Dead (dormant) flower buds of pear are an important phenomenon in pear production in the Netherlands. Vigourous or unbalanced tree growth and Pseudomonas syringae pv. syringae (P.s.s.) are mentioned as likely causes of dead flower buds. Pseudomonas syringae pv. syringae was occasionally isolated from diseased flower buds. However, Alternaria alternata was nearly always isolated from diseased buds and also often in symptomless flower buds. By identifying the causal agent of dead flower buds disease, an effective control strategy can be developed. In field trials it was proven that fungicide treatments can reduce disease incidences significantly

Bruggen, A.S., van

Tjou-Tam-Sin, L.T.

Vink, P.

Wenneker, M.

English

Wageningen Yield 

 839
Control of Alternaria alternata, Causal Agent of Dead (Dormant) Flower 
Bud Disease of Pear 
 
M. Wenneker  
Applied Plant Research 
Research Unit Fruit 
The Netherlands  
 
A.S. van Bruggen and L.T. Tjou-Tam-Sin 
Plant Protection Service 
Wageningen 
The Netherlands 
 
P. Vink 
Applied Plant Research 
Research Unit Flower Bulbs 
Lisse 
The Netherlands 
 
Keywords: Pyrus communis, Alternaria alternata, Pseudomonas syringae, fungicides 
 
Abstract 
Dead (dormant) flower buds of pear are an important phenomenon in pear 
production in the Netherlands. Vigourous or unbalanced tree growth and 
Pseudomonas syringae pv. syringae (P.s.s.) are mentioned as likely causes of dead 
flower buds. Pseudomonas syringae pv. syringae was occasionally isolated from 
diseased flower buds. However, Alternaria alternata was nearly always isolated from 
diseased buds and also often in symptomless flower buds. By identifying the causal 
agent of dead flower buds disease, an effective control strategy can be developed. In 
field trials it was proven that fungicide treatments can reduce disease incidences 
significantly. 
 
INTRODUCTION 
Dead flower buds are a common phenomenon in pear culture in the Netherlands, 
Belgium and Mediterranean countries (Deckers and Schoofs, 2001; Montesinos and 
Vilardell, 1991, 2001; Wenneker et al., 2004, 2006). Disease cases are also reported from 
South America; e.g., Uruguay and Brasil (E. Leoni, pers. commun.). The disease is 
characterized by partial or complete necrosis of flower buds during dormancy or 
budbreak. Depending on disease severity, symptoms vary from reduced number of 
flowers per bud to buds completely killed. 
The disease is present in most years but does not cause problems, due to the 
abundance of flower buds in normal years. However, in years with low bud numbers per 
tree, the disease causes significant (financial) losses, which was the case in 2001 in the 
Netherlands. Disease incidences may be as high as 80–90%. First reports about the 
disease in the Netherlands date from the 1960’s. The problem is mostly found in the main 
pear cultivar ‘Conference’, but cultivars such as ‘Doyenne du Comice’, ‘Verdi’ and 
‘Gieser Wildeman’ are also affected. Adequate control strategies are not available. 
Until now, the bacterium Pseudomonas syringae pv. syringae (P.s.s.) was 
commonly regarded as the causal agent of dead flower buds in pear. However, the 
relation between P.s.s. and dead flower buds in orchards has never been proven in the 
Netherlands. It was concluded that population levels of P.s.s. were not significantly 
correlated to the amount of disease, in an extensive study over ten years in Spain 
(Montesinos and Vilardell, 2001). Other possible causes mentioned are unbalanced 
(vigourous) tree growth, abiotic stresses, incompatibility between scion and cultivar, and 
other plant pathogens and pests. 
The effect of tree growth regulation has been examined. However, no positive 
effect of tree growth regulation; i.e. applications of Luxan ethephon (a.i. ethephon 48%; 
100–250 ml ha-1; 4 applications: starting 10–14 days after bloom with one week interval), 
Regalis (a.i. prohexadione-Ca, 10%; 1 kg ha-1; 3 applications: starting in 3-leaf stage with 
three week interval), or root pruning (one side of the tree in May and other side in June) 
Proc. Xth IS on Pear 
Eds.: A.D. Webster and C.M. Oliveira 
Acta Hort. 800, ISHS 2008 
 840 
was observed in the trials (Wenneker et al., 2004, 2006). From these results it was 
concluded that tree growth control is not an effective way to reduce dead flower buds 
incidence on pear. 
In our study P.s.s. was only isolated sporadically from bulk samples and 
individually diseased flower buds. This indicates that the bacterium plays a minor role in 
dead dormant flower bud disease in the Netherlands. However, the fungus A. alternata 
was (nearly in all samples) found in diseased flower buds and also in symptomless flower 
buds. By identifying the causal agent of dead flower bud disease of pear, an effective 
control strategy can be developed, e.g., fungicide schemes with Alternaria specific 
fungicides as Rovral (a.i. iprodione). 
The objectives of the project are (i) to monitor disease development, (ii) to isolate 
possible pathogens and (iii) to develop possible control strategies. 
 
