Managing Phytophthora Diseases
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7 Managing Phytophthora Diseases Diversity and Management of Phytophthora in Southeast Asia Edited by André Drenth and David I. Guest ACIAR Monograph 114 (printed version published in 2004) 7.1 Principles of Phytophthora Disease Management André Drenth1 and David I. Guest2 Abstract In order to limit the incidence and severity of diseases caused by Phytophthora, effective management strategies are needed. Management of phytophthora diseases is based on a number of principles such as avoiding infection through basic hygiene, limiting susceptibility through drainage and irrigation, improving soil health, use of disease-resistant germplasm, and biological and chemical control. Although the components are discussed here in a sequential order, effective control of phytophthora diseases is often only achieved through the integrated use of a number of these strategies. Introduction Cultural practices There are more than 60 described species of The effectiveness of control strategies depends on Phytophthora and all known species are plant the ability of an individual species of Phytophthora to pathogens. Each species can cause disease in from a survive, either as a saprophyte or as dormant few to over a 1000 different plant species. Hence, there spores. Generally, mycelium and zoospores survive are a few thousand diseases in a wide range of plants for only a few weeks, while chlamydospores may caused by the various species within the genus survive for 6 years, and oospores for 13 years (Erwin Phytophthora. Each of these diseases will have its own and Ribeiro 1996). Some species, however, such as characteristics, which makes it difficult to generalise P. cinnamomi, appear to have a high saprophytic ability (Zentmyer 1980) while others such as disease-control methods. However, it is important to P. palmivora do not (Ko 1971). understand the most common contributing factors that underpin the control of phytophthora diseases. Quarantine, nursery and orchard hygiene Only an in-depth understanding of these underlying Quarantine is the only means of preventing the factors, coupled with a detailed understanding of the introduction of a new pathogen into an area. agronomics of the crop will allow one to develop Quarantine is also extremely important in nurseries effective, integrated disease control methods. The aim where millions of plants are produced each year, of this chapter is to provide an underlying basis for providing opportunities for the rapid spread of disease control by discussing a wide range of Phytophthora. In areas where Phytophthora has not management practices available under the following been recorded, exclusion is essential. Exclude headings: (1) cultural practices, (2) resistance animals by fencing, minimise the movement of breeding, (3) biological control, (4) fungicides, and (5) vehicles and people through the orchard, remove phosphonates. soil from vehicles, boots and tools before they are brought into the orchard, plant only disease-free 1 CRC for Tropical Plant Protection, Indooroopilly Research and resistant trees, and divert water run-off from Centre, 80 Meiers Road, Indooroopilly, Queensland 4068, adjacent orchards (Broadley 1992). Australia. 2 The University of Melbourne, Parkville, Victoria 3010, In nurseries, potting mixes should be steamed to kill Australia. Current address: Faculty of Agriculture, Food Phytophthora inoculum, and only certified and Natural Resources, The University of Sydney, Sydney Phytophthora-free planting material should be used New South Wales 2006, Australia. (Chapter 7.2). Good hygiene in orchards is a 154 Diversity and Management of Phytophthora in Southeast Asia Edited by André Drenth and David I. Guest ACIAR Monograph 114 (printed version published in 2004) Principles of phytophthora disease management 155 fundamental component of effective pest and antagonistic microorganisms in the soil management. It is virtually impossible to eradicate (Lazarovits et al. 2001). While these mechanisms Phytophthora from the soil. Therefore, if Phytophthora suppress the growth of Phytophthora, they may also is present, metalaxyl should be used to minimise create phytotoxicity to the plant roots, making them disease development. Roadways, interrows and less attractive to colonisation by the pathogen equipment should be kept clean. The site should be (Erwin and Ribeiro 1996). Aryantha et al. 2000 well-drained to avoid water from forming ponds showed that the addition of fresh or composted that may subsequently allow Phytophthora to chicken manure to potting mix significantly reduced proliferate. Orchards should also be kept free of the survival of P. cinnamomi and the development of rotting plant debris that may be infected with disease in lupin