Selection of Kentucky Bluegrass for Drechslera poae Resistance under Greenhouse Conditions

Resistance to leaf spot and melting out caused by Drechslera poae was evaluated in eight cultivars and seven breeding lines of Poa pratensis under greenhouse conditions. The results obtained in this study were compared with levels of resistance scored after natural infection under field conditions during 1994–1997 in sun and shade lawn maintenance, as described before. It was found that resistance assessed in the greenhouse as leaf spots in the three‐leaf seedling stage 10–14 days after the first inoculation (assessment I) and resistance assessed as severity of melting out on the crown 56 days after the first inoculation (assessment IV) showed the highest repeatability with the scores obtained in the field under sun and shade conditions during 1994–1997.     

Association of the Production of Phenylpropanoid Compounds at the Infection Sites of Corynespora cassiicola with Soybean Resistance against Target Spot

This study investigated, at the microscopic level, whether the differential defence responses of soybean cultivars that are resistant (Fundacep 59) and susceptible (TMG 132) to target spot, caused by Corynespora cassiicola, could be associated with an increase in the production of phenolics, flavonoids and lignin at the infection sites. Many larger necrotic lesions with yellow halos were noticed on the leaves of plants from cultivar TMG 132, in contrast to the leaves of plants from cultivar Fundacep 59. Necrotic lesions also developed on the petioles of leaves of plants from cultivar TMG 132, while on the petioles and veins of leaves of plants from cultivar Fundacep 59, the lesions were of purple colour. The growth of fungal hyphae was reduced on the leaves of plants from cultivar Fundacep 59, and an apparently high density of trichomes was found in comparison with the leaves of plants from cultivar TMG 132. An appressorium‐like structure was produced at one or both extremities of the conidium of C. cassiicola, preferentially at the major and minor veins on the adaxial leaf surface of plants from both cultivars. Most cells on the leaves of plants from cultivar Fundacep 59 reacted against C. cassiicola infection by accumulating phenolic‐like compounds, which contributed to the death of many fungal hyphae and a greater maintenance of cell integrity. In contrast, fungal hyphae grew without any impedance in the leaf cells of plants from cultivar TMG 132, which was associated with signs of intense leaf tissue disorganization. Stronger autofluorescence and deposition of lignin and flavonoids were found in the cells of leaves of plants from cultivar Fundacep 59, in contrast to cultivar TMG 132. It can be concluded that soybean resistance to target spot is probably dependent on the activation of the phenylpropanoid pathway.     

Molecular Characterization of Phytoplasmas Associated with Peach Diseases in Iran

Recently, peach trees showing leaf rolling, little leaf, rosetting, yellowing, bronzing of foliage and tattered and shot‐holed leaves symptoms were observed in peach growing areas in the central and north‐western regions of Iran. Polymerase chain reaction (PCR) and nested PCR using phytoplasma universal primer pairs P1/Tint, R16F2/R2, PA2F/R and NPA2F/R were employed to detect phytoplasmas. The nested PCR assays detected phytoplasma infections in 51% of symptomatic peach trees in the major peach production areas in East Azerbaijan, Isfahan, ChaharMahal‐O‐Bakhtiari and Tehran provinces. Restriction fragment length polymorphism (RFLP) analyses of 485 bp fragments amplified using primer pair NPA2F/R in nested PCR revealed that the phytoplasmas associated with infected peaches were genetically different and they were distinct from phytoplasmas that have been associated with peach and almond witches’‐broom diseases in the south of Iran. Sequence analyses of partial 16S rDNA and 16S–23S rDNA intergenic spacer regions demonstrated that ‘Candidatus Phytoplasma aurantifolia’, ‘Ca. Phytoplasma solani’ and ‘Ca. Phytoplasma trifolii’ are prevalent in peach growing areas in the central and north‐western regions of Iran.     

