Os proefschrift 27-08-2003 11:43 Pagina 1 Ecology and control of Pythium root rot in flower bulb culture Gera van Os UB E UITGEVERIJ EIGEN BEHEER Os proefschrift 27-08-2003 11:43 Pagina 2 Voor mijn ouders Os proefschrift 27-08-2003 11:43 Pagina 3 Ecology and control of Pythium root rot in flower bulb culture Proefschrift ter verkrijging van de graad van Doctor aan de Universiteit Leiden, op gezag van de Rector Magnificus Dr. D.D. Breimer, hoogleraar in de faculteit der Wiskunde en Natuurwetenschappen en die der Geneeskunde, volgens het besluit van het College voor Promoties te verdedigen op woensdag 5 november 2003 te klokke 14:15 uur door Geertruida Jeltje van Os geboren te Amstelveen in 1965 Os proefschrift 27-08-2003 11:43 Pagina 4 Promotiecommissie: Promotor Prof. dr. J.A. van Veen Co-promotor Dr. J. van Aartrijk (Koninklijke Algemeene Vereeniging voor Bloembollencultuur) Referent Prof. J.M. Whipps (Horticultural Research International) Overige leden Prof. dr. E. van der Meijden Prof. dr. E.J.J. Lugtenberg Prof. dr. A.H.C. van Bruggen (Wageningen Agricultural University) Dr. ir. J.M. Raaijmakers (Wageningen Agricultural University) Uitgeverij Eigen Beheer Amsterdam ISBN: 90 73838 14 2 The research described in this thesis was conducted at the Bulb Research Centre in Lisse, the Netherlands. Part of this research was financed by the Dutch Urgency Program for Research on Diseases and Breeding of Flower Bulbs, financed jointly by the Commodity Board for Ornamental Plants (PT) and the Netherlands Ministry of Agriculture, Nature Conservation and Fisheries. Os proefschrift 27-08-2003 11:43 Pagina 5 5 Contents CHAPTER 1 General introduction 7 CHAPTER 2 Disease development op Pythium root rot in bulbous iris and 19 crocus CHAPTER 3 Effects of soil fumigation and flooding on suppression of 35 Pythium root rot in ornamental bulb culture CHAPTER 4 Effects of composted organic household waste on Pythium 53 root rot in iris and crocus CHAPTER 5 Suppression of Pythium root rot in bulbous iris in relation to 69 biomass and activity of the soil microflora CHAPTER 6 Microbial community responses to disease management soil 83 treatments used in flower bulb cultivation CHAPTER 7 Identification and pathogenicity of Pythium spp. in flower 103 bulb culture CHAPTER 8 Summary and concluding remarks 123 References 133 Samenvatting147 Curriculum vitae 153 List of publications 155 Nawoord 157 Os proefschrift 27-08-2003 11:43 Pagina 6 Os proefschrift 27-08-2003 11:43 Pagina 7 7 CHAPTER 1 General Introduction Os proefschrift 27-08-2003 11:43 Pagina 8 . Photograph Bulb field with premature die-off caused by Pythium infection in iris. Os proefschrift 27-08-2003 11:43 Pagina 9 9 CHAPTER 1 General introduction Introduction Root rot caused by Pythium spp. is an important disease in ornamental bulb culture in the Netherlands. Current legislation about the registration of pesticides, the decreasing use of chemical control methods, and increasing pressures for the use of environmentally friendly methods, make it difficult to recommend chemical disease control methods. These can rapidly become out of date because of the non- availability of the chemicals, or the withdrawal of approval for the use of a chemical on a certain crop. The development and registration of new, environmentally acceptable chemicals is expensive and time consuming and the use of a limited number of specific pesticides increases the risk that pathogens develop resistance. The long juvenile period and the slow vegetative propagation of bulb crops make them very vulnerable for diseases in general, and seriously hamper breeding. Since the 17th century, breeders selected hybrids according to flower characteristics and forcing ability, rather than to disease resistance (Le Nard & De Hertogh, 1993). Alternative control measures, therefore, have to be developed. This thesis describes research on Pythium root rot of ornamental bulbs, designed to obtain knowledge for the development of alternative control measures in order to reduce the use of and dependence on chemical control. Bulb culture in the Netherlands Ornamental bulb culture in the Netherlands comprises the cultivation of several flower crops with different kinds of perennating structures. For example, tulip, crocus and dahlia produce bulbs, corms and tubers respectively. In general, these crops are all indicated as bulb crops. Bulb cultivation, i.e. tulips, probably occurred as early as the 12th century near Persia (Botschantzeva, 1982). These bulbs originated from Central Asia (Hoog, 1973) and adjacent territories such as the Middle East and the Mediterranean. Bulbs were introduced into Europe via Turkey after the middle of the 16th century and brought to the Netherlands in 1571 (Botschantzeva, 1982). Today, the Netherlands is the centre of flower bulb production and trade in the world. Around 70% of the world production originates from the c. 22.000 hectares within its borders used for bulb cultivation. In 2001, the production value of the total flower bulb production was approximately 680 million Euro. The sandy soil behind the dunes in the western part of the country along the North Sea coast provides a well-suited substratum and the mild winters and summers create the optimum climate for the bulb growing “industry”. Outside the Netherlands, production areas of flower bulbs are increasing. Because the auction and trading firms still reside in the Netherlands, Os proefschrift 27-08-2003 11:43 Pagina 10 10 Chapter 1 most of the foreign production is sold via the Netherlands. 