title / Biology and epidemiology of rose downy mildew (Peronospora sparsa)
/ DEFRA
project code / HH1749SHN
Department for Environment, Food and Rural Affairs CSG 15
Research and Development
Final Project Report
(Not to be used for LINK projects)
Two hard copies of this form should be returned to:Research Policy and International Division, Final Reports Unit
DEFRA, Area 301
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Project title / Biology and epidemiology of rose downy mildew (Peronospora sparsa)
DEFRA project code / HH1749SHN
Contractor organisation and location / Horticulture Research International
East Malling, West Malling
Kent, ME19 6BJ
Total DEFRA project costs / £ 253,050
Project start date / 01/04/99 / Project end date / 31/03/02
Executive summary (maximum 2 sides A4)
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CSG 15 (9/01) 3
Projecttitle / Biology and epidemiology of rose downy mildew (Peronospora sparsa)
/ DEFRA
project code / HH1749SHN
Downy mildew, caused by Peronospora sparsa, is a serious disease on roses. Most published research on rose downy mildew has been concerned with its distribution and on attempts at its control. Research on the epidemiology of the disease is very sparse and often based on casual observations, rather than systematic studies. As a result, the biology and epidemiology of this disease are poorly understood and this hinders the development of a truely rational disease management strategy. Currently, control of downy mildew in roses relies almost totally on routine fungicide applications, especially in crops grown outdoors. Successful control with fungicides usually only results from the implementation of routine programmes with a maximum of 10-14 day intervals between applications. In U.K. container-grown crops this can mean 10-15 sprays applied over the period from leaf emergence in spring to the onset of dormancy in autumn.
The project reported here aimed to provide a sound base of knowledge for the development of sustainable management of rose downy mildew. It also aimed to provide essential techniques for future studies of host-pathogen genetics and for breeding cultivars with durable resistance. These techniques may also enable similar work on downy mildews of other HONS or ornamental crops to be carried out. By providing the basis for development of more rational disease control, the research was aimed at assisting growers in increasing the quantity and quality of rose production whilst minimising the use of fungicides to satisfy public demand, which directly contributes to one of DEFRA main objectives of sustainable pest managemnet.
The main results relating to the five specific objectives are given below:
1. Overwintering. Field observations on disease development and histological examinations of infected leaves suggest that oospores are likely to be the primary overwintering inoculum, in addition to mycelia on the wood. Thus, it is important to dispose of infected leaf litter as one of measures to reduce the level of primary inoculum. Further research is needed to understand the formation and viability/pathogenicity of oospores in relation to environmental conditions and to determine the potential of biological control treatments of infected leaves to destroy oospores or prevent oospores from forming.
2. Host resistance/susceptibility. Micro-propagated plants were shown not to be suitable for use in testing host resistance because of their frequent surface contamination by bacteria and their generally enhanced mortality after inoculation with downy mildew conidia irrespective of cultivar. An in vitro detached leaf technique was developed for assessing host susceptibility or resistance to downy mildew. Preliminary results on the usefulness of the method are promising, however further confirmatory studies are needed using a wider range of rose cultivars and species.
3. Systemic infection spreading from normal foliar symptoms. Histological observations showed that the fungus failed to invade the vascular tissues in either leaves or petioles and thus establish systemic infection. Indeed, few fungal structures, other than oospores, were observed in infected leaves. When this is considered in combination with the results on overwintering studies, it appears that systemic infection is unlikely for rose downy mildew under normal UK conditions. Consequently, molecular detection of P. sparsa should not be treated as a priority research area. This work also indicates that primary inoculum sources are likely to be of local origin.
4a. Sporulation and isolate storage. Rose downy mildew is renowned for its generally sparse asexual sporulation, hence its species epithet sparsa. Present studies showed that abundant sporulation is possible under the right conditions and furthermore, free water is not necessary for sporulation. Detached leaves were shown to be effective for in vitro spore production and storage. However, one of the big problems in using detached leaves is the frequent contamination by other fungi, especially on leaves stored for more than several weeks, suggesting that P. sparsa is a weak competitor, compared with other fungi. One of best ways to maintain rose downy mildew is using whole plants under intermittent wetting regimes, thus avoiding conditions suitable for growth of saprophytes and maintaining the downy mildew’s biotrophic advantage.