MATERIALS AND METHODS 
 
Alternaria alternata (Pathogen) Assessment in Commercial Orchards 
In 2004 in 8 commercial pear orchards (cultivar Conference) random samples of 
100 flower buds per orchard were taken. In the laboratory 50 buds were individually 
assessed for the presence of symptoms, and 50 buds were individually tested for infection 
with Alternaria. The buds used for determination of infections were surface sterilized (30 
minutes in 2.5% formaldehyde-solution (a.i. 40%) and thoroughly washed in sterile 
demineralized water to remove sterilizing agent) and cut into two pieces. The flower 
primordia of each bud were plated onto Potato Dextrose Agar (PDA). 
 
Fungicide Trial 
Trials were performed in 2005 and 2006, in a pear orchard located at the 
experimental station at Randwijk, the Netherlands. The orchard was of spindle shaped 
pear trees (cultivar Conference) on Quince MC rootstock. Trees were planted in 1999 in a 
single row planting system (3.5 m x 1.5 m). The efficacy of Rovral aquaflow (a.i. 
iprodione 50%) was tested at a dosage of 1500 ml per 1000 liter (0.15% v/v). 
Spray applications were carried out with a handheld spray gun (manufacturer 
EMPAS) with a 1.2 mm ceramic hollow cone nozzle at 1.1–1.2 Mpa and a spraying 
volume of 1000 L ha-1. The experiment was done in a randomised block design with four 
replicates. Each replicate consisted of 7 trees. Observations were made on the middle 5 
trees.  
The experiment consisted of the following treatments: 
1) Untreated control. 
2) Fourteen spray applications: starting end of May 2005 – till harvest, after harvest and 
before bloom 2006. 
3) Nine spray applications: starting end of May 2005 – till harvest. 
4) Three spray applications: starting end of May 2005 with two weeks interval. 
5) Three spray applications: starting early July 2005 with two weeks interval. 
6) Three spray applications: starting early August 2005 with two weeks interval. 
7) Two spray applications: October 2005 with two weeks interval. 
8) Three spray applications: March and April 2006 with two weeks interval. 
 
Control of Alternaria alternata and Disease Assessment 
Before bloom (February 2006) per treatment 50 dormant flower buds (randomly 
taken over replicates) were individually assessed for infection with Alternaria. The buds 
used for determination of infections were surface sterilized and cut into two pieces. The 
flower primordia of each bud were plated onto PDA. Disease incidence was assessed at 
the beginning of bloom (April 2006). All flower buds per tree were counted and the 
disease incidence per tree was calculated from the overall count (and expressed as 
percentage dead flower buds). 
Mean disease incidence of all trees for each replicate was used for statistical 
 841
analysis. Effect of treatments was determined with ANOVA at a 0.05 probability level. 
The relation between infection rate and dead flower buds was determined. 
 
RESULTS 
 
Alternaria Assessment in Ccommercial Orchards 
In the commercial orchards the internal visual symptoms (i.e. necrotic spots and 
dead flower primordia) ranged from 2–50%. The infection rate ranged from 10–85%. The 
fungus Alternaria alternata was found in nearly all diseased flower buds and also often in 
symptomless flower buds. There was a very good correlation between the occurrence of 
visible symptoms and infection with Alternaria alternata (Fig. 1). 
 
Fungicide Trial 
1. Control of Alternaria Infection. Alternaria infection of the dormant flower buds 
ranged from 4–64% (Fig. 2). Lowest infection rates were observed in the most frequently 
sprayed treatments (14–9 spray applications; treatments 2 and 3, respectively). Less spray 
applications resulted in higher infections rates. Infection rates of late spray applications 
(treatments 7 and 8) were comparable to the untreated control. 
2. Control of Dead Flower Buds. Dead dormant flower bud incidences ranged from 14% 
- 48% (Fig. 3). Lowest dead flower bud incidences were observed in the most frequently 
sprayed treatments (9–14 applications; treatments 2 and 3, respectively). Less spray 
applications resulted in higher dead flower bud incidences. Late spray applications 
(treatments 7 and 8, with 2 or 3 applications, respectively) were comparable to the 
untreated control. 
3. Relation Alternaria Infection and Dead Flower Buds Incidence. Figure 4 shows the 
correlation between the infection rate of flower buds with Alternaria and the occurrence 
of dead flower buds. This figure shows that control of Alternaria reduced dead flower 
buds incidences significantly. 
 
