cause premature fruit drop and the retention of fruit calyces. This study was designed to characterize the spatial and temporal dynamics of PFD in commercial citrus‐growing areas to better understand the disease spread. Experiments were carried out in three young orchards (500 trees each) in two municipalities in Sao Paulo State, Brazil. Symptoms of PFD on the flowers and presence of persistent calyces were assessed in each of three orchards for three years. Logistic, Gompertz and monomolecular models were fitted to the incidence data over time from the trees with symptoms. The spatial pattern of diseased trees was characterized by a dispersion index and by Taylor′s power law. An autologistic model was used for the spatiotemporal analysis. The logistic model provided the best fit to the disease incidence data, which had a fast progress rate of 0·53 per day. During the early epidemic of PFD, the spatial pattern of diseased trees was random, which suggested that inoculum spread was due to mechanisms other than rain splash. As the disease incidence increased (up to 12·6%), the spatial pattern of diseased trees became aggregated. The rapid rate of disease progress and the distribution of PFD suggest that dispersal of the pathogen is possibly related to a mechanism other than splash dispersal, which is more typical of other fruit diseases caused by Colletotrichum spp.     

Relationship between visual estimates of fusarium head blight intensity and deoxynivalenol accumulation in harvested wheat grain: a meta-analysis

ABSTRACT The association between Fusarium head blight (FHB) intensity and deoxynivalenol (DON) accumulation in harvested grain is not fully understood. A quantitative review of research findings was performed to determine if there was a consistent and significant relationship between measures of Fusarium head blight intensity and DON in harvested wheat grain. Results from published and unpublished studies reporting correlations between DON and Fusarium head blight "index" (IND; field or plot-level disease severity), incidence (INC), diseased-head severity (DHS), and Fusarium-damaged kernels (FDK) were analyzed using meta-analysis to determine the overall magnitude, significance, and precision of these associations. A total of 163 studies was analyzed, with estimated correlation coefficients (r) between -0.58 and 0.99. More than 65% of all r values were >0.50, whereas less that 7% were <0. The overall mean correlation coefficients for all relationships between DON and disease intensity were significantly different from zero (P < 0.001). Based on the analysis of Fisher-transformed r values ( z(r) values), FDK had the strongest relationship with DON, with a mean r of 0.73, followed by IND (r = 0.62), DHS (r = 0.53), and INC (r = 0.52). The mean difference between pairs of transformed z(r) values (z(d) ) was significantly different from zero for all pairwise comparisons, except the comparison between INC and DHS. Transformed correlations were significantly affected by wheat type (spring versus winter wheat), study type (fungicide versus genotype trials), and study location (U.S. spring- and winter-wheat-growing regions, and other wheat-growing regions). The strongest correlations were observed in studies with spring wheat cultivars, in fungicide trials, and in studies conducted in U.S. spring-wheat-growing regions. There were minor effects of magnitude of disease intensity (and indirectly, environment) on the transformed correlations.     

Response of black rot in cabbage to spatial distribution of inoculum

Disease progress of black rot in cabbage crop was studied over three years in field plots to compare the effects of uni-focal and multi-focal inoculum applied in equal amounts per plot. Disease progress (plant incidence and leaf incidence) was plotted over time, three dimensional maps were made, and disease aggregation was studied by means of geostatistics, black-black counts and Moran's I statistic. Black rot progress was primarily due to focus expansion. Secondary foci may appear at short distances from the initial focus but they usually merge with the expanding initial focus. Anisotropy occurred occasionally but was of minor importance. Disease proceeds faster in plots with multi-focal inoculation than in those with uni-focal inoculation. Probably, serious epidemics in Dutch cabbage fields originate from large numbers of foci.     

