Factsheet 02/08 Horticultural Bradbourne House Development East Malling Hebe Council Kent ME19 6DZ T: 01732 848383 F: 01732 848498 Project HNS 128 E: [email protected]

Stemphylium leaf-spot and other foliar diseases of hebe Dr Martin McPherson, Stockbridge Technology Centre Ltd

This Factsheet provides information on the newly identified leaf-spot disease of hebe caused by a sp. It also provides information on leaf spot and other foliar diseases affecting the crop. It should assist in their accurate identification and help in the implementation of appropriate control measures.

Background The initial studies showed that Septoria insensitive (resistant) to carbendazim. was not present in the leaf-spot lesions Also, preliminary inoculation studies Following the occurrence of an unusual but instead a different , sub- at STC successfully reproduced leaf- and severe leaf-spot affecting several sequently identified as a Stemphylium spot symptoms and indicated that hebe cultivars in the North of England species (Figure 1), was consistently the fungus may be the primary cause in autumn 2003, an HDC project (HNS recovered. This latter fungus was of the new leaf-spot problem. 128) was commissioned to investigate the distribution of the problem, its cause, biology and control. Previously, the predominant leaf-spot found on hebe has been caused by the fungus Septoria exotica which is characterised by the development of distinctive fungal fruiting bodies (pycnidia) in the centre of the lesions. For many years, Septoria leaf spot has been successfully controlled using fungicides containing carbendazim (eg Delsene 50 Flo – see fungicide section). However, industry reports of poor control with conventional spray programmes containing carbendazim prompted further investigation to determine if the problem was arising as a result of fungicide resistance or whether there was some other cause. 1 Hebe ‘Red Edge’ with a severe infection by Stemphylium

Stemphylium Septoria which develops a dry pale leaf-spot problem was in the UK. The centre to individual lesions, and which study also requested growers to submit leaf-spot have a tendency to coalesce. The samples of hebe with leaf-spot symp- infected leaves either shrivel prema- toms to ascertain the primary cause of Symptoms turely and hang on the plant or drop off. the problem. Over 70% of the hebe samples subsequently received were The fungus produces large numbers confirmed as being infected with of small (1– 3mm) dark brown/purple Distribution Stemphylium leaf-spot. Yet, none of spots on the leaves (Figure 2 overleaf). the samples were found to be infected These neither coalesce (merge) nor A nationwide questionnaire was with Septoria exotica. This indicates a enlarge significantly. The centre of the distributed to commercial hebe growers significant shift in the pathogen popu- lesion remains the same colour, unlike to determine how widespread the lation on this host in the last few years. The precise reason for such a dynamic are not known precisely though the be regarded as immune, some cultivars shift is not clear, though, it has been pathogen does appear to spread more do appear to be much less susceptible speculated that it could have arisen as rapidly during periods of warm wet to Stemphylium leaf-spot. a result of selection pressure following weather (above 25ºC). Therefore, the repeated use of carbendazim-based disease is more likely to be problematic products, over several years. This during summer and early autumn and Cultural control hypothesis was later supported when this is when fungicides need to be it was demonstrated in fungicide applied more frequently to protect crops Attention to cultural control is impor- sensitivity tests in the laboratory that from infection. Spore trapping studies tant to minimise disease risk. the Stemphylium species was highly also supported this observation with Plants should be spaced out well to resistant to carbendazim; and hence few spores being detected during the encourage good air circulation. This not controlled by the fungicide pro- cooler winter months. allows the leaves to dry out more gramme applied routinely on this crop. quickly and reduces their vulnerability to infection from this, and other, leaf Nomenclature infecting pathogens. Similarly, on Infection and spread protected crops, overhead watering A molecular examination of the new should be avoided, where possible, The fungus produces moderate leaf-spot pathogen showed a 96% by using drip or sub-irrigation. Where numbers of asexual spores (conidia) similarity with Stemphylium solani this is not practical or economically on infected leaves and these are and this name will be used on a viable, any overhead watering should believed to be the primary means of presumptive basis to describe this be timed to ensure there is a chance disease spread between plants on new pathogen of Hebe. for the foliage to thoroughly dry out the nursery. during the day. Some Stemphylium species are For outdoor crops, subject to inter- also reported to produce a perfect Varietal susceptibility mittent rainfall, this is naturally more or sexual stage () under difficult and it is likely that alternative certain conditions and spores From observations on nurseries and techniques to minimise disease (ascospores) liberated from fruiting in replicated studies it is evident that risk will need to be deployed. As there bodies (perithecia) could potentially there are considerable differences in continues to be a potential risk of account for over-wintering survival relative susceptibility between different carry-over on leaf litter, efforts should of the fungus and long distance hebe cultivars and types. Whilst no be made to remove infected leaf dissemination. However, during the cultivars have been found which could debris between crops. two-year investigation, the sexual stage of the fungus was not found associated with infected hebe plants Graph 1 Relative Performance of Different Fungicide Products against or leaf litter material retained under Stemphylium leaf-spot on Hebe ‘Red Edge’ a range of different temperature and moisture conditions. 4 The conditions under which the spores are formed, liberated and spread 3.5