Rhizobacterial induction of systemic resistance in tomato plants: non-specific protection and increase in enzyme activities

Rhizobacteria B101R, B212R, and A068R, selected as inducers of systemic resistance against Pseudomonas syringae pv. tomato, were tested individually for biological control of multiple pathogens causing foliar diseases in tomato plants. Greenhouse bioassays were carried with five pathogens—Alternaria solani (early blight), Corynespora cassiicola (foliar blight), Oidium lycopersici (powdery mildew), Stemphilium solani (leaf spot), and Xanthomonas campestris pv. vesicatoria (bacterial spot). The level of control achieved by each rhizobacterium varied with the pathosystem studied. Isolate B101R afforded reduced disease intensity in terms of average number of leaf lesions as compared to the treatment control, protection against A. solani, S. solani, and O. lycopersici. Lipoxygenase, phenylalanine amonia-lyase, and peroxidase activities were estimated spectrophotometrically in extracts of plants grown from seeds that were microbiolized with rhizobacterium B101R, and inoculated with P. syringae pv. tomato. Increases in peroxidase and lipoxygenase activities were detected in foliar extracts from plants whose seeds had been microbiolized, while no increase in phenylalanine amonia-lyase activity was observed.     

Suppression by Red Light Irradiation of Corynespora Leaf Spot of Cucumber Caused by Corynespora cassiicola

The effect of red light irradiation on development of Corynespora leaf spot of cucumber plants (Cucumis sativus L. cv. Hokushin) caused by Corynespora cassiicola (Berk. & Court.) was investigated in greenhouses. In a greenhouse without red light (?Red), lesions enlarged, coalesced, and finally covered the entire leaves of cucumber. In a greenhouse with red light (+Red), however, lesion appearance was delayed relative to that under ?Red and its development was also significantly suppressed. Such difference in disease development was also observed in cucumbers grown under +Red and ?Red in the same greenhouse. Disease suppression under red light was also observed in glasshouse‐grown C. cassiicola‐inoculated cucumbers. Red light did not inhibit the infection behaviour of the pathogen. Our results indicated that the delay and suppression of Corynespora leaf spot of cucumber under +Red were due to induction of resistance in cucumber, and not to differences in environmental conditions or fungal population between the two greenhouses. Red light‐induced resistance might contribute to the development of new methods for controlling Corynespora leaf spot of greenhouse‐grown cucumber.     

Perpetuation of Cherry Leaf Spot Disease in Ornamental Cherry

Cherry leaf spot disease, caused by Blumeriella jaapii (Rehm) Arx., is an increasing concern to nursery producers of ornamental cherry in the south‐eastern United States. Spores were trapped starting in late March before symptoms were observed in the field, which indicates that leaf debris from diseased trees are an important source of primary inoculum. Previously infected trees of six cultivars (‘Kwanzan’, ‘Yoshino’, ‘Okami’, ‘Snowgoose’, ‘Autumnalis’ and ‘Akebono’), which were overwintered in a controlled environment protected from airborne spores, developed disease symptoms in late spring, indicating that dormant buds may also be a source of primary inoculum. Because ornamental cherry trees are propagated by budding and cuttings, disease management should incorporate cultural practices that focus on propagation from disease‐free trees and fungicide applications beginning at petal drop to protect emerging leaves.     

Mapping quantitative trait loci (QTLs) for resistance to Cercospora leaf spot disease (Cercospora beticola Sacc.) in sugar beet (Beta vulgaris L.)

