Management of laurel wilt of avocado,caused by <Emphasis Type="Italic">Raffaelea lauricola</Emphasis>

Laurel wilt is caused by Raffaelea lauricola, a nutritional symbiont of an Asian ambrosia beetle, Xyleborus glabratus. American members of the Lauraceae plant family are most susceptible and 300 million trees have been killed by the disease in the southeastern USA since 2003. Recently, commercial production of an important crop in the laurel family, avocado (Persea americana), has been affected in southern Florida. We summarize studies in which diverse measures were tested for managing the disease. In all studies, trees were treated with potential laurel wilt control measures and subsequently inoculated with R. lauricola. On potted plants in greenhouse experiments, commercial nutritional products (Greenstim and Keyplex 350) and SAR products (Agri-Fos and Nutri-Phite), when applied as soil drenches or foliar sprays, had either no impact on, or increased laurel wilt symptom development compared to non-treated control treatments. Bark applications of Tilt (a propiconazole fungicide for which emergency registration had been obtained in 2010) in a surfactant (Pentrabark) enabled significant laurel wilt protection in greenhouse studies on small potted plants, but Pentrabark and other surfactants moved little propiconazole into the xylem of fruit-bearing trees in field studies. In efficacy studies in the field, Propiconazole Pro (an injectable formulation of propiconazole), Tilt, and two experimental formulations of another triazole fungicide, tebuconazole, decreased the development of laurel wilt compared to nontreated control trees when applied as undiluted injections into branches and scaffold limbs (microinjection), or injection of dilute fungicide into tree flare roots (macroinfusion). However, symptoms developed in all treated trees by 10–11 months after inoculation with R. lauricola, indicating that trees would need to be re-treated at least on an annual basis. Regardless of how fungicides were applied, insignificant levels of different active ingredients entered fruit. Although fungicide treatment of fruit-bearing avocado trees is not a concern for food safety and several triazole and DMI fungicides can protect avocado trees from laurel wilt, cost-effective measures with which the xylem could be loaded with and protected by these products remains a challenge. Management of laurel wilt in commercial avocado production areas is discussed.     

Phylogenetic relationship and fungicide sensitivity of members of the <Emphasis Type="Italic">Colletotrichum gloeosporioides</Emphasis> species complex from apple

The members of the Colletotrichum gloeosporioides species complex (CGSC), the dominant pathogens of apple bitter rot in Nagano prefecture, Japan, were reidentified and the relationship between the species and fungicide sensitivity was revealed. Based on phylogenetic analysis of the ApMat locus with the neighbor-joining (NJ) method, isolates from apple contained three species of the CGSC; C. fructicola, C. aenigma, C. siamense, and three clades of the CGSC: Clade V, S and K. Colletotrichum fructicola and Clade S dominated in Nagano Prefecture. Isolates of C. siamense, C. aenigma and Clade V, S and K remained sensitive to benomyl and quinone outside inhibitor (QoI) fungicides, while C. fructicola often developed resistance to benomyl and QoI fungicides. These results suggest that the development of fungicide resistance differs among members of the CGSC.     

Evaluation of benomyl and thiophanate-methyl for the control of Verticillium wilt of strawberry in the Netherlands

In pot experiments the fungicides benomyl and thiophanate-methyl controlledVerticillium wilt of strawberry when applied as a soil drench after planting. Both compounds were ineffective as foliar sprays and as root dips prior to planting. Soil drenches applied to commercially grown runner plants in the waiting field (August) and to the same plants in the greenhouse (December or January) increased the yield. On infested ground, a soil drench with thiophanate-methyl promoted the occurrence of crown rot caused byPhytophthora cactorum.     