93% of the world trade in flower bulbs is controlled by Dutch firms. More than 75% of the Dutch production is exported. An important number of these exported bulbs is used in gardens and parks. From the bulbs that are not exported, around 90% is used for the production of cut flowers. More than 50% of the flowers thus produced are exported as well (International Flowerbulb Centre, 2002). In the Netherlands, the majority of spring flowering bulb crops, like tulip, hyacinth, iris, crocus and narcissus, is grown on sandy soil in the provinces of Noord- Holland en Zuid-Holland, in rotations with bulb crops only. In other parts of the Netherlands and on the heavier soils in Noord-Holland, bulbs are grown in rotations with e.g. grass, potato, wheat, sugar beet and vegetables. Bulb crops have a complex annual cycle of growth and senescence. For commercial bulb production, propagation is based on the generally slow vegetative production of daughter bulbs in the field. For example, bulbs of tulip, iris, crocus and narcissus can produce one to four new bulbs annually. These crops are planted in autumn, the bulbs root immediately and leaf emergence can be observed in early spring. The new (daughter) bulbs are harvested in June until August after senescence of the leaves. Bulbs of a sufficiently large size to produce a flower are saleable. Smaller bulbs need to grow for one or more years before they can be sold. Many cultivars of major bulb crops are sterile and do not set seed. Seeds from fertile cultivars produce a highly variable offspring and are used for breeding purposes only. Breeders depend on vegetative propagation of their seedlings in order to produce a uniform stock of sufficient quantity that can be tested for the development of new varieties. The selection of a new cultivar and the production of a reasonably large stock may take 10 to 20 years. Diseases and pests The vegetative propagation cycle of most bulb crops comprises several years of growth in soil. Within this period, bulbs are planted and lifted each season, stored and transported, exposing them frequently to physical damage and various contamination sources. Diseases and pests can accumulate and cause severe losses once introduced in the production cycle. Most of the ornamental bulbs and cut flowers are for export which demands high standards for quality and plant health. Diseases can affect the whole growing plant in a range of ways. For example, pathogens affect the plant in the ‘dormant’ condition, i.e. the storage organ(s), the growing plant, such as the leaves, or the underground parts. Symptom expression does not necessarily reflect the site of the attack, as below-ground damage can result in the wilting of collapse of apparently healthy above-ground parts, or secondary infections of shoots and flowers may result from a bulb or corm infection (Rees, 1992). Important fungal diseases are caused by Fusarium oxysporum, Botrytis spp., Os proefschrift 27-08-2003 11:43 Pagina 11 General introduction 11 Rhizoctonia solani, and Pythium spp. Infestation with these fungi affects both propagation in the field and flower production of several major bulb crops. Penicillium spp. may cause considerable damage to some bulb crops in storage. A few bacterial diseases of bulb crops are serious, ‘yellow disease’ (geelziek in Dutch) of hyacinth, caused by Xanthomonas hyacinthi being the most notorious one. Numerous viruses have been identified as pathogens of bulb crops. For instance, at least 14 viruses are known to infect tulip, and more than 15 are confirmed in narcissus (Rees, 1992). The vegetative propagation of bulb crops allows the spread of virus diseases throughout most bulb stocks unless specific steps are taken by the growers to establish a nucleus of virus-free material as a base for healthy stocks. Nematodes are among the most serious pests of major crops. In addition to the damage they cause directly, some, such as nematodes of the genus Trichodorus, are of importance as vectors of virus diseases. Other migratory nematodes (including root lesion nematodes) of the genera Longidorus, Pratylenchus and Xiphinema can be important for a range of bulb crops in light, sandy soils. Stem and tuber nematodes of the genus Ditylenchus are major pests of most commercially grown bulb crops (Rees, 1992).