Spatial analysis of the risk of major forest diseases in Monterey pine plantations

Diplodia pinea (causing diplodia blight), Fusarium circinatum (causing pitch canker), and Mycosphaerella spp. (causing needle blight) are examples of major fungal agents that damage Pinus spp. These diseases have a major negative impact on commercial plantation production, in addition to the risk of being introduced to native forest ecosystems. This study aimed to model the spatial risk of major forest diseases to Monterey pine plantations in northern Spain, to provide a basis for more focused management strategies. Generalized linear mixed‐effects models were used to identify empirical relationships between environmental variables and disease occurrence. Field surveys and laboratory tests confirmed that D. pinea caused shoot blight in 24% of sampled plantations. Fusarium circinatum and Mycosphaerella spp. were detected in 15 and 71% of analysed plantations, respectively. The results confirmed that disease risk was strongly related to summer precipitation, moderately related to solar radiation, followed by summer temperature and tree age. The predicted probability of disease incidence was >20, 98 and 85% for Mycosphaerella spp., F. circinatum and D. pinea, respectively. The resultant maps from this study may be used to identify high‐risk areas for increased monitoring and awareness of fungal diseases.     

Survival of Fusarium circinatum in soil and Pinus radiata needle and branch segments

Survival of Fusarium circinatum in colonized pine needles and wood pieces was measured. Naturally colonized branches and their needles were cut into small pieces and placed in mesh bags on the soil surface at two locations in northern Spain. Pieces were recovered periodically, cultured on a selective medium, and microscopically examined to identify the species. After 507 days, F. circinatum was recovered from 0 to 27% of the wood pieces and from none of the needles. After 858 days, F. circinatum was not recovered from any wood pieces but was found to be present on 1 out of 220 needle pieces analysed. Artificially infested pieces of wood and needles were placed on 5‐mm sieved soil either in plastic boxes at controlled temperature or in mesh bags under field conditions. No survival was recorded after 794 days under field conditions and the decline over time occurred more rapidly in inoculated pieces under field conditions. Soil was also infested with conidia of F. circinatum and survival was estimated. No conidia were recovered after 224 days at 30 °C, although at 20 and 5 °C the respective populations were 20 and 3700 cfu/g soil. Fusarium circinatum was not recovered from 2‐mm‐sieved soil collected under pitch canker‐infected pines. Results indicate that branch segments and needles naturally colonized by F. circinatum will not be a potential source of inoculum, and the fungus in soil is not likely to contribute to reinfection of new plantations after 2 years.     

Effect of Trichoderma viride pre-inoculation in pine species with different levels of susceptibility to Fusarium circinatum: physiological and hormonal responses

Pine pitch canker (PPC), caused by Fusarium circinatum, affects Pinus species worldwide. Although no effective solutions have yet been found to control it, there is a growing interest in using biological control agents (BCA) such as Trichoderma to avoid the application of chemical-based products. Using species with an increasing level of susceptibility to PPC (Pinus pinea, Pinus pinaster and Pinus radiata), this study aimed to evaluate the effect of Trichoderma viride pre-inoculation on disease development, assessing several physiological and hormonal parameters. A 2-week period elapsed between T. viride and F. circinatum inoculation. Sampling for each species was performed independently when at least 50% of the plants of one of the inoculated groups developed disease symptoms. Fusarium circinatum infection reduced water status and photosynthesis, but increased proline, abscisic acid and jasmonic acid concentrations in plants of P. radiata and P. pinaster with symptoms; while in P. pinea water relations were maintained and anthocyanin accumulation occurred in the presence of F. circinatum. In P. radiata, T. viride pre-inoculation accelerated disease progression, with some PPC-induced responses augmented (decreased water potential and photosynthesis; increased substomatal CO₂ concentration) and novel changes not found in seedlings inoculated exclusively with F. circinatum (increased electrolyte leakage and salicylic acid; decreased relative water content). This suggests that T. viride may be initially recognized as an invading organism, subverting the plant defence mechanisms for successful root colonization. If seedlings are not allowed to recover from this state, pathogen infection may thus be facilitated, highlighting the importance of application timing in BCA strategies.     