Soil application of imidacloprid and related SAR-inducing compounds produces effective and persistent control of citrus canker

Soil application of the systemic insecticide imidacloprid (Admire®, Bayer Crop Science) produced season-long control of citrus canker caused by Xanthomonas citri sbsp. citri. Imidacloprid is a neo-nicotinoid that breaks down in planta into 6-chloronicotinic acid, a compound closely related to the systemic acquired resistance (SAR) inducer isonicotinic acid. Potted Swingle citrumelo seedlings (Citrus paradisi × Poncirus trifoliata) were treated with imidacloprid and the SAR inducers, isonicotinic acid, and acibenzolar-s-methyl as soil drenches or with acibenzolar-s-methyl as a foliar spray 1week prior to inoculation of immature leaves with X. citri sbsp. citri. Seedlings were re-inoculated four times over a 24-week period. SAR induction was confirmed by expression of the PR-2 gene (β-1,3 glucanase). Soil drenches of imidacloprid, isonicotinic acid, and acibenzolar-s-methyl induced a high and persistent up-regulation of PR-2 gene expression and reduced the number of canker lesions for up to 24 weeks compared to 4 weeks for foliar acibenzolar-s-methyl. Soil applied inducers of SAR reduced canker lesions up to 70% compared with the untreated inoculated plants. Lesions on leaves were small, necrotic, and flat compared to pustular lesions on inoculated untreated plants. Populations of X. citri sbsp. citri per leaf were reduced 1–3 log units in soil-treated plants compared to inoculated untreated plants.     

Efficacy of phosphonate in controlling white powdery rot of fig caused by <Emphasis Type="Italic">Phytophthora palmivora</Emphasis>

Pre- and post-infection activities of phosphonate and fungicides (azoxystrobin, chlorothalonil, cyazofamid, copper hydroxide, and copper sulfate) against white powdery rot on fig leaves caused by Phytophthora palmivora were determined. Phosphonate and fungicides were applied at 3, 7, or 14 days before inoculation and 16, 24, or 40 h after inoculation. Phosphonate and four fungicides (except for chlorothalonil) had protectant activity for up to 14 days before inoculation. Protectant activity between phosphonate and the four fungicides did not differ significantly. These fungicides did not have a curative activity, whereas only phosphonate had a curative activity when applied 16 h after inoculation. The pre-infection activity of phosphonate was determined in fig fields. Phosphonate provided excellent protectant activity, and its activity did not differ significantly from that of the copper fungicides. The activity of phosphonate in the life cycle of P. palmivora was compared with that of the five fungicides. Half-maximal effective concentration of the inhibition of mycelial growth was low for phosphonate, chlorothalonil, and the two copper fungicides. Mycelial growth was least affected by azoxystrobin and cyazofamid, but n-propyl gallate, an alternative oxidase inhibitor, in combination with these fungicides, significantly inhibited mycelial growth. Sporangium formation was sensitive to all compounds. Germination of encysted zoospores was sensitive to azoxystrobin, cyazofamid, and chlorothalonil and least sensitive to phosphonate and the two copper fungicides. The activity of phosphonate on mycelial growth and sporulation of P. palmivora suggests it has potential for controlling the disease.     

Bacterial wilt symptoms are impacted by host age and involve net downward movement of <Emphasis Type="Italic">Erwinia tracheiphila</Emphasis> in muskmelon

Cucurbit bacterial wilt, caused by Erwinia tracheiphila, is a devastating disease of cucurbit crops in the Midwest and Northeast U.S. Current management of bacterial wilt relies primarily on insecticide applications to control striped and spotted cucumber beetles (Acalymma vittatum and Diabrotica undecimpunctata howardi, respectively), which vector E. tracheiphila. Development of alternative management strategies is constrained by a lack of understanding of bacterial wilt etiology. The impact of host age on rate on symptom development and extent of bacterial movement in the xylem of muskmelon (Cucumis melo cv. Athena) was evaluated following wound inoculation of 2- to 8-week-old plants in growth chamber experiments. Wilting occurred more rapidly in plants after inoculating E. tracheiphila into 2- or 4-week-old plants than 6- or 8-week-old plants. Recovery of viable cells from stem segments revealed that vascular spread of E. tracheiphila was more extensive below than above the inoculation point. These findings provide experimental evidence that host age impacts the rate of symptom development in cucurbit bacterial wilt and that movement of the xylem-inhabiting pathogen E. tracheiphila within muskmelon plants occurs primarily in the downward direction.     

