Scab control in Conference pears is the bottleneck in organic growing. A group of growers,

advisers and researchers in the Netherlands is working on new approaches to organic

pear growing. Central to this approach is a reliable and good scab control. Changes in the

system of cultivation are sought which will enable this scab control with less fungicide.

This has become even more urgent since copper was banned as a fungicide in the

Netherlands. First results from trials on vigour control show that there is potency for

system changes that will enable a better scab control in organic growing. Scab incidence

was reduced in three trials with 34, 58 and 82% respectively. The project, in which also

other cultivation measures are being tested, will be continued

Jansonius, Pieter Jans

English

Organic Eprints

29Conference pears; work on system changes to enable better scab control in organic orchards in the Netherlands P. J. Jansonius 1AbstractScab control in Conference pears is the bottleneck in organic growing. A group of growers, advisers and researchers in the Netherlands is working on new approaches to organic pear growing. Central to this approach is a reliable and good scab control. Changes in the system of cultivation are sought which will enable this scab control with less fungicide. This  has  become  even  more  urgent  since  copper  was  banned  as  a  fungicide  in  the Netherlands.  First  results  from  trials  on  vigour  control  show  that  there  is  potency  for system changes that will enable a better scab control in organic growing. Scab incidence was reduced in three trials with 34, 58 and 82% respectively. The project, in which also other cultivation measures are being tested, will be continued. Keywords: Conference pears, scab, Venturia pirina, vigour control, system approach.IntroductionThe demand for organic pears in North-western Europe is high compared to the limited production. In spite of the good market perspective there are very few pear growers in the Netherlands who dare convert to organic production. The most prominent reason for this is their  fear  of  scab  (Venturia  pirina).  And  indeed  it  is  the  experience  of  those  who  are growing organically that it is very hard to control this important disease. This is even more so since Copper based products were banned as fungicides in the Netherlands. Since 2005 a group of 6 growers have formed an intensive study group that tries to find new ways to make organic pear growing more stable. This group participates in a research project  carried  out  by  the  Louis  Bolk  Institute  and  funded  by  the  Dutch  Ministry  of Agriculture. In a series of on-farm trials various measures are tested that can contribute to a system of growing that enables the grower to keep the scab pressure low without using large amounts of copper. The results from these trials are combined with observations by growers, advisers and researchers in an ongoing discussion during fieldtrips and meetings. Thus far this work has made the group aware that the epidemiology of pear scab clearly differs from that of apple scab and that we need more knowledge to come to a new and more stable system of  growing  Conference  pears.  Of  the  cultivation  measures  tested,  vigour  control  has shown to be a useful measure to help control scab. In this article preliminary results of two trials on vigour control are presented together with more casual observations, questions and hypotheses. Material and Methods Trial 1, started spring 2006. Conference on Quince C rootstock, planted 1995, V-system 3,4 x 1,2 m, young fertile clay soil. Block trial with 4 replications, 8 trees per field. The standard spraying program was applied in both treatments.                                                 1 P.J.(Pieter Jans) Jansonius, M.Sc.,Louis Bolk Institute, Hoofdstraat 24, NL 3972 LA Driebergen, the Netherlands, www.louisbolk.nl , p.jansonius@louisbolk.nl http://orgprints.org/13634/30Stem notching was carried out with chainsaw, pre-bloom, from two sides; only in 2005. Root  pruning  was  carried  out  under  45  degree  angle,  almost  undercutting  the  tree.  