Microsatellite and mating type markers reveal unexpected patterns of genetic diversity in the pine root‐infecting fungus Grosmannia alacris

Grosmannia alacris is a fungus commonly associated with root‐infesting bark beetles occurring on Pinus spp. The fungus has been recorded in South Africa, the USA, France, Portugal and Spain and importantly, has been associated with pine root diseases in South Africa and the USA. Nothing is known regarding the population genetics or origin of G. alacris, although its association with root‐infesting beetles native to Europe suggests that it is an invasive alien in South Africa. In this study, microsatellite markers together with newly developed mating type markers were used to characterize a total of 170 isolates of G. alacris from South Africa and the USA. The results showed that the genotypic diversity of the South African population of G. alacris was very high when compared to the USA populations. Two mating types were also present in South African isolates and the MAT1‐1/MAT1‐2 ratio did not differ from 1:1 (χ² = 1·39, P = 0·24). This suggests that sexual reproduction most probably occurs in the fungus in South Africa, although a sexual state has never been seen in nature. In contrast, the large collection of USA isolates harboured only a single mating type. The results suggest that multiple introductions, followed by random mating, have influenced the population structure in South Africa. In contrast, limited introductions of probably a single mating type (MAT1‐2) may best explain the clonality of USA populations.     

New insights into radiata pine seedling root infection by Fusarium circinatum

Pine root infection by Fusarium circinatum has been reported in the literature, but the underlying pathogenic interaction is poorly understood. A green fluorescent protein (GFP)‐tagged F. circinatum isolate, together with confocal microscopy, was used in order to monitor the events associated with root infection of Pinus radiata seedlings. It was found that in order to reach and successfully infect pine roots, F. circinatum employed features that are similar to those previously described for other root‐infecting pathogens, such as mycelial strands, single runner hyphae and simple hyphopodia as well as other features that are reminiscent of those that are known to be involved in biotrophic invasion, such as bulbous invasive hyphae and filamentous invasive hyphae. Abundant sporulation was observed at the root surface as well as inside tracheids both in roots and in the root collar region. The fungus can spread from the roots to the aerial parts of the plant, and once there, colonization appears to be similar to the process that occurs when the pathogen is inoculated in the stem. Wilting symptoms and plant demise may be the result of a reduction in water uptake by roots and of the blockage of the vascular system by fungal hyphae and resin.     

Inheritance of virulence in Fusarium circinatum,the cause of pitch canker in pines

Wildtype strains of Fusarium circinatum, the causal agent of pitch canker, were crossed to obtain an F₁ generation. Progeny of this cross were tested for virulence by inoculating Pinus radiata seedlings, and were found to induce a wide range of lesion lengths. Two strains from the F₁ generation that induced long lesions (= high virulence) were used as parents to produce an F₂ generation, followed by a second round of selection for high virulence to obtain an F₃ generation. Mean lesion lengths were not significantly different between the three generations (P ≥ 0.196). A parallel set of crosses was performed to select for low virulence by using progeny in the F₁ and F₂ generations that induced short lesions as parents for F₂ and F₃ generations, respectively. In this case, both rounds of selection resulted in a significant reduction in mean lesion length, from 33.8 ± 0.8 mm in the F₁ generation, to 19.7 ± 0.7 and 12.9 ± 0.7 mm in the F₂ and F₃ generations, respectively. Thus it is apparent that F. circinatum retains the genetic capacity for avirulence to pines, which could reflect a lack of strong selection for virulence in nature. Progeny of a cross between high and low virulence parents manifested nearly continuous variation in lesion lengths, consistent with virulence being a quantitatively inherited trait. Based on this cross, broad‐sense heritability (H²) was determined to be 0.74, which suggests that virulence is under strong genetic control.     

Alternative species to replace Monterey pine plantations affected by pitch canker caused by Fusarium circinatum in northern Spain

The pitch canker pathogen Fusarium circinatum was first found to cause damage in nurseries and pine plantations in northern Spain in 2004. Since then, establishment of pine plantations in the region has decreased as a result of the prohibitions placed on planting Pinus spp. and Pseudotsuga menziesii in areas affected by the disease. However, although most pine species have been found to be susceptible to the pathogen under nursery conditions, little is known about how the fungus affects the trees in the field. Furthermore, it is not known whether some of the native or exotic species commonly planted in the area are also susceptible to F. circinatum. The aim of this study was to evaluate the susceptibility of several conifer species commonly planted in northern Spain to the pitch canker pathogen. For this purpose, two different trials were carried out, one under controlled laboratory conditions and the other in the field. Although most of the conifers were affected by the pathogen in the laboratory tests, only Pinus radiata, Pinus nigra, Pinus pinaster and Pinus uncinata were susceptible to the pathogen in the field.     