The breeding of sugar beet varieties that combine resistance to Cercospora and high yield under non-diseased conditions is a major challenge to the breeder. The understanding of the quantitative trait loci (QTLs) contributing to Cercospora resistance offers one route to solving this problem. A QTL analysis of Cercospora resistance in sugar beet was carried out using a linkage map based on AFLP and RFLP markers. Two different screening methods for Cercospora resistance (a field test at Copparo, Italy, under natural infection, and a newly-developed leaf disc test) were used to estimate the level of Cercospora resistance; the correlation between scores from the field (at 162 days after sowing) and the leaf disc test was significant. QTL analysis was based on F₂ and F₃ (half-sib family) generations derived from crosses between diploid single plants of 93164P (resistant to Cercospora leaf spot disease) and 95098P (susceptible). Four QTLs associated with Cercospora resistance (based on Lsmean data of the leaf disc test) on chromosomes III, IV, VII and IX were revealed using Composite interval mapping. To produce populations segregating for leaf spot resistance as a single Mendelian factor, we selected for plants heterozygous for only one of the QTLs (on chromosome IV or IX) but homozygous for the others. Received: 1 September 1999 / Accepted 7 October 1999     

Target leaf spot of tomato incited by Corynespora cassiicola,an emerging disease in tomato production under Gangetic alluvial region of West Bengal,India

Abstract

Tomato (Lycopersicon esculentum Mill.) is a cosmopolitan vegetable and widely cultivated in almost all the countries of the world including India. Irreversible investment – production ratio for tomato cultivation in recent Indian agricultural systems arise the question, is there any biotic backlogs responsible for such a production loss. Our present investigation is based on this fact. Target leaf spot disease of tomato is caused by Corynespora cassiicola, a serious and emerging disease in India. Prolonged real time surveillance of this disease on tomato from 2010–11 to 2016–17, reflects some remarkable features of pathogenic progress in Gangetic alluvial region of West Bengal. This pathogen is the natural barrier for tomato production with a disease severity ranged between 35% and 58% which ultimately causes tremendous loss of tomato foliage and fruits. C. cassiicola was identified on the basis of morpho-cultural (ITCC Accession No. 7542) and molecular characterization (Genbank Accession No. KJ767193). Homology searching of internal transcribed spacer region of CcHaTom isolate was highly matched with Genbank Accession No. KP666184 (Cynodondactylon/India), AB873045 (Vitex negundo/India) and JN541214 (Malvaviscus concinnus/USA) with 95% similarity. Phylogenetic analysis established that KJ767193 and C. cassiicola retrieved sequences were conspecific from a common ancestral origin, which supports its neighbourhood with this fungal pathogen. Optimum temperature between 24 and 25?°C, coupled with 80–85% relative humidity triggered the disease progress. From the emerging scenario, C. cassiicola infecting tomato is the real threat for indigenous cultivars.     

Leaf structure of Syzygium spp. (Myrtaceae) in relation to site affinity within a tropical rain forest

