First record of Botrytis flower blight, caused by Botrytis cinerea on Chamelaucium uncinatum (Geraldton waxflower) in South Africa

Geraldton waxflower (Chamelaucium uncinatum Schauer, family Myrtaceae), indigenous to Western Australia, is cultivated commercially in South Africa as a cut flower crop and exported to markets in the Northern Hemisphere.^6,30,22 Flowers are small with waxy petals and softly scented green pointed leaves, and are popular as fillers.^2,9,31 In September 2000, disease symptoms were observed on 4-year-old plants in a commercial orchard of Chamelaucium uncinatum 'Ofir' in Philippe, South-Western Cape Province, South Africa. The base of the petals and the calyxes of the waxflowers showed brown necrotic lesions. Eventually the calyx and all the petals turned brown, the flowers shrivelled, and abscised. The disease did not move into the receptacle. Botrytis cinerea Pers.: Fr was consistently isolated from affected petal and calyx tissues.¹⁰ When placed in a moist chamber, the pathogen produced profuse conidia and mycelia on the surface of dead and infected tissue. Koch's postulates were confirmed by spraying flower stems of C. uncinatum 'Ofir' with a spore suspension (1x106 conidia ml^-1). Inoculated flower stems were placed in a bottle filled with water, and enclosed in a transparent plastic bag for 24 hours at 21°C. Typical symptoms developed on the petals and calyx within 3 days after inoculation. Botrytis cinerea was reisolated from affected tissues.¹⁰ Botrytis flower blight or grey mould, causing a flower petal disease, has been recorded on Chamelaucium uncinatum in Australia,^{5,16,24,30,31} but this is the first record of Botrytis on Chamelaucium uncinatum in South Africa. Because Geraldton waxflower is a major cut flower crop grown for export, this disease, resulting in flower drop, can have significant losses to the industry, especially under cool, wet growing conditions.

Chamelaucium uncinatum Schauer, Geraldton waxflower, is from the Myrtaceae family.³⁰ This ornamental, woody, perennial shrub belongs to a genus comprising of around 31 species all indigenous to Western Australia.^3,4,6,9 It is one of Australia's most famous wildflowers that are widely cultivated as cut flowers in Australia and in countries such as India, Israel Mexico, Peru, South Africa, Thailand and U.S.A. (California).^6,16,29 The plant is of increasing importance in the international floriculture industry,¹⁸ and is used as a landscape plant, a long-lasting cut flower and as a flowering pot plant.¹⁹ The plants grow well in South Africa's hot dry orchards.¹² Many hectares of waxflowers are grown by commercial flower growers for the export markets of the Northern Hemisphere,^6,22 because of their ease of cultivation and high productivity.²⁰ Chamelaucium uncinatum is usually a medium to large shrub of which the flowers appear in late winter and spring, and may last into the summer. The small flowers with five waxy petals are circular in shape, usually 15 to 20 mm diam.^2,9 (Fig. 1).

Fig. 1 Chamelaucium uncinatum

http://gardenbed.com/plant/pga_cascadebrill1.cfm

The flowering stems are sought after as fillers.^20,31 There are over 400 named hybrids and varieties,³¹ with approximately 50 registered cultivars in Australia.⁶ Colours of the different cultivars vary from pure white to pale pink, variegated white and pink through to dark purple.^12,20 Some species have white flowers that age to red or dark pink.^20,31 At the end of September 2000, disease symptoms were observed on 4-year-old Geraldton waxflower 'Ofir' plants in Philippe, South-Western Cape Province, South Africa. Some of the petals and calyxes of the flowers showed a brown, necrotic discoloration (Fig.2 ).

Fig. 2 Brown discoloration of the outer surface of the calyx and petals

Eventually the entire flower turned brown, shrivelled and abscised (Fig.3).

Fig. 3 Brown discoloration of the inside of the calyx and petals

Botrytis cinerea Pers.: Fr. was consistently isolated from affected tissue. This study was conducted to prove that Botrytis cinerea was the cause of the flower blight of Geraldton waxflowers in South Africa. Brown discoloration of the outer surface of the calyx and petals. Brown discoloration of the inside of the calyx and petals.

Materials and methods

An isolate of Botrytis cinerea from naturally infected Geralton waxflowers 'Ofir' plants was grown on a petri dish containing potato dextrose agar (PDA, Biolab). A spore suspension containing 1x106 spores ml^-1 was prepared from 14-day-old cultures, flooded with sterile distilled water and gently scraping the fungal colony with a flame-sterilised scalpel to dislodge conidia. Flower stems of C. uncinatum 'Ofir' were sprayed with the spore suspension of Botrytis cinerea. Control flowers were sprayed with sterile distilled water only. Inoculated flower stems were placed in a bottle filled with water and flowers were enclosed in a transparent plastic bag for 24 hours at 21°C. Three days after inoculation, plant tissue showing disease symptoms were surface sterilised in 0.35% NaOCl for 30 seconds, rinsed once in sterile distilled water, and plated on PDA. Plates were incubated at 25°C. Infected tissue was also placed in a Petri dish with a few drops of distilled water in order to induce sporulation.