Evidence of low levels of genetic diversity for the Phytophthora austrocedrae population in Patagonia,Argentina

Phytophthora austrocedrae is a recently discovered pathogen that causes severe mortality of Austrocedrus chilensis in Patagonia. The high level of susceptibility of the host tree, together with the distribution pattern of the pathogen, have led to the hypothesis that P. austrocedrae was introduced into Argentina. The aim of this study was to assess the population structure of P. austrocedrae isolates from Argentina in order to gain an understanding of the origin and spread of the pathogen. Genetic diversity was determined based on amplified fragment length polymorphisms (AFLPs). In total, 48 isolates of P. austrocedrae were obtained from infected A. chilensis trees, representing the geographical range of the host. Four primer combinations were used for the AFLP analysis. Of the 332 scored bands, 12% were polymorphic. Gene diversity (h) ranged from 0·01 to 0·03; the Shannon index (I) ranged from 0·01 to 0·04. A high degree of genetic similarity was observed among the isolates (pairwise S values = 0·958–1; 0·993 ± 0·009, mean ± SD). A frequency histogram showed that most of the isolate pairs were identical. Principal coordinate analysis using three‐dimensional plots did not group any of the isolates based on their geographical origin. The low genetic diversity (within and between sites) and absence of population structure linked to geographic origin, together with the aggressiveness of the pathogen and the disease progression pattern, suggest that P. austrocedrae might have been introduced into Argentina.     

Long‐term survival of Acidovorax citrulli in citron melon (Citrullus lanatus var. citroides) seeds

Long‐term survival of Acidovorax citrulli in citron melon (Citrullus lanatus var. citroides) seeds was investigated. Citron melon seed lots infected with A. citrulli were generated in the field by inoculating either the pistils (stigma) or pericarps (ovary wall) of the female blossoms. Seventeen A. citrulli isolates from 14 different haplotypes belonging to two different groups (group I and II) were used for inoculation. After confirming that 100% of seed lots were infected, they were stored at 4°C and 50% RH for 7 years. After storage, the viability of A. citrulli cells from individual lots was determined by plating macerated seeds on semiselective medium as well as growing seeds for 14 days and scoring for bacterial fruit blotch symptoms. The type of A. citrulli isolate (group I or group II) used did not significantly influence bacterial survival. However, A. citrulli survival was significantly greater in seed lots generated via pistil inoculation (52·9 and 29·4%) than via pericarp inoculation (23·5 and 17·6%). Repetitive extragenic palindrome (rep)‐PCR on A. citrulli isolated from citron melon seed lots after storage displayed similar fingerprinting patterns to those of the reference strains originally used for blossom inoculation, indicating that cross‐contamination did not occur. The results indicate that A. citrulli may survive/overwinter in citron melon seeds for at least 7 years and bacterial survival in seed was influenced more by method of blossom inoculation than by the type of bacterial isolate.     

Temporal and spatial propagule deposition patterns of the emerging fungal pathogen of chestnut Gnomoniopsis castaneae in orchards of north-western Italy

Two chestnut (Castanea sativa) orchards of north-western Italy were sampled with passive spore traps 35 times over 24 months. Samples were analysed through a newly developed quantitative PCR assay to quantify propagule loads of the emerging fungal pathogen Gnomoniopsis castaneae. Average propagule deposition patterns were assessed along with temporal and climatic variables, including sampling month and season, temperatures, relative humidity, precipitations, and wind. Machine learning algorithms combining information theory, fractal analysis, unbiased recursive partitioning, ordinary least squares and logistic regressions, were used to model propagule deposition patterns. The trained models were validated on independent data gathered from 24 samplings conducted in a third chestnut orchard during the same timeframe. Results showed that propagule deposition rate (DR) was variable within and among sites, with a site average ranging from 173 to 765 spores ⋅m⁻² ⋅h⁻¹. Propagule deposition was observed across all seasons, although the DR dropped substantially during wintertime (p < 0.05). Mean, maximum, and minimum temperatures, the growing degree days at 0 and 5℃ thresholds, and wind gust were all positively correlated (p < 0.05) with DR of G. castaneae. The trained models were all significant (p < 0.05), as well as their validation (p < 0.05). Fluctuations of propagule deposition throughout the year were consistent among sites and proved to be driven by temperatures. Wind gust was associated with the overall amount of propagules deposited at site level. In future, the increase in temperatures and strong winds as a result of climate change may boost the spread of G. castaneae.     