This study examined four species of Syzygium (S. firmum, S. makul, S. operculatum, S. rubicundum) Myrtaceae, a tree genus that dominates the canopy of rain forests of south‐west Sri Lanka. Syzygium spp. occupy differing habitats with relation to succession and forest topography. We examined differences in leaf morphology and physiology in response to amount of shade, an important environmental variable affecting Syzygium distribution within the forest. To study change in leaf structure and physiology, environmental shelters were constructed simulating forest shade that differed in quality, quantity and duration. Seedlings were exposed to: (i) 0% shade (full sun, FS), red : far red (R : FR) ratio 1.27; (ii) 65% shade (large opening, LO) with direct sunlight similar to the centre of a large canopy opening, R : FR ratio 1.27; (iii) 82% shade (small opening, SO) with direct sunlight similar to the centre of a small canopy opening, R : FR ratio 1.27; (iv) 58% uniform light shade (LS) with a quality similar to the outside edge of a large canopy opening, R : FR ratio 1.05; (v) 85% uniform medium shade (MS) with a quality similar to the inside forest edge of a large canopy opening, R : FR ratio 0.97; (vi) 99% uniform deep shade (DS) similar to that of the forest understorey, R : FR ratio 0.23. The shelters were constructed in a large open area at the field station of the Sinharaja World Heritage site, Sri Lanka. Seedlings of each species were grown for two years in their respective shade treatments before physiological, morphological and anatomical measurements were made on leaves. Variation in leaf structure and physiology between the species was associated with differences in shade‐tolerance and water‐use. All species increased in photosynthesis rates and dimensions in leaf structure (leaf blade and cuticle thickness, stomatal density, thickness of upper and lower epidermis, and thickness of palisade mesophyll) with decrease in shade. In contrast, stomatal conductivity was highest in the DS (99% shade) treatment. Leaves of Syzygium firmum were thickest and largest in area. S. firmum also had highest photosynthesis in the SO (82% shade) treatment. S. firmum was the most shade‐tolerant of all species: it grows well in low shade and its leaf structure suggests it to be the most conservative in water‐use of the Syzygium spp. In the forest S. firmum can persist in the forest shade as established seedlings, but grows best within canopy openings of late‐seral rain forest. Leaves of S. operculatum were thinnest but had highest stomatal densities of the four species. S. operculatum is considered shade‐intolerant, with a leaf structure suggesting it to be prone to desiccation, and by implication susceptible to drought. S. operculatum is found along streams within early seral rain forest habitat, often originating on stream banks after land clearance for cultivation. In the FS (0% shade) treatment, S. rubicundun had highest photosynthesis rates and greatest number of leaves but smallest leaf area of the Syzygium species. S. rubicundum is more shade‐intolerant but more efficient in water‐use than S. operculatum. S. rubicundum is a mid‐seral canopy tree of the midslope stands that are thought to have originated after catastrophic windthrows or swidden cultivation. The leaf physiology and structure of S. makul suggests it to be both moderately shade‐tolerant and conservative in water‐use. It is the most widely distributed Syzygium species across the topography of late‐seral rain forest. We suggest forest disturbance and hydrology are important environmental factors that influence distribution of Syzygium species across the topography. Results from this study contribute to a body of knowledge suggesting that canopy tree species of rain forests in south‐west Sri Lanka have discrete affinities to topography and differences in successional status, and that adaptations in leaf structure and physiology are indicative of such phenomena. © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society, 2003, 141 , 365–377.     

Variation in Aggressiveness of Stagonospora nodorum Isolates in North Dakota

Stagonospora nodorum blotch (SNB), caused by Stagonospora nodorum, is an important disease in the northern Great Plains of the United States and in other wheat‐producing regions in the world. SNB can be managed by different strategies including the use of resistant cultivars. Genetic variation in the pathogen populations is one of the important factors in the development of durable resistant cultivars. Our main objective was to determine variation in aggressiveness/virulence in the 40 isolates of S. nodorum collected from various locations in North Dakota. To achieve this goal, we tested the isolates on two susceptible wheat cultivars (cvs ‘ND495’ and ‘Alsen’) and two resistant wheat cultivars (cvs ‘Erik’ and ‘Salamouni’) – two‐leaf‐stage seedlings under controlled conditions. Aggressiveness of each isolate was characterized by the two epidemiological parameters: percent necrotic leaf area (% NLA) and lesion type (LT) 8 days post‐inoculation. The isolates differed significantly (P ≤ 0.05) for % NLA and LT, and were grouped into three aggressiveness groups (AG): low, medium and highly aggressive. Four isolates (S50, S57, S66 and S89) induced 18–26% NLA and were included into the low aggressive group (AG 1). Three isolates (S15, S39 and S89) induced 57–59% NLA and were considered highly aggressive (AG 3). Thirty‐three isolates were medium aggressive (AG 2). No relationship between AG and mating types was observed. There were significant (P ≤ 0.05) differences in % NLA and LT among wheat cultivars. Significant wheat cultivars by isolates interaction was also demonstrated, suggesting evidence for the existence of host specificity in this system. Overall, our results indicate that S. nodorum isolates prevalent in North Dakota varied greatly in their aggressiveness and that AG 3 isolates can be utilized in breeding wheat for resistance to SNB.     

Rhizobium radiobacter conjugation and callus-independent shoot regeneration used to introduce the cercosporin export gene cfp from Cercospora into sugar beet (Beta vulgaris L.)

