Integrated disease management in organic farming combines the use of various measures. The usefulness of certain measures depends on the specific crop-pathogen combination. In many crops, preventative measures can control diseases without the need of plant protection products. However, for certain disease problems, preventative measures are not sufficient. For example, organic apple production strongly depends on the multiple use plant protection products

Köhl, Jürgen

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Köhl, J.

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Integrated disease management

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Organic Eprints

Integrated disease management  Jürgen Köhl   Abstract - Integrated disease management in organic farming  combines  the  use  of  various  measures.  The usefulness of  certain measures depends on the spe-cific  crop-pathogen  combination.  In  many  crops, preventative  measures  can  control  diseases  without the  need  of  plant  protection  products.  However,  for certain disease problems, preventative measures are not sufficient. For example, organic apple production strongly depends on the multiple use plant protection products.1  INTRODUCTION Disease  management  in  organic  cropping  systems combines various components which can be divided into  strategic  preventative  measures,  tactical  pre-ventative measures and control measures. For each crop-pathogen  relationship  and  cropping  system such components will contribute to different extent to disease management (Termorshuizen, 2002). The development  of  integrated  disease  management systems  depends  on  thorough  knowledge  of  the cropping systems as well as of the pathogen and can only be achieved by interdisciplinary research.   PATHOGEN CHARACTERISTICS AND DISEASE MANAGE-MENT Host-specificity and mobility are the two main char-acteristics  of  pathogens  determining  the  choice  of disease  management  measures  (Wijnands  et  al., 2000). Strictly host-specific pathogens which are not mobile can be controlled by using cropping systems with low frequencies of the susceptible crop. Exam-ples  are  cyst  nematodes  of  potato  or  sugar  beet. Pathogens which are not host-specific and not mo-bile can be controlled by the choice and sequence of crops grown in the rotation supported by preventa-tive  measures  increasing  soil  suppressiveness  and plant health. Examples are the soil borne pathogens Sclerotinia  sclerotiorum  and  Rhizoctonia  solani. Host-specific  pathogens  with  high  mobility  such  as Phytophthora  infestans  in  potato  cannot  be  con-trolled by crop rotation. Preventative measures are sanitation in a cropping area and the choice of crop structure  and  planting  date  in  combination  with resistant  varieties.  In  many  situations  also  control measures  such  as  applications  of  plant  protection products may be needed to achieve sufficient yield. Also  pathogens  which  are  not  host-specific  but highly mobile cannot be controlled by crop rotation. Disease  prevention  depends  on  strengthening  the                                                  Jürgen Köhl is with Plant Research International, Biointeractions & Plant Health, NL-6700 AA Wageningen, The Netherlands (jurgen.kohl@wur.nl).    crop, escaping the disease by choosing proper seed-ing dates and creating an open crop structure. Dis-ease control by using crop protection products may be  needed  in  many  cases.  Example  for  a  mobile pathogen with a broad host range is Botrytis cinerea causing grey mould in various crops such as beans, peas, strawberries, grapes and many other crops. How differently various measures contribute to dis-ease  management  in  different  crop-pathogen  rela-tionships will be illustrated by the comparison of two systems. In wheat, various Fusarium spp. can cause Fusarium Head Blight (FHB) leading to a decrease of yield  and,  more  important,  the  production  of  my-cotoxins  in  the  grain.  Fusarium  spp.  have  a  broad host  range  and  also  can  survive  saprophytically. Mobility of spores of most Fusarium spp. is low. In apple, Venturia inaequalis can cause apple scab on leaves and fruit resulting in reduced yields and qual-ity of fruit. The pathogen is strictly host-specific and can  survive  only  on  apple  tissues.  The  mobility  of spores is low.  STRATEGIC PREVENTATIVE MEASURES Many  measures  for  preventative  disease  control have  a  long-term  strategic  character.  Various  as-pects  of  the  farm  management  and  the  long-term cropping system as well as of the location including the farm neighbourhood have impact on diseases of crops  and  thus  should  generally  be  considered  in integrated disease management.  Avoidance of pathogen sources in neighbourhood of field  and  crop  rotation  in  neighbouring  field.  Since most Fusarium spores travel only a few centimetres, sources  in  the  crop  neighbourhood  will  not  cause epidemics. Ascospores of V. inaequalis produced in neighbouring  orchards  may  reach  the  crop.  Aban-doned  orchards  and  orchards  with  high  apple  scab pressure should not be found in the neighbourhood of an apple orchard.   Soil  structure,  soil  suppressiveness,  biological  soil disinfection,  and  catch  crops.  These  measures  are important for managing soil borne diseases but will have no direct effect on the above-ground develop-ment of Fusarium spp. and V. inaequalis.  Crop rotation. Main inoculum source of FHB are crop residues  of  preceding  diseased  crops.  