Temporal spore dispersal patterns of grapevine trunk pathogens in South Africa

Trunk disease pathogens of grapevines, viz. Phaeomoniella chlamydospora, Eutypa lata and several species in Botryosphaeriaceae, Phaeoacremonium and Phomopsis are known to infect fresh pruning wounds by means of air-borne inoculum released after rainfall or prolonged periods of high relative humidity. Recent surveys have demonstrated that most or all of these pathogens are present in climatically diverse grape growing regions of South Africa. However, the factors controlling spore dispersal of these pathogens in vineyards were largely unknown. To address this question, spore trapping was done in a Chenin Blanc vineyard in the Stellenbosch area, South Africa, for 14 weeks during the grapevine pruning period from June to mid-September of 2004 and 2005. Hourly recordings of weather data were done by a weather station in the row adjacent to the spore trap. Spores of E. lata and Phomopsis and species in Botryosphaeriaceae were trapped throughout the trapping periods of 2004 and 2005, with higher levels of trapped spores recorded in 2005. The spores of all three pathogens were trapped during or after periods of rainfall and/or high relative humidity. In neither of the 2 years were spores of Pa. chlamydospora or Phaeoacremonium spp. trapped. Results indicated that spore event incidence, as well as the amount of spores released during a spore event of above-mentioned pathogens, were governed by rainfall, relative humidity, temperature and wind speed prior to and during the spore events.     

Potential for eradication of the exotic plant pathogens Phytophthora kernoviae and Phytophthora ramorum during composting

Temperature and exposure time effects on Phytophthora kernoviae and Phytophthora ramorum viability were examined in flasks of compost and in a large‐scale composting system containing plant waste. Cellophane, rhododendron leaf and peat‐based inoculum of P. kernoviae and P. ramorum isolates were used in flasks; naturally infected leaves were inserted into a large‐scale system. Exposures of 5 and 10 days respectively at a mean temperature of 35°C in flask and large‐scale composts reduced P. kernoviae and P. ramorum inocula to below detection limits using semi‐selective culturing. Although P. ramorum was undetectable after a 1‐day exposure of inoculum to compost at 40°C in flasks, it survived on leaves exposed to a mean temperature of 40·9°C for 5 days in a large‐scale composting system. No survival of P. ramorum was detected after exposure of infected leaves for 5 days to a mean temperature of ≥41·9°C (32·8°C for P. kernoviae) or for 10 days at ≥31·8°C (25·9°C for Phytophthora pseudosyringae on infected bilberry stems) in large‐scale systems. Fitted survival probabilities of P. ramorum on infected leaves exposed in a large‐scale system for 5 days at 45°C or for 10 days at 35°C were <3%, for an average initial infection level of leaves of 59·2%. RNA quantification to measure viability was shown to be unreliable in environments that favour RNA preservation: high levels of ITS1 RNA were recovered from P. kernoviae‐ and P. ramorum‐infected leaves exposed to composting plant wastes at >53°C, when all culture results were negative.     

Sulphate and sulphurous acid alter the relative susceptibility of wheat to Phaeosphaeria nodorum and Mycosphaerella graminicola

The relative abundances of DNA of Mycosphaerella graminicola and Phaeosphaeria nodorum in archived wheat samples are closely correlated with UK anthropogenic emissions of oxidized sulphur over the last 160 years. To test whether this could be a causal relationship, possible modes of action of sulphur on the two fungi were examined. Mycelial growth of the two fungi in solutions of sulphurous acid was similar. Sulphurous acid at pH 4 reduced percentage germination of P. nodorum conidia more strongly than M. graminicola conidia. In spray inoculations of wheat cv. Squarehead's Master, Cappelle Desprez and Riband with water or sulphurous acid (pH 4), the ratio of leaves infected by P. nodorum to leaves infected by M. graminicola was increased by factors of 2·5, 2·1 and 0·6, respectively at pH 4. The same three cultivars of wheat were grown in sand and vermiculite and fertilized with nutrient solution containing 2·5 or 0·5 mm sulphate. Both pathogens infected less frequently at 2·5 mm sulphate, by a factor of about 2. The severity of infection by M. graminicola was reduced on all three cultivars by a factor of about 4·5 at 2·5 mm sulphate, but severity of P. nodorum was reduced only by a factor of about 2. Both elevated free sulphate concentrations in soil and sulphite in rainwater could therefore increase the prevalence of P. nodorum relative to M. graminicola, which is consistent with the historical changes in abundance.     