seedlings. Chicken manure more Phytophthora (Broadley 1992). effectively suppressed P. cinnamomi and plant- disease symptoms than did cow or sheep manure. Drainage and irrigation All composts increased soil organic matter, total Excess irrigation and rainfall are considered to be the biological activity, and populations of antagonistic most important factors that increase the severity and actinomycetes, fluorescent pseudomonads, and spread of Phytophthora-incited diseases. In turn, the fungi. However, chicken manure also stimulated the duration of free water, in soil or on foliage or fruit is production of endospore-forming bacteria, which the most important environmental factor in the was positively correlated with lupin seedling development of disease caused by Phytophthora survival. The addition of composted manures is because it is during this time that propagules necessary for disease development but it is not proliferate and infect (Erwin and Ribeiro 1996). In sufficient for biological control. Mulches may also addition, zoospores, cysts and chlamydospores reduce the impact of phytophthora root rot if used travel in the soil in irrigation water, rainfall run-off from the time of orchard establishment or if the and movement of soil. Orchards should be disease is not too far advanced. The ‘Ashburner established on land that is well-drained and not system’, based on improved drainage and mulches, subject to flooding. Therefore, sloping ground is has been successfully employed to manage preferable. Ideally, the soil should be drained to a phytophthora root rot of avocados (Broadley 1992). depth of 1.5 metres. Mounding of the soil around the Chapter 7.3 reports that mulches are also effective in tree promotes good drainage (Broadley 1992). Row managing phytophthora root rot of papaya. crops should be planted on raised beds to prevent Companion and cover cropping free water from contacting the plants (Erwin and Ribeiro 1996). To reduce the rate and extent of build- Companion cropping can reduce the impact of up of inoculum, plants should be irrigated less phytophthora diseases. For example, in the frequently so that free water drains away (Lutz et al. subtropics of Australia, banana and avocado are 1989). In areas where rainfall is the main source of planted together. The bananas provide mulch and water, optimal horizontal and vertical drainage are reduce soil water after heavy rain. This system necessary to prevent water-logging. Spraying water reduces the impact of root rot caused by P. cinnamomi. on the trunks of trees should be avoided as Care must be taken to choose a companion crop that constantly wet bark may encourage the does not compete too heavily with the orchard crop. development of cankers. Cover crops, when incorporated into the soil, increase the amount of organic material, which encourages the Organic amendments and mulching growth of microbes that suppress Phytophthora Mulching stimulates plant root growth, increases (Broadley 1992). nutrient uptake, decreases evaporation from the soil, Fertilisers increases soil-water holding capacity, reduces surface water run-off, facilitates drainage, regulates Some forms of nitrogen have been shown to favour soil temperature, and provides a high level of an increase in disease, while other forms suppress nutrients for soil microbes (Aryantha et al. 2000). disease (Schmitthenner and Canaday 1983). Amendments can either enhance or suppress Generally, the role of fertilisers or nutrients in disease, depending on their nature. Phytophthora is controlling or suppressing phytophthora diseases is inhibited by alfalfa meal, cotton waste, soybean unclear. Some reports indicate that fertilising meal, wheat straw, chicken manure and urea. improves plant vigour and hence resistance to Ammonia and volatile organic acids released by disease, while others indicate that pathogen decomposing organic matter kill Phytophthora, and infection is favoured because of improved plant the residual organic matter stimulates competitive vigour (Erwin and Ribeiro 1996). Diversity and Management of Phytophthora in Southeast Asia Edited by André Drenth and David I. Guest ACIAR Monograph 114 (printed version published in 2004) 156 Diversity and management of Phytophthora in Southeast Asia Suppressive soils in others, infection of the root is minimised due to natural resistance mechanisms (Broadley 1992). Soils that favour the expression of disease are General resistance to P. citrophthora and P. nicotianae conducive, while those that are inhospitable to plant has been developed in many rootstocks onto which pathogens are suppressive. The principal cause of grafts of commercial citrus species can be made suppressiveness is an increase in the population of (Erwin and Ribeiro 1996). antagonistic bacteria, fungi and actinomycetes.