The  best documented example is the high risk of FHB when wheat is grown after maize. Maize stubble are often colonised by the same Fusarium spp. affecting wheat and such Fusarium spp. can survive and multiply on maize stubble for several years. Avoiding growth of maize in rotation with wheat will substantially reduce risks of FHB epidemics. Rotation schemes with cere-als grown after cereals should generally be avoided. In the perennial apple production crop rotation is no issue.   Tillage. Primary inoculum of FHB are crop residues left  on  the  soil  after  tillage.  Using  reduced  tillage systems  will  increase  FHB  risks  since  much  more residues will be present on the soil surface. In apple orchards, tillage is not an option.   TACTICAL PREVENTATIVE MEASURES Tactical  preventative  measures  deal  with  the  plan-ning and realisation of a certain crop. Typical meas-ures are the choice of variety, seed quality, seeding time and crop structure.  Resistant  varieties.  In  wheat,  resistance  breeding made  considerable  progress  and  partly  resistant cultivars are used in practise. In apple, partly resis-tant varieties are available. However, the pathogen has  the  potential  to  adapt.  Furthermore,  changing varieties in a perennial crop needs high investments.   Removal of crop residues from field. Fusarium spp. threatening  wheat  crops  are  surviving  primarily  in stubble  of  cereals  including  maize.  Removing  this potential inoculum sources is not feasible, although physical  removal  especially  of  maize  stubble  may have  a  significant  impact  on  disease  development. In  apple,  removal  of  fallen  leaves  as  the  principle inoculum source of apple scab in spring is an inter-esting option. Removal of leaves by using specially designed vacuum cleaners has been demonstrated. However,  mechanisation  is  difficult,  cost  intensive and  application  depends  much  on  orchard  circum-stances.  Biological crop residue treatments. Microbial decom-position of crop residues is a natural process which can be supported by adding stimulating nutrients or selected  micro-organisms.  Also  earthworms  can  be protected and stimulated to consume plant residues on the soil surface. Both possibilities are under in-vestigation for apple scab  prevention in  REPCO. In arable crops, stimulation of resident microbial popu-lations on residues may be  achieved by  creating a suitable microclimate, e.g. by using mulches.   Healthy seeds and planting material. Seeds of wheat can be infected by Fusarium spp. Producing healthy seeds is important to guarantee the establishment of a  vigorous  crop.  For  the  development  of  FHB  epi-demics after flowering, the major inoculum sources are  infested  crop  residues  and  thus  field-borne. Reducing  the  seed-borne  fraction  of  the  disease inoculum may only have very limited effect against FHB. Using clean planting material of apple will not result in any disease prevention since V. inaequalis overwinters on the orchard floor and easily can enter disease–free young trees.  Sowing time. For infections of wheat ears by Fusa-rium spp., the crucial factor are the climatic condi-tions during the short window of flowering. Choosing early or late sowing times is not an option for dis-ease  prevention  since  weather  during  flowering cannot  be  predicted.  Also  for  apple,  no  effect  of planting time on apple scab can be expected.  Crop structure. Crop structure affects microclimatic conditions  within  the  canopy  and  determines  the distance  pathogen  spores  have  to  spread  to  reach susceptible  host  tissue.  A  dense  wheat  crop  will favour  pathogen  sporulation  on  the  soil,  but  may block  spore  flights  of  Fusarium  spp.  depending mainly  on  splash  dispersal  during  rainfalls.  Vertical leaf positions may also block spore flights. Ears on taller  plants  may  have  a  better  change  to  escape infections  which  may  have  a  moderate  effect  on FHB. The canopy structure of apple trees is managed to  obtain  sufficient  yield  and  possibilities  to  create more  open  canopies  are  limited.  Spores  of  V.  in-aequalis are very much adapted to infect trees and spread  from  orchards  floors  and  within  canopies. Possibilities to manage the apple scab by crop struc-ture are low.  DISEASE CONTROL MEASURES Disease control measures are used to control a cer-tain disease of a crop. Physical, chemical or biologi-cal control measures may be used.   Physical treatments. Fusarium spp. on seeds can be controlled by warm water treatments. However, the effect on FHB will be limited.  Natural  compounds  and  biocontrol  agents  as  plant protection  products.  The  control  of  FHB  does  not depend on plant protection products since preventa-tive  measures  such  as  rotation,  and  tillage  can  be used.  In  apple,  preventative  measures  such  as  re-moval  of  fallen  leaves  can  delay  the  outbreak  of epidemics.  However,  epidemics  need  to  be  con-trolled  by  multiple  applications  of  plant  protection products  such  as  copper.  Environmentally  friendly new products are strongly needed.    ACKNOWLEDGEMENT This  work  is  funded  by  the  European  Commission (Project No 501452; REPCO) and the Dutch Ministry of Agriculture, Nature and Food quality.  REFERENCES Termorshuizen,  A.J.  (2002).  In  "Plant  Pathologist’s Pocketbook",  pp.  318-327,  eds,  J.M.  Waller,  J.M. Lenné and S.J. Waller. (CAB International, Walling-ford, UK).  Wijnands, F.G., Sukkel, W. and Booij, C. (2000). In "Biologisch bedrijf onder de loep: Biologische akker-bouw en vollegrondsgroenteteelt in perspectief", pp. 65-71, eds. F.G. Wijnands, J.J. Schröder, W. Sukkel and  R.  Booij.  (Praktijk  Onderzoek  Plant  en  Om-geving, Lelystad, The Netherlands).  

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