3

2.5

2

1.5

1

0.5 Mean number of lesions/shoot 0 Octave Shirlan Amistar Rovral Plover Frupica Systhane Scala Bavistin Folicur Untreated Signum

18 Oct 2004 4 Feb 2005 15 Nov 2004

2 Distinctive leaf spotting on Hebe caused NB: Three assessments of leaf-spot incidence were conducted during the period October 2004 to by Stemphylium February 2005. Fungicide control effective control of Stemphylium with Action points for the a limited number of well-timed sprays. control of Stemphylium As early studies demonstrated that Ideally this would need to be aligned leaf-spot the Stemphylium pathogen was not with information on climatic factors controlled by carbendazim, it was and disease risk eg inoculum (spore) • Growers should acquaint themselves important to find alternative effective load for optimum results. with this new disease and monitor fungicides. A number of potential crops closely. products were identified in a primary screen and subsequently assessed Crop and cultivar safety • Where leaf-spotting appears, submit on the susceptible cultivar ‘Red Edge’. a sample to a diagnostic laboratory Several products, including Signum The four products that significantly to confirm the primary cause. (pyraclostrobin & boscalid), Octave reduced Stemphyliuim (Amistar, (prochloraz-Mn), Amistar (azoxystrobin) Octave, Rovral & Signum) were tested • Once the pathogen has been and Rovral (iprodione), proved to be at different rates of use (including identified accurately, consider the very effective in reducing the incidence double the recommended rate) and various options for cultural control. and severity of Stemphylium leaf-spot produced no evidence of crop in these studies (Graph 1). Whilst damage on a broad range of com- • Adjust the fungicide spray program- Shirlan (fluazinam) was also moderately mercially available Hebe types me, as necessary, to secure effective, reports of skin sensitisation and cultivars. However, such tests improved efficacy against this fungus in particularly sensitive individuals may were limited to one season, so (see fungicide section). preclude its use where plants are likely growers are advised to test treat a to be handled regularly. small batch of plants to satisfy • Reduce the risk of resistance deve- Limited studies undertaken to themselves of product safety on the loping by alternating at least two but investigate fungicide timing suggest cultivars being grown before making preferably three fungicide products that it may be possible to secure an application. with different modes of action.

Other leaf diseases important pathogens that affect (Botrytis cinerea) and web blight the quality of hebe plants. The (Rhizoctonia solani). in hebe most economically important of these are Septoria exotica (as In addition to Stemphylium leaf- mentioned above), downy mildew spot there are a number of other (Peronospora grisea), grey mould

Septoria leaf-spot with a tendency to merge or coalesce. The centre of the lesion is pale with Background small fungal fruiting bodies (pycnidia) embedded in the decaying leaf tissues Septoria leaf-spot (Septoria exotica) (Figure 3); though these may only be has been regarded for the last 25 – 30 visible with a hand-lens. years as the most common leaf-spot disease in commercial hebe crops. Consultant plant pathologists have Infection and spread previously advocated the use of carbendazim-based fungicides to The fruiting bodies produce prolific maintain effective control (see fungicide numbers of spores which are dispersed section). When leaf-spot problems by water-splash and hence the disease recently increased in severity, it was is most severe in wet seasons. initially assumed that it was caused by the same pathogen, but investigations showed that it was not. Control