Results

Brown, necrotic lesions developed on the petals and calyx within 3 days after inoculation. Abscission occurred at the base of the receptacle. Within one week after inoculation, the flowers were totally necrotic. No necrosis or abscission was observed in control non-inoculated flowers. Botrytis cinerea was reisolated and identified from affected tissues on PDA¹⁰ .The fungus sporulated on the diseased tissue in the Petri dish. The isolate has been lodged in the culture collection of the University of Stellenbosch (STE-U).

Discussion

Inoculation studies proved that Botrytis cinerea was the cause of flower blight of Geraldton waxflowers in South Africa. In Australia, it has been reported that Botrytis cinerea causes brownish lesions on the petals of Geraldton waxflowers in the field, and in severe cases, shedding of buds and flowers,⁵ or petal drop after harvest, especially in transit.²⁴ The abscission of buds and flowers during post-harvest handling, is one of the major problems encountered in marketing cut Geraldton waxflower overseas.¹⁵ Geraldton waxflower buds and flowers each have a well-developed abscission zone at the base of the calyx tube. This is where premature flower abscission usually occurs in cut, packaged Geraldton waxflower.^16,30 Physical injury, exogenous ethylene, and endogenous ethylene, associated with water stress and pathogen infection (e.g. Botrytis cinerea), induce flower fall in Geraldton waxflower.¹⁶

Botrytis cinerea is a common post-harvest pathogen, causing grey mould of many types of cut flowers,^11,23 including rose,⁸ gerbera²⁵ and carnation.⁷ Infection with Botrytis cinerea takes place through wounds, through decaying or dead plant tissue, and by direct penetration of the undamaged host.^14,24 It has been shown that B. cinerea expanded from latent infections of floral parts into the receptacle, and concluded that the calyx is the primary pathway for B. cinerea.²¹ For Geraldton waxflower, there are many potential sites for colonisation, for example the bracteole,³ which is forced off the expanding flower bud and then senesces, or when the stigma and anthers die as the flower ages. Fungal growth may resume after harvest as flowers age under high humidity, or even wet conditions, during handling and transport.¹⁶ Postharvest it can develop in export cartons as a hairy growth on buds and flowers.⁵ At advanced stages, affected tissue withers and turns grey-brown to black.⁵

A dieback, caused by Botrytis cinerea, has also been reported.²⁴ Initial symptoms on the plant include water soaked spots on the open flowers. The tissue later becomes soft and watery. The affected parts of the plant wilt and collapse. If the humidity remains high, a grey mould of spores and mycelia develops over the surface of the infected tissue. In cases of severe infection, the infection move down into the stems, resulting in dieback. Stem lesions usually appear on succulent stems or stalks and may be either dark, sunken, elongated lesions with a definite outline, or may spread through the stalk and cause it to weaken and break over at the point of infection. The tissue may then become wrinkled, dry and flat. Black sclerotia may develop on the surface or sunken within the tissue.²⁴ This is in contrast to the symptoms observed on Geraldton waxflowers in South Africa, where the infection stopped in the calyx, at the base of the receptacle. Dead areas or necrotic lesions occur on buds and petals and are caused by early infections.²⁴ The latter symptoms are similar to that on the Geraldton waxflowers observed during this study.

Botrytis flower blight or grey mould can cause significant losses in Geraldton waxflowers, both in the field during cool wet weather conditions, and postharvest during shipment to export markets.^5,24 Control of this disease therefore needs to be addressed. Control must be based on a preventative programme starting as one spray every 2 weeks with the first rains, or six weeks before harvesting.^5,31 During wet weather, sprays should be conducted every 7-10 days.⁵ A post-harvest dip is also recommended, but flowers must be thoroughly dried before packing, and maintained at a constant low temperature (2°C) to reduce moisture build-up in the carton.^5,26,31 Each cultivar should, however, be tested for phytotoxicity, as cultivars of Geraldton waxflower may respond differently to chemicals. When dipped flowers are not dried before packing and shipping, an ideal environment may develop for fungal infection (e.g. Botrytis).¹³ The level of Botrytis inoculum on waxflower can be further reduced in the field by sanitation,¹ good ventilation of the orchard by means of the correct plant spacing and weed control, and the avoidance of overhead irrigation.²⁴ Packing shed sanitation, the maintenance of low product temperature after harvest,¹⁷ and control of in-package relative humidity²⁷ should effectively limit disease severity and flower fall.²⁸