In 2005: one side pre-bloom, the other side in July. In 2006: both sides pre-bloom. Scab  incidence  and  severity  on  the  fruit  were  measured  indoors  after  harvesting separately per tree. Scab was scored in 4 categories: 0= no scab; 1= < 10 small lesions or 0,5 cm2 (=very little scab); 3= 10-30 small lesions or 0,5-1 cm2 (= clearly visible but still marketable); 4= > 30 small lesions or >1 cm2 (=no longer marketable as good quality) Scab on shoots was counted on year-old twigs cut in late winter. The reduction in shoot growth was not measured exactly but judged visually by the group. Trial 2, started spring 2007. Conference on Quince C rootstock, planted 1991, slender spindle 3,25 x 1,4 m, Light clay on sand. Block trial with 4 replications, 8 trees per field. The standard spraying program was applied in all treatments. Stem notching was carried out with chainsaw, pre-bloom, from two sides. Root pruning was carried out under 90 degree angle, 40 cm from the stem. Scab incidence and severity on the fruit were measured as in trial 1. Trial 3, 2006-2007 On-farm trial sprayed by the growers. Full-grown Conference on QC. On location 1 without replications, on location 2 in four replications. Fields several rows wide and around 20 trees long with observations made on trees in the middle. Scab was determined as in trial 1.ResultsTrial 1. Soon after conversion a heavy scab pressure developed in this orchard. With hindsight the grower,  from  his  conventional  experience  knowing  Conference  as  being  not  very susceptible  to  scab,  has  underestimated  scab  and  maybe  underestimated  the  spray products used in organic growing (at that period low dosages of copper). At the start of the trial this high pressure shows itself in almost all year-old twigs having scab. Combined with the knowledge that incidence on the fruit in 2005 was also very high we may assume that also  the  ascospore  dose  from  overwintering  leaves  was  high.  Leaf degradation  in  this orchard was relatively bad. Vigour was judged by growers as high. The owner wanted to cut the stems from 2 opposite sides about halfway with 30 cm between cuts. We created extra vigour control by combining this with root pruning. The aim was to suppress shoot growth drastically with to enable better scab control. Because of the severity of the scab problem and the fact that the overall vigour was clearly too high it was unacceptable for the grower to leaf part of the trees untreated. The question addressed here is not if vigour control has to be applied but how much vigour control is needed to bring scab under control again. In the first season in mid July the vigour control was clearly visible. Shoots at the top stopped  growing  earlier  and  the  tree  was  visibly  more  open.  Considering  the  drastic measures taken this effect was rather mild. Weather in summer was very dry which made scab control relatively easy. http://orgprints.org/13634/31Surprisingly there was a clear effect on both scab incidence and severity at the fruit. Most of  the  scab  found  comprised  of  small  lesions.  Root  pruning  reduced  the  number  of scabbed fruits with 58%. Scab incidence on the new wood was very low overall, but still showing a clear effect of the treatment. Table 1: Trial 1: Twig scab at start of trial and after one year. March, 2006 (start of trial) March, 2007    lesions/m  incidence  lesions/m  incidence 1. trunk notching  51  94%  0,8  10% 2. trunk notching   + root pruning  46  92%  0,2  6% Table 2. Trial 1: Fruitscab incidence.      Harvest 2006     Harvest 2007       total incidencecategory 1 incidence  total incidence category 1 incidence1. trunk notching  19 %  16 %  11 %  10 % 2. trunk notching   + root pruning  8 %  7 %  2 %  2 % Table 3. Trial 1: Mean production and fruit weight.    Harvest 2006     Harvest 2007       kg/tree number of fruit/treemeanfruitweight(g) kg/treenumberoffruit/treemeanfruitweight(g)1.trunk notching  28.6  256  112  20.9  171  123 2. trunk notching   + root pruning  28.7  265  111  21.7  194  113 In the second season we can assume that we started with a lower scab pressure than the year  before.  Again  we  carried  out  root  pruning  and  again  we  saw  the  same  effect  on vigour. This year the summer was very wet. Scab incidence on the fruit was lower than in 2006 but again the effect of vigour control was marked. Root pruning reduced the number of scab fruits with 82%. Scab incidence on the shoots has not been measured yet. Production in both years was roughly the same for both treatments (65 and 52 ton/ha). The average fruit weight was below optimum in both years. In 2006 no hand thinning was carried out to see the effect of the root pruning on fruit set and June drop. In 2006 the orchard had a severe infestation with pear suckers (Psylla piri) which led to suboptimal fruit growth. The impression of the group is that this level of growth control does not have to lead to suboptimal fruit size if hand thinning is carried out properly as has been done