Whilst it is likely that carbendazim or Symptoms other benzimidazole (mbc) fungicides would still provide effective control of The symptoms caused by Septoria Septoria, no tests have been under- are slightly different to those of taken recently to confirm this. However, Stemphylium. The lesions are more it is important to note that as part of 3 Typical symptoms caused by Septoria exotica variable in size, generally being larger, the EU pesticide review process, with distinctive fruiting bodies in centre of lesions carbendazim has been revoked it is not possible to make specific to control Stemphylium leaf-spot may and at the time of printing, is in a recommendations for the future use have some activity against Septoria, use-up phase. The future availability of mbc fungicides and alternative though this has not been tested (see of alternative mbc fungicides in the products may need to be sought. fungicide section). UK also remains uncertain. As such, Some of the newer products found

Downy mildew examination by looking for the felt-like sible, prolonged periods of leaf wetness fungal growth on the underside of should be avoided. If employing (Peronospora grisea) the leaves, when systemic shoot overhead irrigation (Figure 6), it is best infection occurs sporulation can be to irrigate in the morning on suitable Background limited and closer microscopic days to ensure the foliage has time to examination by a qualified pathologist dry quickly. Alternatively, employing Downy mildew is probably one of the in the laboratory may be necessary capillary irrigation will reduce the risk most common and damaging foliar for accurate diagnosis. of further infection. diseases of hebe with some cultivars For outdoor crops exposed to regular being particularly susceptible (Figure 4). rainfall, space plants out to encourage Infection and spread good air circulation between plants and orientate the rows in line with the Symptoms The fungus requires cool wet weather prevailing wind if possible. As some for infection and spread. Where pos- cultivars are more susceptible than The symptoms of the disease are varied and particularly influenced by the overall susceptibility of the cultivar(s) being grown, the prevailing weather conditions, the inoculum or disease pressure and the age of the plants when they were first infected. Some downy mildew fungi produce a white fungal growth on the under-surface of the leaves (eg Lettuce). In contrast, on Hebe a brown/mauve felt-like fungal growth appears on the under-surface of the leaves (Figure 5). The fungus can also, under some circumstances, cause a systemic shoot infection leading to leaf necrosis and ultimately shoot dieback. Whilst the fungus can usually be identified quite readily during visual 5 Downy mildew produces a white fungal growth in lettuce (left) but a brown growth in Hebe (right)

4 Downy mildew (P. grisea) on Hebe 6 Careful management is required if employing overhead irrigation franciscana variegata others it is advisable, where appro- It is worth noting that if fungicide propamocarb hydrochloride (eg Filex priate, to select those that are more drenches are applied for the control or Proplant) are likely to provide good tolerant of the disease. of root diseases such as Phytophthora, control, though there are several other incidental control of downy mildew effective products that can potentially may be expected due to the systemic be used on non-edible ornamental Control nature of some of the products. crops either through the ‘Long Term It would be inadvisable however to Arrangements for Extension of Use’ Fungicides that control Stemphylium use the same fungicide for drench (LTAEU) (see fungicide section) or and/or Septoria leaf-spot diseases in and foliar application as this could Specific Off-Label Approvals (SOLA’s). hebe are unlikely to provide effective increase the risk of fungicide resist- Such products are used at the growers control of downy mildew. Instead, ance developing. own risk. specific oomycete fungicides are Products containing metalaxyl-M needed to provide the most effective + mancozeb (eg Fubol Gold), control of this disease. fosetyl aluminium (eg Aliette) and