Host‐induced gene silencing in the necrotrophic fungal pathogen Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a necrotrophic fungus that causes a devastating disease called white mould, infecting more than 450 plant species worldwide. Control of this disease with fungicides is limited, so host plant resistance is the preferred alternative for disease management. However, due to the nature of the disease, breeding programmes have had limited success. A potential alternative to developing necrotrophic fungal resistance is the use of host‐induced gene silencing (HIGS) methods, which involves host expression of dsRNA‐generating constructs directed against genes in the pathogen. In this study, the target gene chosen was chitin synthase (chs), which commands the synthesis of chitin, the polysaccharide that is a crucial structural component of the cell walls of many fungi. Tobacco plants were transformed with an interfering intron‐containing hairpin RNA construct for silencing the fungal chs gene. Seventy‐two hours after inoculation, five transgenic lines showed a reduction in disease severity ranging from 55·5 to 86·7% compared with the non‐transgenic lines. The lesion area did not show extensive progress over this time (up to 120 h). Disease resistance and silencing of the fungal chs gene was positively correlated with the presence of detectable siRNA in the transgenic lines. It was demonstrated that expression of endogenous genes from the very aggressive necrotrophic fungus S. sclerotiorum could be prevented by host induced silencing. HIGS of the fungal chitin synthase gene can generate white mould‐tolerant plants. From a biotechnological perspective, these results open new prospects for the development of transgenic plants resistant to necrotrophic fungal pathogens.     

Involvement of a vascular hypersensitive response in quantitative resistance to Ralstonia solanacearum on tomato rootstock cultivar LS‐89

Ralstonia solanacearum causes bacterial wilt disease in Solanaceae spp. Expression of the Phytophthora inhibitor protease 1 (PIP1) gene, which encodes a papain‐like extracellular cysteine protease, is induced in R. solanacearum‐inoculated stem tissues of quantitatively resistant tomato cultivar LS‐89, but not in susceptible cultivar Ponderosa. Phytophthora inhibitor protease 1 is closely related to Rcr3, which is required for the Cf‐2‐mediated hypersensitive response (HR) to the leaf mould fungus Cladosporium fulvum and manifestation of HR cell death. However, up‐regulation of PIP1 in R. solanacearum‐inoculated LS‐89 stems was not accompanied by visible HR cell death. Nevertheless, upon electron microscopic examination of inoculated stem tissues of resistant cultivar LS‐89, several aggregated materials associated with HR cell death were observed in xylem parenchyma and pith cells surrounding xylem vessels. In addition, the accumulation of electron‐dense substances was observed within the xylem vessel lumen of inoculated stems. Moreover, when the leaves of LS‐89 or Ponderosa were infiltrated with 10⁶ cells mL^?1 R. solanacearum, cell death appeared in LS‐89 at 18 and 24 h after infiltration. The proliferation of bacteria in the infiltrated leaf tissues of LS‐89 was suppressed to approximately 10–30% of that in Ponderosa, and expression of the defence‐related gene PR‐2 and HR marker gene hsr203J was induced in the infiltrated tissues. These results indicated that the response of LS‐89 is a true HR, and induction of vascular HR in xylem parenchyma and pith cells surrounding xylem vessels seems to be associated with quantitative resistance of LS‐89 to R. solanacearum.     

Effect of colored shade nets on pepper powdery mildew (<Emphasis Type="Italic">Leveillula taurica</Emphasis>)

The effect of colored shade nets with different shade intensities and qualities of irradiation transmittance on pepper powdery mildew was tested in mini-plots and field experiments. Leaf coverage byLeveillula taurica and leaf shedding due to the disease were more severe in the shade, by up to 275% and 70%, respectively, than in the open field. Leaf coverage byL. taurica symptoms and leaf shedding from plants grown under 25% shade black nets were higher, by up to 70% and 180%, respectively, than under 40% shade nets. The color of the shade nets affected the intensity of photosynthetically active (PAR), ultra-violet (UV), blue, red and far red radiations, the UV/blue light ratio, and percent PAR and UV transmitted. The various nets suppressed the disease differently. Black, blue-silver, green and red nets were associated with lower levels of disease in the field experiments. The red net was also superior in the mini-plots. The other results from the mini-plots were not similar to those from the field, probably reflecting more intensive epidemic development in the mini-plots. No interaction between net type and cultivar was found when two cultivars were grown under the nets. Yield was higher under nets than in the open; nevertheless, the yield from plants grown under the 40% shade black net was not higher than that of the plants under the 25% black net, despite the significantly lower levels of disease at the higher shade intensity. B-quality pepper yield was significantly higher in the plots covered by 25% shade. Yield differences between the different colored nets were also not well correlated with disease levels, probably due to factors negating the direct effect of the nets on the plants and their yield. Implementation of either ‘friendly’ (Ampelomyces quisqualis AQ10/Trichoderma harzianum T39/ sulfur/ neem seed extract) or chemical (sulfur/ pyrifenox/ Polyoxin AL/ myclobutanil/ azoxystrobin) spray regimes successfully reduced disease severity under the different nets. There was no interaction between net type and spray regime. Thus, growing sweet pepper under shade nets results in increased yields and also in higher powdery mildew severity. Disease is negatively associated with the rate of shading and is variably affected by the quality of light filtered through the different colored shade nets. http://www.phytoparasitica.org posting May 8, 2007.     