in the rest of the orchard. Trial 2. This orchard has been organic from planting and has gradually developed a high scab pressure which fluctuates from year to year. Leaf degradation is good and vigour medium. Apart  from  scab,  canker  (Nectria  galligena)  and  dying  of  mixed  buds  (Alternaria?Pseudomonas?) pose problems in this orchard. After the good results with trial 1 a second trial  was  started  here  to  check  the  effect  on  scab  under  different  conditions  and  see whether vigour control also has an effect on canker and dying mixed buds. http://orgprints.org/13634/32Because this location is vulnerable to drought and sweet water is scarce, root pruning is considered risky by the grower. Therefore different vigour control measures are compared. Due to wet conditions in summer, all treatments had a weed cover on the tree strip during summer which minimizes the difference between treatments 1 and 4. Table 4. Trial 2. Percentage of the fruit in different scab categories.   cat 0  cat 1  cat 2  cat 3  Total incidence 1. untreated  33  b  51  b  13  a  3  a  67 % 2. root pruning  57 a  34  a  8   a  1  a  43 % 3. trunk notching  40  b  47  b  10  a  3  a  60 % 4. grasscover treestrip  35  b  51  b  11   a  3  a  65 % Table 5. Trial 2. Average production per tree and mean fruit weight.    kg/ tree  mean fruit weight (g) 1. untreated  10.1  181 2. root pruning  10.5  163 3. trunk notching  11.2  173 4. grasscover treestrip  11.0  168 In July the effect of vigour control was visible but the differences between treatments and between individual plots were not very clear. As in Trial 1. the effect on scab incidence and severity  was  surprisingly  clear  for  the  root  pruning  treatment.  The  reduction  of  scab incidence from 67 to 43 % comprises an effect of 34%. Due to the dying of many mixed buds the production was low with around 23 ton/ha. The effect of root pruning on fruit size was no problem under these circumstances. Trial 3. Aim of this trial was to determine when it is possible to stop spraying so as to save sprays and reduce residue on the fruit. Trials on some farms and in some years gave sometimes incomprehensible results. The trials presented here show a picture that fits the general observations made by the growers. Table 4 Trial 3. Scab incidence on fruit and twigs after ending spraying at different dates.    Location 1 Location 2   fruit  fruit  twigs fruit   2006  2006  2006 2007incidence  incidence  incidence  severity lesions / m incidence1. stop spraying at July 1.  1.6 %  76 %  41 %  12  44 % 2. stop spraying at Sept. 1.  0.4 %  28 %  22 %  2  26 % Location 1 is an orchard where scab has been kept under control since conversion and the pressure is very low. Location 2 is typical for many organic orchards where spraying is done till right before harvest to keep the fruit reasonably “scab-free”. The majority of the fruits show just a few lesions. Ending spraying at July 1 leads to a sharp increase of scab incidence. The summer infections lead to small lesions and mainly a rise in the percentage category 1 fruit. http://orgprints.org/13634/33Because pears with just a few small lesions are still marketable some growers may ignore this. The amount of twig scab shows the same trend though and thereby ignoring late summer infections may lead to a further build up of the scab pressure in an orchard. Discussion Some  15  years  back,  controlling  scab  on  organic  Conference  pears  was  not  really  a problem. Since then it has become very hard to keep scab off the fruit and the twigs. As the results of trial 3 show, it is necessary to keep the fruit covered with fungicides almost until harvest time. These results strengthen the impression that the fruit of Conference pears  stays  very  susceptible  throughout  the  summer.  In  this  pears  are  different  from apples where the susceptibility of the fruit is lower in summer and offers the chance to minimize spraying once you have come (almost) clean through the primary season. With pears summer sprays are needed to prevent fruit scab. Another reason for a tight schedule in summer is the prevention of twig scab. The amount of twig scab is a critical factor in the epidemiology of pear scab. Once an orchard develops more than a little twig scab it becomes very hard to keep the epidemic under control. Already pre-bloom the constant spread of conidiospores from the twig lesions sets in. This may lead to very early infections of the fruit. In fact the fruitlets may be infected already before bloom, at the stage when the buds are opening and the individual