Grey mould be found causing shoot dieback on to ‘ring the changes’ to minimise the weakened or damaged plants as this risk of resistance developing. (Botrytis cinerea) provides ideal conditions for infection Previous use of carbendazim based and subsequent colonisation. products has led to the development Background of resistant strains in the pathogen population and it is now largely Grey mould, caused by the fungus Control ineffective against Botrytis cinerea. Botrytis cinerea, is a ubiquitous air- There is also good evidence of borne pathogen and is very common, As for downy mildew, close attention strains developing reduced sensitivity affecting many plant species to improved air circulation is important to iprodione (Rovral), and again including, occasionally, some specific to ensure the foliage dries out quickly, here effective disease control may hebe cultivars eg ‘Green Glow’. though in this case, minimising damage be compromised. is also important due to the opportunist Fortunately, there are several nature of this fungus. alternative fungicide products with Infection and spread Strict attention to hygiene on the activity against Botrytis eg nursery is also important to minimise pyrimethanil (Scala), mepanipyrim The fungus requires cool wet the risk of carry-over of the patho- (Frupica) and fenhexamid (Teldor) conditions for optimum infection gen on decaying leaf litter and other and these, where possible, should be and it can usually be found in crop debris. used in conjunction with some of the consignments of plants that have In some circumstances, it may be older multi-site inhibitor fungicides like been packed too tightly where air necessary to consider employing pre- chlorothalonil (eg Bravo 500) as this circulation has been reduced. As ventative fungicide applications, but will help to minimise the risk of further an opportunist fungus it can also as with other pathogens it is important resistance problems developing.

Web blight Stirling’, Rhizoctonia solani can be diagnostic laboratory for analysis and problematic where it causes a identification. Where web blight is (Rhizoctonia solani) stem base infection and web blight. confirmed, treatment with an effective Correct diagnosis is important here fungicide eg tolclofos-methyl (Basilex) Occasionally, on particularly and, where necessary, it is advisable may be necessary. susceptible cultivars such as ‘James to submit suspect samples to a

Summary of specific off-label approvals (SOLAs) For the purpose of this factsheet, it was and (at the time of printing) via the considered appropriate to list some of the effective fungicides Long Term Arrangements for Extension more commonly used fungicide products of Use (LTAEU). It should be noted that are known to have activity against for use against however that the future of the LTAEU specific pathogens, that can be used on foliar diseases are uncertain and growers will both outdoor and protected crops, need to check further before using and that offer differing modes of action There are currently a wide range products via the LTAEU to ensure drawn from contrasting fungicide groups of fungicides available to hebe they are operating within the regula- (Table 1 overleaf). The alternation of such producers through full approvals, tory framework. products in a spray programme will effectively reduce the potential for devel- using fungicides on ornamentals do so For environmental protection it is opment of fungicide resistance and at their own risk and to ensure crop important to note all statutory prolong the useful life of the fungicide. safety, it is advisable to test treat a small and advisory information on the batch of plants if the product has not product label. been previously used on the nursery. Crop safety If the product is to be used under protection, ensure there is an approved With the exception of those with a full use for this activity to ensure spray on-label recommendation, growers operators are protected.

Table 1 Examples of fungicides for use either outdoors or under protection1 for the control of the primary leaf diseases on hebe The list is not exhaustive. No criticism is implied of products not included. P = protectant, S = systemic, T = translocated, TL = translaminar, C= contact, Cu = curative, E = eradicant (information based on BCPC UK Pesticide Guide 2007) please see the footnotes at the end of table on the next page for explanations

Primary or Active Product(s) Mode of Fungicide Additional comments Key Target ingredients (examples only) Action Group (examples only) (FRAC Code)

Downy Metalaxyl-M 2 + Fubol Gold WG S, P 4 + M3 A very effective acylalanine or phenyl- mildew mancozeb amide and dithiocarbamate fungicide. Moderate-high resistance risk 2

Fosetyl-aluminium Aliette 80 WG S 33 A useful systemic phosphonate fungicide

Propamocarb-HCl Filex or Proplant P, T 28 An effective carbamate fungicide with a different mode of action to products above

Stemphylium Pyraclostrobin + Signum P, S 11 + 7 Very effective QoI (strobilurin) and leaf-spot boscalid carboxamide fungicide. Likely to be reasonably broad-spectrum

Prochloraz-Mn Scotts Octave P, E 3 A good broad-spectrum DMI or imidazole fungicide

Azoxystrobin Amistar S, TL, P 11 An effective broad-spectrum QoI or strobilurin fungicide