Screening of alternative products for integrated pest management of cucurbit powdery mildew in Sweden

Cucurbit powdery mildew (CPM) is one of the most important plant protection problems in Swedish cucumber production. CPM fungi have developed resistance to the commonly used fungicide (Imazalil) rendering its current use less effective. We therefore screened a selection of alternative products, evaluating their ability to control CPM in seven small-scale, semi-commercial, greenhouse experiments. Products were screened for their ability to suppress CPM on a susceptible cultivar, in 2013. In 2015, the best treatments were tested in different combinations, in different intervals on a susceptible and a partially resistant cultivar. The treatment that gave the best CPM control was Sakalia in combination with wetting agent Yuccah, (based on Reynoutria sachaliensis and Yucca schidigera respectively) applied at 7-day intervals. This treatment was highly efficient on both cucumber cultivars. Sakalia mixed with Yuccah applied at 14-day intervals had an almost equally controlling effect on CPM. The microbial pesticides, Polyversum (Pythium oligandrum) and AQ10 (Ampelomyces quiscalis) and the fungicide Imazalil, provided no to poor control of CPM compared to control treatments. Hortistar (Silicon) partially reduced CPM infections and was more efficient on the partially resistant cultivar. We conclude that Sakalia in combination with wetting agent Yuccah could efficiently control the disease even under the severe conditions caused by artificial inoculation. Further testing of this combination in commercial greenhouses will enable evaluation of the potential effects on yield and beneficial or pest insects. The knowledge gained from this study can be used to develop IPM tools for commercial production systems.     

Fungicide sensitivity,growth rate,aggressiveness and frost hardiness of <Emphasis Type="Italic">Monilinia fructicola</Emphasis> and <Emphasis Type="Italic">Monilinia laxa</Emphasis> isolates

Monilinia fructicola, the most destructive pathogen of the genus Monilinia, has recently been introduced into Serbia and many other European countries. Since then, many studies have been conducted to evaluate the characteristics of Monilinia species that have a role in the establishment and survival of the pathogen in new areas. The present study assessed the capacity of M. fructicola to repress and replace Monilinia laxa in Serbia based on: fungicide sensitivity, growth rate and aggressiveness at different temperatures, as well as frost hardiness of the isolates of both species. The results showed that the isolates of M. fructicola, compared to M. laxa, were significantly less sensitive to the following fungicides: iprodione, tebucanozole, chlorothalonil, azoxystrobin, fluopyram, and boscalid. In addition, M. laxa isolates exhibited little variation in sensitivity to all of the tested fungicides, whereas M. fructicola isolates displayed a wide range of sensitivity. The temperature of 5°C favored M. laxa growth and aggressiveness, while at 30°C M. fructicola grew faster and had higher lesion expansion rate. These results support an assumption that M. fructicola will continue to spread in Serbian orchards in coming years, particularly on stone fruits harvested during hot summer weather.     

Panama disease of banana occurred in Miyakojima Island,Okinawa, Japan

In 2016, in Miyakojima, Okinawa, Japan, banana plants (Musa?×?paradisiaca) ‘Shima-banana’ developed yellowing and wilt associated with vascular discoloration of the pseudostems. Fusarium oxysporum, identified based on morphological characters, was frequently isolated from the vascular tissue of the infected plant and reproduced the original symptoms on ‘Shima-banana’ after drench inoculation with a spore suspension. Thereby, we determined that the disease is Panama disease caused by F. oxysporum. This is the first official report of Panama disease (Panama-byo in Japanese) of banana in Japan.     