Leaf spot disease caused by Cercospora is responsible for crop and profitability losses in sugar beet crops in the US and worldwide. The cfp gene that encodes a protein that exports phytotoxic cercosporins from Cercospora was conjugally transferred to sugar beet using Rhizobium radiobacter (Agrobacterium tumefaciens), to improve Cercospora-induced leafspot resistance. Conditions for shoot regeneration were optimized to increase regeneration/transformation efficiencies. Low-light and room-temperature conditions were favorable to sugar beet regeneration without callus when cytokinin had been added to the tissue culture medium. Using this procedure adventitious shoots from leaf pieces were obtained in a simple, one-step regeneration procedure. T7, a cfp-transgenic clone verified by PCR with gene-specific primers, is being propagated for leaf spot disease resistance evaluation.     

Biological spectrum of Trichoderma harzianum Rifai isolates to control fungal diseases of tomato (Solanum lycopersicon L.)

Three Trichoderma harzianum isolates viz., Th-Sks, Th-Ke and Th-Ar collected from respective states of India viz., Rajasthan, Kerala and Andhra Pradesh were evaluated for the management of six fungal diseases namely damping off, Fusarium wilt, Rhizoctonia wilt, early leaf spot, late blight and Septoria leaf spot in tomato. During in vitro analysis, T. harzianum isolates inhibited the pathogens’ growth. Isolate Th-Sks was the most virulent antagonist against all the test pathogens and exhibited maximum of 79.47% growth inhibition of Phytophthora infestans. Isolate Th-Sks proved most effective at suppression efficacy in the range of 95–100% and 91–100% against all diseases under glasshouse and in the field conditions, respectively. Tomato seeds treatment with isolate Th-Sks also promoted plant height (78.23 cm) and fruits yield (290 g/plant) during field trial and data were found to be not-significantly different from other isolates. Thus, it is concluded that isolate Th-Sks can be utilised as a biocontrol agent for management of fungal diseases in tomato.     

Contributions of diffusional limitation, photoinhibition and photorespiration to midday depression of photosynthesis in Arisaema heterophyllum in natural high light

Diurnal changes in photosynthetic gas exchange and chlorophyll fluorescence were measured under full sunlight to reveal diffusional and non‐diffusional limitations to diurnal assimilation in leaves of Arisaema heterophyllum Blume plants grown either in a riparian forest understorey (shade leaves) or in an adjacent deforested open site (sun leaves). Midday depressions of assimilation rate (A) and leaf conductance of water vapour were remarkably deeper in shade leaves than in sun leaves. To evaluate the diffusional (i.e. stomatal and leaf internal) limitation to assimilation, we used an index [1–A/A₃₅₀], in which A₃₅₀ is A at a chloroplast CO₂ concentration of 350 μ mol mol ^{? 1}. A₃₅₀ was estimated from the electron transport rate (J_T), determined fluorometrically, and the specificity factor of Rubisco (S), determined by gas exchange techniques. In sun leaves under saturating light, the index obtained after the ‘peak’ of diurnal assimilation was 70% greater than that obtained before the ‘peak’, but in shade leaves, it was only 20% greater. The photochemical efficiency of photosystem II ( Δ F/F_m ′ ) and thus J_T was considerably lower in shade leaves than in sun leaves, especially after the ‘peak’. In shade leaves but not in sun leaves, A at a photosynthetically active photon flux density (PPFD) > 500 μ mol m ^{? 2} s ^{? 1} depended positively on J_T throughout the day. Electron flows used by the carboxylation and oxygenation (J_O) of RuBP were estimated from A and J_T. In sun leaves, the J_O/J_T ratio was significantly higher after the ‘peak’, but little difference was found in shade leaves. Photorespiratory CO₂ efflux in the absence of atmospheric CO₂ was about three times higher in sun leaves than in shade leaves. We attribute the midday depression of assimilation in sun leaves to the increased rate of photorespiration caused by stomatal closure, and that in shade leaves to severe photoinhibition. Thus, for sun leaves, increased capacities for photorespiration and non‐photochemical quenching are essential to avoid photoinhibitory damage and to tolerate high leaf temperatures and water stress under excess light. The increased Rubisco content in sun leaves, which has been recognized as raising photosynthetic assimilation capacity, also contributes to increase in the capacity for photorespiration.     