4b. Infection. A linear model satisfactorily described the effect of wetness duration on the incidence of downy mildew; wet periods greater than 16 h are needed to cause a significant amount of infection. Temperature, in the range of 5-20°C, had no significant effects on the incidence of downy mildew. Most importantly, conidia were shown to have a very high potency for infecting leaves: only 40 spores per leaflet resulted in the near maximum incidence of disease under the experimental conditions. Consequently, the comparatively few spores produced in field conditions are likely to be highly infective. These results suggest that a prolonged overhead irrigration regime should be avoided. Furthermore, overhead irrigation should be operated during times when leaf surfaces can dry in a relatively short time. For example at high temperatures, especially since temperature within the normal range (i.e. <= 18°C) had no discernable effects of infection.
5. Sampling. According to a computer simulation model, sampling disease across the prevailing wind direction will generate more informative data for analysis, especially the spatial aspect of the disease dynamics. When considered in conjunction with previous studies, it is clear from this work that selection of appropriate initial conditions and sampling methods is very important in experiments to maximise information on spatial aspects of plant disease epidemics. Caution may also be needed in interpreting the observed differences between spatial statistics reported in different studies since putative differences may only reflect, at least in part, differences in factors such as sampling details, initial conditions and wind conditions, and not biological factors.
CSG 15 (9/01) 3
Projecttitle / Biology and epidemiology of rose downy mildew (Peronospora sparsa)
/ DEFRA
project code / HH1749SHN
Scientific report (maximum 20 sides A4)
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CSG 15 (9/01) 3
Projecttitle / Biology and epidemiology of rose downy mildew (Peronospora sparsa)
/ MAFF
project code / HH1749SHN
1. INTRODUCTION
Downy mildew (Peronospora sparsa) on rose is found virtually everywhere that roses are cultivated. It is generally more common on glasshouse-grown roses than on plants growing outdoors. The disease was first described in 1862 in England and from then into the early 1900s was reported throughout Europe. Downy mildew on roses had spread to the Midwestern United States by 1880s and has more recently been seen in all parts of the USA. It also occurs on roses in Canada, New Zealand and Australia. In recent years its geographical distribution has expanded further with reports on its occurrence in both China and South Africa.
The disease can be very destructive but tends to be sporadic in its occurrence. Although not as common as powdery mildew, downy mildew is still considered in the UK to rank in the ‘top four’ most important foliar diseases of cultivated roses along with rust, powdery mildew and black spot. Symptoms of downy mildew occur on leaves, stems, peduncles, calyxes, and petals. Infection is generally restricted to young plant growth. Severe leaf abscission may occur. Purplish to black areas varying from small spots to areas 2 cm or more in diameter appear on stems and peduncles. Similar spots and dead tips develop on calyxes, and infected twigs may be killed. Leaf symptoms vary from angular blotches (yellow, purple to brown) to a scorch like burn.
Under humid, cool conditions, conidia and conidiophores appear copiously on the lower surfaces of leaves but, under less favourable conditions, spore production is sparse and difficult to detect. Regular overhead watering and dense plant growth from close spacing create ideal conditions for the disease to spread quickly under cool, humid conditions. Rose plants infected with the pathogen may not be obvious and the disease may occur only when environmental conditions are ideal. Oospores may be found in infected leaves, sepals, flower buds, and stems. The fungus may overwinter in stems as dormant mycelia without oospores. Sporangia may be produced for long periods of time when high humidity and cool temperatures persist. Sporangia germinate within four hours in water, and sporulation may occur on leaf surfaces in three days under ideal conditions. Spores may survive and be viable on dried fallen leaves for as long as one month. More detailed information on conditions for spore production and infection is lacking, which is essential for adopting an integrated approach for managing this disease.