<Emphasis Type="Italic">Aphelenchoides gorganensis</Emphasis> n. sp. (Nematoda: Aphelenchoididae)<Emphasis Type="Italic">,</Emphasis> a new species from Iran

Phytophthora capsici infection of chili pepper seedlings can cause substantial losses due to damping-off and collar rot diseases. Chemical control is no longer effective due to reported resistance development, on top of the related environmental concerns and the consumer demands for reduced use of fungicides. Biological control is a sustainable option, with several agents having been reported to be effective against this pathogen. This research focused on optimizing the application of strain THSW13 of Trichoderma hamatum and a bacterial isolate BJ10–86 with the objectives of improving chili pepper seed germination, reduce damping-off disease incidence, and improve the growth of the seedlings. Bacterial isolate BJ10–86 was subjected to molecular identification and found to be Pseudomonas aeruginosa. Chili pepper seeds treated with the biocontrol agents, individually or in combination, were seeded into commercial nursery media that had been pre-inoculated with P. capsici zoospores. Over a period of 35 days the chili pepper seed treatments significantly (P = 0.008) reduced the disease incidence of seedlings damping-off. Combined application of T. hamatum and P. aeruginosa was the best biocontrol treatment with an area under disease curve of only 36.61 units compared to 92.87 units for the control treatment. Similar results were observed in vitro where T. hamatum and P. aeruginosa synergistically inhibited P. capsici growth by 73.2 %. The inhibition activity of this treatment was similar to mefenoxam treatment, which implies that it is an effective and sustainable alternative for chili pepper seed treatment. The biocontrol seed treatment had no effect on seed germination and seedling growth.     

<Emphasis Type="Italic">Fusarium oxysporum</Emphasis> and <Emphasis Type="Italic">Fusarium avenaceum</Emphasis> associated with yield-decline of pyrethrum in Australia

Fusarium wilt, one of the destructive diseases of cucumber can be effectively controlled by using biocontrol agents such as Trichoderma harzianum. However, the mechanisms controlling T. harzianum-induced enhanced resistance remain largely unknown in cucumber plants. Here we screened the potent T. harzianum isolate TH58 that could effectively control F. oxysporum (FO). Glasshouse efficacy trials also showed that TH58 decreased disease incidence by 69.7 %. FO induced ROS over accumulation, while TH58 inoculation suppressed ROS over accumulation and improved root cell viability under F. oxysporum infection. TH58 inoculation could reverse the FO-induced cell division block and regulate the proportional distribution of nuclear DNA content through inducing 2C fraction. Moreover, the expression levels of cell cycle-related genes such as CDKA, CDKB, CycA, CycB, CycD3;1 and CycD3;2 in TH58 - pre-inoculated seedlings were up-regulated compared with those infected with FO alone. Taken together, these results suggest that T. harzianum improved plant resistance against Fusarium wilt disease via alterations in nuclear DNA content and cell cycle-related genes expression that might maintain a lower ROS accumulation and higher root cell viability in cucumber seedlings.     

<Emphasis Type="Italic">Dittrichia viscosa</Emphasis> and<Emphasis Type="Italic">Rubus ulmifolius</Emphasis> as reservoirs of aphid parasitoids (Hymenoptera: Braconidae: Aphidiinae) and the role of certain Coccinellid species

The role of the self-sown shrubsDittrichia viscosa (L.) W. Greuter andRubus ulmifolius Schott as reservoirs of aphid parasitoids was investigated. In the field studies conducted,D. viscosa grew adjacent to crops of durum wheat and barley andR. ulmifolius grew adjacent to cotton. The relative abundance of the parasitoids of(a) Capitophorus inulae (Passerini) onD. viscosa, (b) Rhopalosiphum padi (Linnaeus) on durum wheat and barley,(c) Aphis ruborum (Börner) onR. ulmifolius, and(d) Aphis gossypii Glover on cotton in various parts of Greece, was assessed during the years 1996–2000. In 2000, the fluctuation of parasitization of the above four aphid species was recorded and the action of the aphidophagous predators of the family Coccinellidae was studied. It was observed thatAphidius matricariae Haliday predominated onC. inulae andR. padi in all sampling cases. In contrast,Lysiphlebus fabarum (Marshall) was the dominant species parasitizingA. ruborum onR. ulmifolius andA. gossypii on cotton in Thessaly (central Greece) and Macedonia (northern Greece), whereasLysiphlebus confusus Tremblay et Eady andBinodoxys acalephae (Marshall) were the dominant parasitoid species in Thrace (northern Greece).Coccinella septempunctata Linnaeus was the most abundant coccinellid species on durum wheat, whereasAdonia variegata (Goeze) predominated on cotton. However, coccinellid individuals were scarce on bothD. viscosa andR. ulmifolius. The present study indicated that these two shrubs can be regarded as useful reservoirs of aphid parasitoids.     