Identification and detection of Bougainvillea spectabilis chlorotic vein‐banding virus in different bougainvillea cultivars in Taiwan

A new virus disease of bougainvillea occurred in Taiwan and proved to be caused by a Badnavirus, which is similar to the pathogen tentatively named ‘Bougainvillea spectabilis chlorotic vein‐banding virus (BsCVBV)’ in Brazil according to pathological and molecular results. In electron microscopic observations, typical bacilliform virions measuring 130–158 × 27–42 nm were observed in infected bougainvillea tissues. The transmission tests demonstrated that the virus could be easily transmitted among different bougainvillea cultivars by bud grafting but not by mechanical inoculation. BsCVBV showed different pathogenicity to various bougainvillea cultivars in our inoculation tests. The Taipei‐Red and Thimma cultivars showed the apparent foliar symptoms of chlorosis, chlorotic spots, wrinkling and leaf‐distortion; the original species of Bougainvillea glabra produced chlorotic spots and vein clearing on leaves without wrinkling or leaf distortion; both ‘Mrs. Eva Mauve Variegata’ and Hati Gadis showed mild mottling and faint spots of leaves; Helen Johnson was tolerant to BsCVBV. Our devised PCR‐based assays demonstrated that BsCVBV could replicate and persistently survived in all tested bougainvillea cultivars used in this study although it induced different symptoms in them. The BsCVBV‐1 primer pair devised from our cloned BsCVBV‐specific DNA fragments proved to be efficient in the PCR assays for the rapid and specific detection of BsCVBV in Taiwan, and this PCR‐based method is helpful in the quarantine, inspection and ecological studies for BsCVBV in Taiwan.     

Effects of foliar apple trichomes on Galendromus occidentalis (Nesbitt) (Acari: Phytoseiidae): preferences,fecundity and prey consumption

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Defence‐Related Enzymes in Soybean Resistance to Target Spot

Target spot, caused by the fungus Corynespora cassiicola, has become a serious foliar disease in soybean production in the Brazilian Cerrado. Information in the literature regarding the biochemical defence responses of soybean to C. cassiicola infection is rare. Therefore, the objective of this study was to determine the biochemical features associated with soybean resistance to target spot. The activities of chitinases (CHI), β‐1‐3‐glucanases (GLU), phenylalanine ammonia‐lyases (PAL), peroxidases (POX), polyphenol oxidases (PPO) and lipoxygenases (LOX), as well as the concentrations of total soluble phenolics (TSP) and lignin‐thioglycolic acid (LTGA) derivatives, were determined in soybean leaves from both a resistant (FUNDACEP 59) and a susceptible (TMG 132) cultivar. The target spot severity, number of lesions per cm² of leaflet and area under the disease progress curve were significantly lower for plants from cv. FUNDACEP 59 compared to plants from cv. TMG 132. The GLU, CHI, PAL, POX and PPO activities and the concentration of LTGA derivatives increased significantly, whereas LOX activity decreased significantly on the leaves infected by C. cassiicola. Inoculated plants from cv. FUNDACEP 59 showed a higher PPO activity and concentrations of TSP and LTGA derivatives at 4 and 6 days after inoculation compared to plants from cv. TMG 132. In conclusion, the results of this study demonstrated that the defence‐related enzyme activities increased upon C. cassiicola infection, regardless of the basal level of resistance of the cultivar studied. The increases in PPO activity and concentrations of TSP and LTGA derivatives, but lower LOX activity, at early stages of C. cassiicola infection were highly associated with soybean resistance to target spot.     