Current status of red palm weevil in Turkey

The red palm weevil Rhynchophorus ferrugineus (Olivier) is the most serious pest of palms in Turkey. Weevil infestation was first detected in Turkey in summer 2005 in parks and gardens of Mersin province, along the Mediterreanean coast of Turkey. Following the discovery of the pest, destruction of infested plant material, prophylactic insecticide chemical treatments, as well as adult weevil trapping were carried out on palm plantations. Traps containing a commercial aggregation pheromone were hung on palm trees at a high density, in order to monitor the pest infestation and reduce the weevil population by mass trapping. A significant decrease in the number of trapped beetles and destruction of infested plant material was observed in 2009 and continued in the following years in several cities in Turkey. Therefore, it has been observed that mass trapping and curative pesticide applications have played a significant role in the suppression of R. ferrugineus populations in palm plantations.     

Plum pox virus as a stress factor in the vegetative growth, fruit growth and yield of plum (Prunus domestica) cv. ‘Cacanska Rodna’

During 2005–2007, under the environmental conditions of Cacak (43°53’N; 20°21’E), western Serbia, the effect of Plum pox virus (PPV) on the phenology of vegetative shoot and fruit growth was investigated in a plum orchard of cv. ‘Cacanska Rodna’ including trees non-infected with PPV and those with infection and clearly visible symptoms, as verified by RT-PCR. The results showed that PPV did not affect the growth phenology of the vegetative shoot (length and thickness) and the fruit growth. However, it caused negative effects on the average length (22.31 ± 0.28 cm in non-infected trees, 18.35 ± 0.07 cm in infected trees) and thickness (4.21 ± 0.03 mm in non-infected trees, 4.01 ± 0.02 mm in infected trees) of vegetative shoots as well as on the average fruit weight (20.43 ± 0.16 g in non-infected fruits, 17.58 ± 0.18 g in infected fruits) and fruit dimensions. The greatest effect of PPV during the trial was the induction of a massive premature fruit drop and, consequently, a 52.94% decrease in total yields per tree (18.0 ± 1.10 kg) and unit area (12.0 ± 0.89 t ha^-1) in infected vs non-infected trees.     

Relationship between disease severity and escape of Pseudoperonospora cubensis sporangia from a cucumber canopy during downy mildew epidemics

Fundamental to the development of models to predict the spread of cucurbit downy mildew is the ability to determine the escape of Pseudoperonospora cubensis sporangia from infected fields. Aerial concentrations of sporangia, C (sporangia m^?3), were monitored using Rotorod samplers deployed at 0·5 to 3·0 m above a naturally infected cucumber canopy in two sites in central and eastern North Carolina in 2011, where disease severity ranged from 1 to 40%. Standing crop of sporangia was assessed each morning at 07·00 h EDT and ranged from 320 to 16 170 sporangia m^?2. Disease severity and height above the canopy significantly (P < 0·0001) affected C with mean concentration (C_m) being high at moderate disease. Values of C_m decreased rapidly with canopy height and at a height of 2·0 m, C_m was only 7% of values measured at 0·5 m when disease was moderate. Daily total flux (F_D) was dependent on disease severity and ranged from 5·9 to 2242·3 sporangia m^?2. The fraction of available sporangia that escaped the canopy increased from 0·028 to 0·171 as average wind speed above the canopy for periods of high C increased from 1·7 to 3·6 m s^?1. Variations of C_m and F_D with increasing disease were well described (P < 0·0001) by a log‐normal model with 15% as the threshold above which C_m and F_D decreased as disease severity increased. These results indicate that disease severity should be used to adjust sporangia escape in spore transport simulation models that are used to predict the risk of spread of cucurbit downy mildew.     

Spatial and temporal dynamics of Meloidogyne minor on creeping bentgrass in golf greens

Meloidogyne minor, first reported on potatoes in the Netherlands in 2004, is an emerging nematode pest in Europe. It damages turfgrass, particularly creeping bentgrass (Agrostis stolonifera) grown on sandy soils such as those of golf greens. However, little is known of the nematode's life history and pathology. In this study, the spatial and temporal distribution of M. minor on a creeping bentgrass green in Ireland was determined over a 15 month period. Cores were taken on transects across yellowing patches of grass caused by nematode damage. Second‐stage juveniles (J2) were absent from the soil from November to February, when soil temperatures were below 10°C. Both galls and egg masses were present throughout the year but were more abundant in late summer and early autumn. More J2, galls and egg masses were present in the top 10 cm of soil than at a depth of 11–20 cm. The nematode population tended to decrease as distance from the centre of the yellow patches increased. The diameter of visual symptoms (yellow patches) was also recorded over the 15 months. The mean diameter of five sampled patches increased from 23·7 cm in June 2003 to 45·2 cm in August 2004. There were 158–193 galls per 100 cm³ soil at the margin of the visible infested area, indicating that this could be the threshold level for visible symptoms.     