Iprodione Rovral WP P, E 2 An effective dicarboximide protectant product

Septoria Carbendazim 3 Delsene 50 Flo S, P, Cu 1 An older but effective systemic leaf-spot MBC or benzimidazole fungicide – but ineffective against Stemphylium

Azoxystrobin Amistar S, TL, P 11 Potential for activity against this target – but untested

Prochloraz-Mn Scotts Octave P, E 3 Potential for activity against this target – but untested

Pyraclostrobin + Signum P, S 11 + 7 Potential for activity against this boscalid target – but untested

Botrytis grey Iprodione Rovral WP P, E 2 Partially effective – but some strains mould with reduced sensitivity

Table continued… Primary or Active Product(s) Mode of Fungicide Additional comments Key Target ingredients (examples only) Action Group (examples only) (FRAC Code)

Botrytis grey Prochloraz-Mn Scotts Octave P, E 3 Some activity against this target mould expected

Mepanipyrim Frupica SC P 9 An anilino-pyrimidine (AP) fungicide with good activity against Botrytis

Pyrimethanil Scala P 9 An anilino-pyrimidine (AP) fungicide with good activity against Botrytis

fenhexamid Teldor P 17 A good protectant hydroxyl-analide fungicide with a different mode of action to AP products

chlorothalonil Bravo 500 P M5 A multi-site protectant chloronitrile (phthalonitrile) fungicide useful in the early stages of disease development

Rhizoctonia Tolcolofos-methyl Basilex P 14 An effective aromatic hydrocarbon web blight fungicide

azoxystrobin Amistar S, TL, P 11 Some activity against Rhizoctonia demonstrated on other crops

1 Subject to the statutory conditions of high. Avoid repeated use of this fungicide 3 It should be noted that as part of the EU approval and, where SOLA’s and ‘Long Term by alternating with different mode of action pesticide review process, carbendazim has Arrangements for Extension of Use’ apply, at products. If poor control of downy mildew is been revoked and at the time of printing, is the growers own risk. observed following the use of this fungicide in a use-up phase. Growers should check the then switch to an alternative product from approval status carefully before use. 2 Resistance to fungicides containing metalaxyl- a different FRAC group. Report the incident M has been reported in some downy mildews to FRAG-UK using the on-line resistance (eg Bremia lactucae in lettuce) and the risk of reporting form: www.pesticides.gov.uk/rags. breakdown should be considered moderate- asp?id=644#Reporting