Spectral effects of LEDs on chlorophyll fluorescence and pigmentation in Phalaenopsis ‘Vivien’ and ‘Purple Star’

We examined the effect of light emitting diode (LED) lighting in greenhouse facilities on growth, chlorophyll fluorescence and pigmentation in Phalaenopsis ‘Vivien’ and ‘Purple Star’ under purpose‐built LED arrays yielding c. 200 µmol m^?2 s^?1 at plant height for 14 h per day and 24/18°C day/night temperature, respectively, from January to April 2013. The light treatments were (1) 40% blue in 60% red (40% B/R), (2) 0% blue in 100% red (0% B/R) and (3) white LEDs with 32% blue in white (32% B/W, control), with background daylight under shade screens. The plants were harvested twice for leaf growth and pigmentation. There was no clear pattern in the spectral effect on growth since the order of leaf size differed between harvests in March and April. F_v/F_m was in the range of 0.52–0.72, but overall slightly higher in the control, which indicated a permanent downregulation of PSII in the colored treatments. The fluorescence quenching showed no acclimation to color in ‘Purple Star’, while ‘Vivien’ had lower ETR and higher NPQ in the 40% B/R, resembling low light acclimation. The pigmentation showed corresponding spectral response with increasing concentration of lutein while increasing the fraction of blue light, which increased the light absorption in the green/yellow part of the spectrum. The permanent downregulation of PSII moved a substantial part of the thermal dissipation from the light regulated NPQ to non‐regulated energy losses estimated by Φ_NPQ and Φ_NO and the difference found in the balance between Φ_PSII and Φ_NPQ in ‘Vivien’ disappeared when Φ_NO was included in the thermal dissipation.     

The Effects of Co-Infection by Sunn-Hemp Mosaic Virus (SHMV) and Fusarium oxysporum on the Growth of French Bean

In the seedlings of three cultivars of French bean (Phaseolus vulgaris), ‘Prince,’‘Masterpiece’ and ‘Pinto’, co-infection by Sunn-hemp mosaic virus (SHMV) with either of the vascular wilt pathogens, Fusarium oxysporum f. sp. phaseoh (Fop) or f. sp. tracbeiphilum (Fot) caused greater losses in total fresh weight and in leaf area compared with uninfected plants or plants infected singly with any one of these pathogens. Co-infection of a fourth cultivar (‘Canadian Wonder’) had no greater effects on growth reduction than single infection. The concentration of SHMV in the leaves of ‘Prince’ and to a greater extent in the leaves of ‘Masterpiece’ increased more after double infection than with infection by the virus alone. The nature and possible mechanisms of the pathogenic effects in French bean are discussed.     

Temporal patterns of citrus greasy spot‐induced defoliation of sweet orange cultivars in Brazil

Recôncavo Baiano is an area favourable for the occurrence of citrus greasy spot (CGS) (Mycosphaerella citri), but there has been no study of this pathosystem in Brazil. This work aimed to characterise the temporal patterns of CGS‐induced defoliation in sweet orange cultivars ‘Bahia’ (Washington Navel) and ‘Pêra’. Temperature, rainfall and relative humidity were recorded, as well as weekly defoliation (fallen leaves/canopy m² or m³). Considering the mean of fallen leaves per canopy m², and mean canopy area, the total annual defoliation was estimated to be around 32 000 leaves per plant for ‘Bahia’ and 18 500 for ‘Pêra’ sweet orange. Spectral density analysis showed that defoliation has a 5‐week‐long main cycle for both cultivars. The proportion of symptomatic fallen leaves was never below 0.97. The monthly number of fallen leaves per canopy area was positively correlated with the mean CGS incidence on leaves. Defoliation was significant, resulting in a low leaf density throughout the year. Many defoliation cycles and the very high proportion of symptomatic fallen leaves assure a constant inoculum supply. Based on these results, CGS cannot be considered a minor disease, at least in Recôncavo Baiano.