Evidence for a new introduction of the pitch canker fungus Fusarium circinatum in South Africa

Fusarium circinatum causes pitch canker of Pinus species in many parts of the world. The fungus was first recorded in South Africa in 1990 as a pathogen of P. patula seedlings and emerged later as a pathogen of established plantation trees, especially P. radiata in the Western Cape Province (WCP). In this study the population biology of F. circinatum in the WCP was explored. The aim was to determine the possible origin and reproductive mode of the pathogen, with the ultimate intention of informing disease management strategies in the region. Vegetative compatibility assays, sexual mating studies and amplified fragment length polymorphism analyses were used. For comparative purposes, an isolate collection obtained from diseased P. radiata seedlings in a commercial nursery in the region, as well as a set of isolates from commercial seedling nurseries in the central and northern parts of South Africa, were included. The results showed that the WCP population of F. circinatum employs a predominantly asexual mode of reproduction and that it is highly differentiated from populations of the fungus elsewhere in South Africa. However, limited genetic structure was found within the respective WCP isolate collections. Overall these findings suggest that pitch canker in the WCP originates from one or more separate introductions of the pathogen and that its movement in the region is not restricted. More effective strategies are thus required to limit and manage the effects of F. circinatum in plantations in this region of South Africa.     

Evidence that wheat cultivars differ in their ability to build up inoculum of the take‐all fungus,Gaeumannomyces graminis var. tritici,under a first wheat crop

The effect of wheat cultivar on the build‐up of take‐all inoculum during a first wheat crop was measured after harvest using a soil core bioassay in field experiments over five growing seasons (2003–2008). Cultivar differences in individual years were explored by analysis of variance and a cross‐season Residual Maximum Likelihood (REML) variance components analysis was used to compare differences in those cultivars present in all years. Differences between cultivars in the build‐up of inoculum were close to or at significance in two of the five trial years (2004 P < 0·05; 2006 P < 0·07), and current commercially listed cultivars were represented at both extremes of the range. In 2007 and 2008, when environmental conditions were most favourable for inoculum build‐up, differences were not significant (P < 0·3). In 2005 the presence of Phialophora spp. at the trial site restricted the build‐up of take‐all inoculum under all cultivars. The cross season REML variance components analysis detected significant differences (range: 3·4–47·8% roots infected in the soil core bioassay; P < 0·01) between the nine cultivars present in all years (excluding 2005). This is the first evidence of relatively consistent differences between hexaploid wheat cultivars in their interactions with the take‐all fungus, and this could give an indication of those cultivars that could be grown as a first wheat crop, in order to reduce the risk of damaging take‐all in a second wheat crop. This phenomenon has been named the take‐all inoculum build‐up (TAB) trait.     

Infection routes for Sclerotinia sclerotiorum in apetalous and fully petalled winter oilseed rape

Near‐isogenic lines (NILs) of apetalous (AP) and fully petalled (FP) winter oilseed rape were used to investigate infection by Sclerotinia sclerotiorum, which occurs mainly via infected petals adhering to leaves in FP oilseed rape. AP1 flowers had an average of 1·4 and 0·8 petals per flower in field and polytunnel experiments, respectively. In field experiments there were no significant differences between counts of FP1 petals, FP1 stamens and AP1 stamens adhered to leaves during flowering. At any one sample time, significantly more stamens tested positive for S. sclerotiorum on AP1 than FP1 NILs, e.g. in 2004, at early flowering 37·5% and 24·2% of stamens tested positive on AP1 and FP1 NILs, respectively. In polytunnel experiments, there were significantly more sclerotinia lesions per plant in the FP1 than in the AP1 NIL. The AP1 NIL did not avoid infection completely, probably because it produced some petals, and lesions were initiated from adhered stamens as well as petals. However, while 8·5% and 16·3% of petals initiated lesions in FP1 and AP1 NILs, respectively, only 2·5% and 1·0% of stamens initiated lesions in FP1 and AP1 NILs, which suggests stamens may be less infective than petals. In field experiments the AP1 NIL had significantly less incidence of sclerotinia stem rot than the FP1 NIL in 2004 (4·9% and 7·0%, respectively). However, there was no significant difference in stem rot incidence between AP and FP lines in 2005 (3·6% and 4·3%, respectively) or 2006 (5·5% and 3·9%, respectively).