Suggested fungicide can have a significant bearing on 50 Flo) to maintain effective control infection levels in propagation and of Septoria exotica. However, it programmes every effort should be made to ensure is important to note that as part of the mother stock is free from foliar the EU pesticide review process, Different circumstances on different (and root) pathogens at the outset. carbendazim has been revoked and nurseries mean that it is not possible • Apply Aliette 80 WG either as a at the time of printing, is in a use-up to be prescriptive with respect to a drench or coarse spray or via phase (see fungicide section). single fungicide programme but the compost incorporation.This will following provides an example spray provide control of downy mildew • If Botrytis becomes evident it may programme as a guide that may be and Pythium/Phytophthora root be necessary to adjust the pro- helpful for some growers. rots. Follow this with foliar sprays gramme slightly to include specific of Fubol Gold or Filex/Proplant Botrytis fungicides such as at 14 – 21 day intervals during prochloraz-Mn (Octave), pyrimethanil Propagation and liner stage conditions of high disease pressure. (Scala), mepanipyrim (Frupica) or fenhexamid (Teldor). It is particularly important to protect • For leaf-spot control, apply Rovral crops during propagation from downy WP followed by Signum to maintain • If Rhizoctonia is diagnosed as a mildew & Stemphylium leaf-spot good protection through the problem on specific cultivars during infection, although consideration propagation period, repeating appli- the propagation stage then also should also be given to the other cations at approximately four week introduce either tolclofos-methyl pathogens mentioned, especially on intervals. Also, while it remains avail- (Basilex) or azoxystrobin (Amistar) cultivars known to be susceptible. able for use, introduce occasional into the programme. The source of the cutting material sprays of carbendazim (eg Delsene Potted plants Alternatively it may be possible to • Assuming the temperatures drop in consider some of the newer blight the autumn it should be possible to With potted plants, consider the sea- fungicides as approved on potatoes, significantly reduce the frequency sonal nature of some of the pathogens. subject to statutory conditions of of spray applications for leaf-spot Downy mildew is rarely a problem during use as specified on the product label control, though continued monitoring the summer, unless it remains cool, wet and the continued availability of will remain necessary. and humid, whereas Stemphylium leaf- the Long Term Arrangements for spot spreads most aggressively during Extension of Use (LTAEU). • From late August and September, the this period. Spray programmes should risk of downy mildew (and Botrytis) be constructed to take account of this • From late spring onwards, the focus increases again and the frequency and target specific pathogens when the should turn to controlling leaf-spot of appropriate fungicide applications crops are most vulnerable. However, a fungi (assuming downy mildew should be increased accordingly. degree of flexibility should be built in to has not carried through from early take account of the vagaries of the UK spring). Use alternating applications • On Botrytis susceptible cultivars, it is weather and the possible occurrence of Rovral WP, Signum & Octave at likely to be necessary to strengthen of unexpected or new problems in the approximately 2 – 4 week intervals. the fungicide programme through crop. Unusual leaf symptoms should Amistar may also be substituted the inclusion of Botrytis-specific be examined by a plant pathologist here if a broader spectrum of activity fungicides such as Scala, Frupica by submitting samples to a diagnostic is required eg Rhizoctonia control. or Teldor. plant clinic (see below). If Septoria is considered a risk then • In early spring the risk of downy substitute occasional sprays with • Continue to check susceptible mildew infection is high, so Aliette 80 carbendazim (eg Delsene 50 Flo). cultivars for web blight and if WG should be applied as a drench However, it is important to note Rhizoctonia is diagnosed then use (also to provide control of Pythium that as part of the EU pesticide either tolclofos-methyl (Basilex) or and Phytophthora root rots), followed review process, carbendazim has azoxystrobin (Amistar) as necessary. by a foliar application of a product been revoked and at the time of with a different mode of action such printing, is in a use-up phase (see as Fubol Gold or Filex/Proplant. fungicide section).

Further information • HDC Factsheet 24/02 – Project Plant diagnostic laboratories: PC/HNS 121 – Control of grey The Plant Clinic Other useful publications mould (Botrytis cinerea) in Stockbridge Technology Centre Ltd container-grown ornamentals: Cawood, Selby • HDC Factsheet 04/04 – Projects HNS heated glasshouse crops. North Yorkshire YO8 3TZ 53 and HNS 79: Control of downy Tel. (01757) 268275 mildew diseases on hardy nursery • HDC Factsheet 23/02 – Project www.stc-nyorks.co.uk stock and herbaceous perennials. PC/HNS 121 – Control of grey mould (Botrytis) in container-grown Central Science Laboratory • HDC Factsheet 25/02 – Project PC/ ornamentals: unheated green- Sand Hutton HNS 121 – Controlling humidity to house crops. York YO41 1LZ minimise the incidence of grey mould Tel. (01904) 462000 (Botrytis cinerea) in container-grown • HDC Pesticide checklist www.csl.gov.uk ornamentals: heated glasshouse crops. 2007 – shrubs.

Acknowledgements Beales, CSL for collaboration on both for their constructive comments on Project HNS 128 and this factsheet. the manuscript. Special thanks go to Cathryn Dr Tim O’Neill, ADAS, Mr Danny Lambourne, STC Research foundation, Elliott, HNS consultant and Ian Nelson, Dr Steve Parker, CSL and Dr Paul Johnsons of Whixley are also thanked

Further information: A full copy of the final Whilst publications issued under the auspices © 2008 Horticultural Development Council. report for HDC Project HNS 128 is available of the HDC are prepared from the best available No part of this publication may be reproduced in from the HDC office (01732 848383). information, neither the authors nor the HDC can any form or by any means without prior permis- accept any responsibility for inaccuracy or sion of the Horticultural Development Council. liability for loss, damage or injury from the appli- January 2008 cation of any concept or procedure discussed. Design and production: HDR Visual Communication