Spatial and temporal spread of powdery mildew (Erysiphe pisi) in peas (Pisum sativum) varying in quantitative resistance

A field experiment was conducted to assess the progress in time and spread in space of powdery mildew (caused by Erysiphe pisi ) in pea ( Pisum sativum ) cultivars differing in resistance to the disease. Disease severity (proportion of leaf area infected) was measured in 19 × 23 m plots of cultivars Pania and Bolero (both susceptible) and Quantum (quantitatively resistant). Inoculum on infected plants was introduced into the centre of each plot. Leaves (nodes) were divided into three groups within the canopy (lower, middle, upper) at each assessment because of the large range in disease severity vertically within the plants. Disease severity on leaves at upper nodes was less than 4% until the final assessment 35 days after inoculation. Exponential disease progress curves were fitted to disease severity data from leaves at middle nodes. The mean disease relative growth rate was greater on Quantum than on Pania or Bolero, but it was delayed, resulting in an overall lower disease severity on Quantum. Gompertz growth curves were fitted to disease progress on leaves at lower nodes. Disease progress on Quantum was delayed compared with Pania and Bolero. The average daily rates of increase in disease severity from Gompertz curves did not differ between the cultivars on these leaves. Disease gradients in the plots from the inoculum focus to 12 m were detected at early stages of the epidemic, but the effects of background inoculum inputs and the rate of disease progress meant that these gradients decreased with time as the disease epidemic intensified. Spread was rapid, and there were no statistically significant differences between cultivar isopathic rates (Pania 2.2, Quantum 2.9 and Bolero 4.0 m d⁻¹).     

The role of <Emphasis Type="Italic">Euwallacea</Emphasis> nr. <Emphasis Type="Italic">fornicatus</Emphasis> (Coleoptera: Scolytinae) in the wilt syndrome of avocado trees in Israel

The polyphagous shot hole borer (PSHB), Euwallacea nr. fornicatus (Coleoptera; Scolytinae) has become a serious threat to the avocado industry and several shade tree species in Israel. Branch wilting and tree mortality is the outcome of PSHB galleries. Understanding the relationship between avocado trees and the PSHB is required for considering management strategies. In Israel, 52 tree species from 26 botanical families were attacked by the PSHB, but only 12 species were suitable for beetle reproduction. All examined avocado cultivars were attacked, but ‘Hass’ most severely. Large and medium diameter avocado branches were more resistant to PSHB, compared to thin branches. Effectively, gallery density increased as branch diameter decreased. Concomitantly, in large and medium diameter branches, extensive sugar exudation occurred and beetle attack rarely progressed to the formation of natal galleries, whereas minimal sugar exudation was observed in thin branches. This was more evident in those that were weakened by repeated attacks followed by successful beetle colonization. PSHB prefers and successfully colonized branches that had been previously attacked by its conspecifics, and reproduction was much higher in these branches, as opposed to initial attacks. Lesion frequencies increased from late spring (April) until late summer (September). Avocado branches at the early stages of beetle colonization may be identified by sugar exudation at the base of the thin branches. The main approach for reducing damage caused by the PSHB is sanitation, achieved by the removal of colonized branches and intact infested pruned slash.     