The fungus is believed to overwinter as dormant mycelium in cuttings and plants. However, the role of oospores in overwintering and initiating infections is less certain. The ability of downy mildews to form systemic infections in a wide range of herbaceous plants is well known (Spencer, 1981). This is especially important in woody perennials which are vegetatively propagated from clonal stock. On raspberry, the downy mildew fungus (Peronospora rubi) readily infects leaves, flowers, developing fruits and stems. However, the spread of this pathogen into the vascular tissues of veins in raspberry was very limited. The fungus was confined to the cortex, extending only a few centimetres beneath infected nodes, whilst oospores were not seen in this tissue (Williamson et al., 1995). Mycelium of P. sparsa may survive the winter in the cortex of rose stems (Wheeler, 1981). Rose downy mildew sometimes starts on bare-rooted, apparently healthy stocks. However, it is not known to what extent the systemic infection occurs in roses under UK conditions, or what the role of oospores is in the survival of the pathogen.
Many statistical methods have been developed and used to analyse the spatio-temporal dynamics of plant disease epidemics and these have been reviewed recently (Zadoks & van den Bosch, 1994; Madden & Hughes, 1995; Nelson, 1995; Ferrandino, 1996; Hughes et al., 1997; Ferrandino, 1998). These spatial statistics include parameter estimates from fitting distributions, such as beta-binomial parameter q, spatial autocorrelation, intraclass correlation, estimated parameters of the binary power law model, and statistics associated with distance class analysis. Their usefulness depends critically on our understanding of their dependence on underlying biological and physical factors. Previous theoretical research (Madden & Hughes, 1995; Yamamura, 1990; Li & Reynolds, 1995; Yang, 1995; Xu & Ridout, 1998) has shown the importance of initial epidemic conditions, spore dispersal gradient and sampling schemes in determining these spatial statistics. Wind speed/turbulence is known to affect spore dispersal greatly for both rain-splashed and dry dispersed fungal spores (Fitt et al., 1987; Bock et al., 1997; Aylor, 1999). However, it is not known how important wind is in affecting spatial dynamics compared to other factors such as initial epidemic conditions. Furthermore, under field conditions where there is a prevailing wind direction, the orientation of quadrats is likely to affect estimated spatial statistics.
2. OBJECTIVES
Most published research on rose downy mildew has been concerned with its distribution and on rudimentary attempts at its control. Research on the epidemiology of the disease is very sparse and often based on casual observations, rather than systematic studies. As a result, current knowledge on the biology and epidemiology of the rose downy mildew fungus is very limited. Thus it is not surprising that current control of downy mildew in roses relies almost totally on routine fungicide applications, especially in crops grown outdoors. Successful control with fungicides usually only results from the implementation of routine programmes with 10-14 day intervals between applications. In U.K. container-grown crops this can mean 10-15 sprays applied over the period from leaf emergence in spring to the onset of dormancy in autumn.
This study aims to fill such knowledge gaps and to provide information for underpinning development of sustainable disease management strategies. Particularly, this study has five specific objectives:
1) To observe and determine the overwintering structures.
2) To develop an efficient technique for assessing plant susceptibility/resistance.
3) To determine the possibility of the fungus invading vascular tissues and thus establishing the probability of systemic infection by conducting histological studies.
4) To determine the effects of environmental conditions on infection and sporulation processes.
5) To design an optimum scheme for sampling disease using a stochastic simulation model, especially considering the effects of wind direction and speed on pathogen dispersal.
3. INOCULUM COLLECTION, PREPARATION AND STORAGE (key to Objectives 2, 3 & 4)
3.1 Collection of rose P. sparsa isolates
Initially, we had great difficulties in obtaining isolates. Although our request for infected material for isolates was taken on seriously by the industry and the Rose Growers’ Association, the disease was apparently not observed in nurseries during most of late Spring and Summer 1999 despite severe epidemics in the previous season. Two samples of suspected downy mildew were received early in the 1999 season, but no successful isolations were made from these (both had received sprays already), and the producers concerned were subsequently able to control the disease with intensive spray programmes (implemented partly to avoid similar losses to downy mildew to those experienced in severe epidemics during Spring 1998). In addition, several samples were received which proved not to be downy mildew. Indeed, it appears that on occasion, some growers may have difficulties in distinguishing between rose downy mildew and certain types of black spot (Diplocarpon rosae) lesion. This can have serious repercussions as the effective fungicides for the two pathogens are quite different. Identification photographs and instruction on inspection of suspect lesions using a hand lens have subsequently been distributed directly to the growers involved with helping the project.