flowers are still tightly together. Our hypotheses is that during heavy rainfall spores are washed in and then have optimal condition to germinate on tissue of which we can assume that it is not well covered with fungicides. An experiment to test this hypothesis failed two years in a row because of dry weather conditions during the critical period. These early fruit infections become visible shortly after bloom. At this moment we seldom find scab at the leaves. With ascospore discharge almost finished these early lesions on the fruit may for a short period be an important source of conidiospores, next to lesions on year-old twigs which were not cut away during winter pruning. In 2006 we labelled these infected fruitlets to find out how long they last on the tree and observe the development of the lesions. We found that within 4 weeks 50% of the infected fruitlets had aborted, after 8 weeks 20% were left and at harvest less than 5 %. While lesions at fruitlets of 15-20 mm in cross section are sporulating very actively, this seems to diminish rapidly. Our hypothesis is that these early lesions on fruit can be an important motor in the scab epidemic, just after bloom when ascospore discharge is at its end. To keep scab under control we think the following conditions must be met, which may be reached by certain measures: Condition  Measures Low ascospore pressure in spring  Leaf shredding, degradation enhancing sprays, antagonistic fungi? Low conidiospore pressure in spring  Pruning  away  infected  twigs,  high  dosage limesulpur sprays at bud break. Preventing early fruit infections  ??? An open canopy and timely cessation of shoot growthSufficient crop load: vigour control measures http://orgprints.org/13634/34The most important factor determining this year’s scab epidemic is the successfulness of last  years  prevention  programme.  A  build-up  of  pressure  must  be  prevented.  Of  the different measures that can be thought of vigour control is one that can have far reaching consequences, both positive and negative. On the positive side we hope that a more open canopy will positively affect the microclimate inside the tree and enable better coverage when spraying. Vigour control leads to an earlier cessation of shoot growth which may shorten the period in which susceptible tissue is available in the form of young leaves and shoots. On the negative side we may have diminished fruit size and thereby income loss. The general idea amongst pear growers is that pear trees need to be rather vigorous to be productive.  Training  and  pruning  are  aimed  at  concentrating  this  growth  at  the  end  of branches. Finding the balance between fruit size and vegetative growth is an important task.From trials 1 and 2 we can conclude that vigour control can contribute substantially to controlling scab. We were surprised that relatively minor differences in growth level can lead to such big differences in scab incidence and severity. Not only does this measure lead to less scab on the fruit but more important we have the impression it is an important tool to bring scab under control again once it has gotten out of hand as can be seen in the example  of  trial  1.  In  this  trial,  which  is  still  being  continued  we  are  now  going  to supplement the vigour control measure by a different pruning strategy to make the trees even more open. Of course other measures should also be applied here to bring scab under control again. A question of the long term is whether the current planting systems are really suited for organic growing. Maybe other tree shapes and according planting systems offer better opportunities to combine good scab control with the desired productivity and fruit quality.  Currently a lot is awaited from new spray products bases on potassium bicarbonate for instance Armicarb.  In an extensive on farm trial in 2006 we found that during summer this product worked reasonably well provided the scab pressure was low (unpublished results). If European organic growers want to diminish their use of copper by switching to other products all other conditions must be met to keep scab pressure low.Acknowledgements Thanks are due to the Dutch Organic Pear Grower group for there active participation in this research project: in helping to carry out the trials, making observations, discussing the results and formulating new questions and hypotheses. http://orgprints.org/13634/

Conference pears; work on system changes to enable better scab control in organic orchards in the Netherlands

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