Pathogenicity of isolates of Magnaporthe spp. from wheat and grasses infecting seedlings and mature wheat plants in Argentina

Wheat blast of wheat (Triticum aestivum), caused by Magnaporthe oryzae pathotype triticum (MoT; anamorph Pyricularia oryzae) is a destructive disease in the South American countries of Brazil, Paraguay and Bolivia. In Argentina, the fungus was recently recorded on wheat and barley plants in the northeast part of the country, Buenos Aires and Corrientes Provinces, with a potential for spreading. This work aimed to study, for the first time, the morphocultural and pathogenic characteristics of Magnaporthe isolates collected from wheat and other herbaceous species in Argentina and three neighbouring countries (Paraguay, Brazil and Bolivia) and determine their aggressiveness on wheat varieties. Statistical differences among isolates, culture media, and development conditions were found for conidia colour, growth rate, size and sporulation rate. Pathogenicity tests performed on seedlings with 19 isolates of Magnaporthe spp. under greenhouse conditions showed a maximum disease severity of 55.3% and 66.7% for varieties BIOINTA 3004 and Baguette 18, respectively. Weed and grass isolates were infectious on wheat, demonstrating their potential epidemiological role on the disease. Spike disease severity was 34.6% for the host × pathogen interaction of BIOINTA 3004 × PY22. Observed symptoms included partial or total spike bleaching, and glume and rachis discolouration. The 1000‐grain weight was significantly reduced to 38.5% and 63.1% for cultivars BIOINTA 3004 and Baguette 18, respectively. The disease affected grain germination, which fell to 65.9% for seeds infected with the PYAR22 isolate. Symptoms observed in infected grains were partial spotting, grain softening, and rot symptoms with the presence of a greyish mould.     

Present status of verticillium wilt of olive in Andalucía (southern Spain)1

Verticillium wilt caused by Verticillium dahliae is a disease highly prevalent in newly established olive orchards in Andalucía, southern Spain. Two syndromes of the disease occur in Andalucia, namely apoplexy and slow decline. Apoplexy is characterized by quick dieback of twigs and branches while slow decline consists of rapid drying out of inflorescences together with leaf chlorosis and necrosis. Systematic disease observations carried out in two experimental orchards planted with susceptible cv. Picual indicated that natural recovery of diseased trees occurred over time. Infection and vascular colonization of olive plants by V. dahliae were studied in susceptible (Picual) and resistant (Oblonga) cultivars inoculated with a mildly virulent or a highly virulent cotton-defoliating isolate of V. dahliae. Disease symptoms developed 24–32 days after inoculation in cv. Picual, but at that time plants of cv. Oblonga remained free from symptoms. However anatomical observations and isolations indicated that systemic infections by the two isolates had occurred to a large extent in both cultivars.     

Effects of abiotic variables on the response of carnation cultivars to Fusarium oxysporum f.sp. dianthi

The effects of abiotic variables on the response of carnation cultivars to Fusarium oxysporum f.sp. dianthi ( F.o. dianthi  ) were examined in experiments conducted under semi-controlled environments. The abiotic variables examined were solar radiation intensity, temperature and growth substrate. Temperature was not controlled, but differed markedly among experiments, thus, its effect was not determined quantitatively. Disease incidence and disease severity varied significantly among the experiments (due mainly to differences in temperature), among the solar radiation treatments and among the cultivars tested. The three-way interaction term (i.e. cultivar × shade treatment × experiment) was highly significant ( P  < 0.001) when both disease incidence and disease severity were considered, indicating that no single variable was predominant in determining disease intensity. The effects of the growth substrate on disease progress was examined in plants grown in tuff or in tuff mixed with peat (1 : 1 and 1 : 3) substrates. The growth substrate had a potent effect on disease development in the less susceptible cultivars. Severe epidemics developed in all cultivars when they were grown in the tuff/peat mixture, although some were resistant when grown in tuff alone. These results led to the conclusion that the carnation response to F.o. dianthi is substantially influenced by the environmental conditions of the test.