Azoxystrobin and propiconazole offer significant potential for rice blast (<Emphasis Type="Italic">Pyricularia oryzae</Emphasis>) management in Australia

Fungicides are the preferred rice blast (Pyricularia oryzae) control option by farmers. However, no fungicides are yet registered for this purpose in Australia. Hence, it is important to test the baseline sensitivity of P. oryzae isolates collected from blast-affected regions across northern Australia, which have not yet been exposed to the fungicides, as part of a resistance management strategy. Further, it is also important to investigate and compare effect of application timing of fungicides on conidial development, including germination and germ tube growth, and penetration on susceptible rice. The EC₅₀ of a collection of fungicide-sensitive blast isolates were within the range of 0.02–2.02 and 0.06–1.91 mg L^?1 for azoxystrobin and propiconazole, respectively. Azoxystrobin was shown to have greater inhibitory effect on conidial germination than propiconazole. In addition, for pre-inoculation application, only germ tubes in the presence of external nutrients continued to grow from 24 to 48 hpi. On susceptible seedlings, both fungicides completely controlled blast disease when applied the same day as inoculation. However, for pre- or post-inoculation application of fungicide, the extent of disease control was reduced, with azoxystrobin more efficacious than propiconazole. A stimulatory effect of both fungicides at low dose was observed on certain P. oryzae isolates. This is the first study to assess the baseline sensitivity of the P. oryzae population in Australia and the first to report a stimulatory effect of low azoxystrobin concentration on growth of P. oryzae. The study highlights, for the first time, the critical role of external nutrients in promoting germ tube growth under fungicide stress conditions. Lastly, it demonstrates the high degree of efficacy of the fungicides and their potential for future rice blast management in Australia.     

Studies on the therapeutic effects of pectolytic enzyme inhibitors

A few polyphenols (tannic acid and digallic acid), anthraquinone derivatives (rufianic acid and quinalizarin), and detergents (an alkylaryl sulfonate) proved to be especially effective inhibitors of pectolytic and cellulolytic enzymes of four phytopathogenic fungi. Some of these substances prevented the infection of tomato leaves withAlternaria solani. It appears, however, that inhibition of the germination of conidia was the crucial effect rather than inhibition of the enzymes. Alkylaryl sulfonate and rufianic acid were able to reduce the maceration of bean hypocotyls byRhizoctonia solani. Rufianic acid also counteracted tomato wilt, caused byFusarium oxysporum f.lycopersici, especially in the case of postinfectional application. On the other hand preinfectional rather than postinfectional application of rufianic acid had effect onVerticillium wilt of tomato.In one case, viz.Fusarium wilt, the mode of action of rufianic acid was investigated in greater detail. The presence of this substance could be demonstrated in the exudates of healthy and infected plants after application to the roots. The pectolytic and cellulolytic activity of the exudates and the pectinmethylesterase activity of stem extracts of infected, rufianic acid treated plants were reduced, as well as the quantity of mycelium in the stems. Possibly other mechanisms are also involved in the mode of action of rufianic acid.The prospects of using pectolytic enzyme inhibitors for the therapy of plant diseases are discussed.     

Egg and larval parasitoids of the beet armyworm<Emphasis Type="Italic">Spodoptera exigua</Emphasis> on maize in Turkey

The present study was conducted to determine egg and larval parasitoids of the beet armywormSpodoptera exigua Hübner (Lepidoptera: Noctuidae), which is an important but sporadic pest in Turkey. High beet armyworm population levels were recently observed in fields of first and second crop maize in the southeast Mediterranean region of Turkey. The parasitoid species complex and its impact on the pest were analyzed in a 4-year study in first and second crop maize. The braconid larval parasitoidsMicroplitis rufiventris Kokujev,M. tuberculifer Wesmael,Meteorus ictericus Nees,Chelonus obscuratus (Herrich Schäffer) (an egg-larval parasitoid),Apanteles ruficrus (Haliday); the ichneumonid larval parasitoidsHyposoter didymator (Thunberg) andSinophorus xanthostomus Gravenhorst; and the egg parasitoidTrichogramma evanescens (Westwood) were found to be the natural enemies attacking the pest. Among the parasitoid species the solitary endoparasitoidH. didymator was the most prevalent species, being reared from 40.5% of the parasitized larvae found. Higher parasitism rates were recorded on first crop than on second crop maize in every year. Possible reasons for this difference in larval parasitism between two growing seasons include lower population of the pest and reduced insecticide applications in first crop maize fields which permitted higher parasitism. However, parasitoid activity was insufficient to counterbalance the population growth of the pest on subsequent second crop maize.     

Post-infection activity of fungicides against <Emphasis Type="Italic">Phytophthora infestans</Emphasis> on tomato (<Emphasis Type="Italic">Solanum lycopersicum</Emphasis> L.)

Late blight caused by Phytophthora infestans is the most devastating disease of tomato (Solanum lycopersicum L.) and causes important economically losses if not properly controlled. Control is achieved mainly by preventive fungicide applications. However, even if curative applications are discouraged because they increase the risk of resistance development in the target pathogens, in practice fungicides may be applied also when the disease is already present, a situation that commonly occurs in the field. The aim of this work was to study the curative activity of several fungicides toward P. infestans to determine their efficacy when applied after the infection process. Nine trials were performed in greenhouse using potted tomato plants that were treated 24 h after inoculation. Disease severity was assessed three times from the development of the symptoms on the untreated plants and data analysed using a linear mixed model. Differences in post-infection control between the different chemical classes were found. Metalaxyl-M and cymoxanil showed the best curative activity while among the CAA fungicides, a good efficacy was expressed by dimethomorph. Interestingly, evidence of synergy between active ingredients having different modes of action was observed such as in the mixtures containing dimethomorph?+?ametoctradin, dimethomorph?+?pyraclostrobin and fosetyl-Al?+?propamocarb. This study provided useful information on the post-infection activity of some fungicides used to control tomato late blight and should be taken into account to perform more in depth studies at the field level and to improve the management strategies of the disease.     

Sensitivity of <Emphasis Type="Italic">Cochliobolus heterostrophus</Emphasis> to three demethylation inhibitor fungicides,propiconazole, diniconazole and prochloraz,and their efficacy against southern corn leaf blight in Fujian Province,China

Southern corn leaf blight (SCLB) caused by Cochliobolus heterostrophus is a fungal disease that impacts production of corn in China. Fungicides have been the main strategy to manage SCLB. In this study, 276 isolates of C. heterostrophus from seven locations in Fujian Province of China were tested for sensitivity to three demethylation inhibitor (DMI) fungicides. The results indicated that most of the isolates of C. heterostrophus tested were exceptionally sensitive to the three DMI fungicides. Correlation analysis revealed positive association between propiconazole and diniconazole (r?=?0.8145, P?<?0.0001), propiconazole and prochloraz (r?=?0.6190, P?<?0.0001), and diniconazole and prochloraz (r?=?0.5784, P?<?0.0001). However, there was no cross-resistance between these three DMI fungicides and the other six fungicides tested, which included carbendazol, chlorothalonil, mancozeb, iprodione, fluazinam, and pyraclostrobin. In a preventive pot experiment, one spray of 25% propiconazole emulsifiable concentrate (EC) with 250 μg active ingredient (a.i.) mL^?1 applied 12 and 24 h before inoculation at the seedling (V6) stage reduced severity of SCLB by 85.60–89.21%. Nevertheless, the curative activity of propiconazole was much weaker (P?<?0.05) than its preventive efficacy. In greenhouse pot assays, one dose of propiconazole at 250 μg a.i. mL^?1 was the most efficacious for controlling SCLB at the seedling and tasseling (VT) stages of corn, decreasing severity by 80.31%–84.85%, which was higher (P?<?0.05) compared to diniconazole, prochloraz, and other reference fungicides. Therefore, propiconazole appears to be very effective in reducing SCLB and should be applied as a preventive rather than therapeutic fungicide. Our findings provide essential information on the evolution of DMI resistance in C. heterostrophus in Fujian Province of China and may serve as a guide for early resistance monitoring in the future.     

Towards a mechanistic model for the Verticillium dahliaepotato system

Development ofVerticillium dahliae in potato was modelled by coupling aVerticillium subroutine to an existing potato crop growth model, using mainly biologically meaningful and measurable parameters. The model, written in FORTRAN77, describes disease development in single plants. Stem base infection is modelled as a stochastic process. The average effect on the crop is calculated by taking the average of model reruns. The parameters of the potato crop growth model were calibrated in advance of those of theVerticillium subroutine. By doing so, corrections can be made for lack of fit in the potato growth model. Calibration of theVerticillium subroutine was performed using experimental field data of two years at two microsclerotial density levels. The results of the calibration procedure of theVerticillium subroutine improved strongly when the results of runs of both microsclerotial densities were expressed in one combined goodness of fit value. Simulation of root infection and incidence of stem colonization byV. dahliae were in line with experimental data. However, deviances from data from the literature at inoculum densities which were not used in the calibration procedure indicate that calibration has to be performed using more field data and also incorporating more parameters like those describing fungal dynamics. Optimal runs were obtained at pathozones of c. 30 μm, which may indicate that infections occur only when a microsclerotium is very close to the root.     

Effects of triazole fungicides on sterol biosynthesis during spore germination of Botrytis cinerea,Venturia inaequalis and Puccinia graminis f. sp. tritici

With three plant pathogens,Botrytis cinerea, Venturia inaequalis and Puccinia graminis f. sp.tritici, the time course of sterol biosynthesis during spore germination was examined by labeling experiments along with the question whether this pathway could be inhibited by triazole fungicides. Conidia ofB. cinerea andV. inaequalis are able to synthesize sterols immediately after the beginning of the germination process when the germ tubes have not yet emerged. On the contrary uredospores ofP. graminis start sterol biosynthesis after 6 to 8 h germination time almost at the end of the germ tube phase, indicating that sterol reserves of the spores are likely to be used for the germ tube growth.The sterol C-14 demethylation appeared to be the rate limiting step within the sterol biosynthetic pathway: the half life of 24-methylenedihydrolanosterol was less than 1 h forB. cinerea. It was more than 1 h forV. inaequalis and 3 h forP. graminis. Independent of these differences in the time course of sterol biosynthesis and in the C-14 demethylation rate, the synthesis of sterols in germinating spores was strongly inhibited by triazole fungicides in all three pathogens examined. In contrast toP. graminis, this inhibition could be demonstrated withB. cinerea andV. inaequalis even in ungerminated conidia, indicating that the fungicides were rapidly taken up and reached their target within 1 or 2 h. These results are discussed along with the question whether spore germination can be used as a bioassay for the estimation of sensitivities of triazole fungicides.     

A new method for inoculation of fruit with <Emphasis Type="Italic">Guignardia citricarpa</Emphasis>, the causal agent of citrus black spot

Citrus black spot (CBS) is a fungal disease, caused by Guignardia citricarpa, that has a high economic impact on citrus. Although G. citricarpa has been associated with black spot of citrus, an adequate pathogenicity test is still not available. Thus, our objective was to develop and evaluate a simple, safe, and practical pathogenicity test. We used fruits from Pera-Rio and Valencia sweet orange trees from two different orchards, located in the State of São Paulo, Brazil. Inoculation was performed by placing six disks colonized by G. citricarpa, onto the peel of healthy fruits, previously bagged. In the Pera-Rio sweet orange grove, initial symptoms of the false melanose type resulting from the inoculations were observed 55 days after inoculation (dai). In the Valencia grove, initial symptoms also of the false melanose type resulting from the inoculations occurred 73 dai. A total of 92.8% and 86.6% of the Pera Rio and Valencia fruits inoculated, respectively, showed symptoms of CBS. Citrus black spot symptoms were not observed in any of the control fruits.