DISCUSSION AND CONCLUSIONS 
Until recently, it was commonly accepted that the bacterium Pseudomonas 
syringae pv. syringae was the causal agent of dead flower buds of pear. This was partly 
due to the fact that Pseudomonas syringae is proven to be the causal agent of blossom 
blast. The symptoms of blossom blast are characterized by blast of blossom and leaves 
which occur in periods of cool wet weather during bloom and post-bloom stages. 
However, these symptoms differ from the symptoms of dead flower bud disease; which 
are characterized by partial or complete necrosis of flower buds during dormancy or 
budbreak. 
Extensive research in Spain (Montesinos and Vilardell, 2001) did not reveal a 
significant relation between dead flower bud incidence and Pseudomonas levels. In 
addition, antibacterial treatments control (copper and kasugamycin) did not prevent the 
occurrence of dead flower buds. Also, in the Netherlands a relation could not be proven 
(Wenneker et al., in prep.). However, the fungus A. alternata was (nearly in all samples) 
found in diseased flower buds and also in symtomless flower buds. In laboratory tests the 
pathogenicity of A. alternata was proven on flower buds of detached pear twigs 
(Wenneker et al., in prep.). Therefore, it is assumed that A. alternata is the causal agent of 
dead flower buds of pear in the Netherlands. 
The genus Alternaria encompasses both nonpathogenic and pathogenic species. 
Most Alternaria species are saprophytes (Thomma, 2003). Some species are 
(opportunistic) plant pathogens that cause a range of diseases on crops as cereals, 
ornamentals, vegetables and fruits. A. alternata is known to cause late blight in pistachio 
(Pryor and Michailides, 2002; Evans et al., 1999) and several diseases in fruit crops such 
as moldy-core in apple (Reuveni et al., 2002) and brown rot in citrus (Timmer et al., 
1998). Pathotypes per species are found with a distinct and limited host range, 
characterized by the production of host-specific toxins essential for pathogenesis 
 842 
(Johnson et al., 2000). 
The results in this paper indicate a significant relationship between disease 
symptoms in dormant flower buds and infection rate of A. alternata. Apparantely, A. 
alternata is capable of penetrating and infecting pear flower buds. The survey in 
commercial orchard revealed that the infection rate of dormant flower buds with A. 
alternata can be over 80%. This shows the potential (financial) risk for individual 
growers in years with favourable conditions for infection and disease expression. 
Important issues for further research are the differences in infection rates and disease 
expression between orchards; e.g., effect of inoculation pressure, spraying scheme, and 
cultural practices.  
Apparantely, choice of fungicides is important in achieving good control of dead 
flower bud disease. Timmer et al. (2000) noted that, though, some cultural measures can 
help to control Alternaria brown spot in citrus, fungicide applications are essential to 
produce blemish free fruit. However, choice of fungicides is important. According to 
Reuveni (2006) attempts to control Alternaria and moldy-core in apple by using foliar 
sprays of several fungicides, e.g., benomyl, captan, dodine, iprodione, mancozeb or some 
of their combinations have been unsuccessful in the past, probably due to low efficacy. 
Multiple spray applications with iprodione reduced Alternaria infections in the 
present study. Also, a significant correlation with dead flower buds disease was proven. 
Surveys in commercial pear orchards in the Netherlands revealed high infection rates in 
dormant flower buds. It is possible that standard registered fungicides in pear growing 
control most fungi, with the exception of Alternaria alternata, and thereby creating 
conditions for massive growth of Alternaria on pear buds. The registration of effective 
fungicides against Alternaria, such as Rovral (a.i. iprodione; this study), Sygnum (a 
premix fungicide containing pyraclostrobin + nicobifen (BASF); Reuveni, 2005) or 
Switch (a premix fungicide containing cyprodinil + fludioxinil (Syngenta); M. Wenneker, 
unpublished data) would be useful to control dead flower bud disease of pears in the 
Netherlands. 
 
ACKNOWLEDGEMENTS 
This research is funded by the Dutch Product Board for Horticulture. The authors 
wish to thank Ron Anbergen, Marian van Dieren and Nina Joosten for their excellent co-
operation in this research. 
 
Literature Cited 
Deckers, T. and Schoofs, H. 2001. Bacterial Problems in Belgian pear growing. The 
compact fruit tree 34(4):121–124. 
Evans, N., Michailides, T.J., Morgan, D. and Felts, D. 1999. Studies on sources of 
inoculum of Alternaria Late Blight of Pistachio. KAC Plant Protection Quarterly 
9(2):4–6. 
Johnson, R.D., Johnson, L., Kohmoto, K., Otani, H., Lane, C.R. and Komada M. 2000. A 
polymerase chain reaction-based method to specifically detect Alternaria alternata 
apple pathotype (A. mali), the causal agent of Alternaria blotch of apple. 
Phytopathology (90)9:973–976. 
Montesinos, E. and Vilardell, P. 1991. Relationship among population levels of 
Pseudomonas syringae, amount of ice nuclei, and incidence of blast on dormant 
flower buds in commercial pear orchards in Cataluna, Spain. Phytopathology 
81(1):113–119. 
Montesinos, E. and Vilardell, P. 2001. Effect of bactericides, phoshonates and nutrient 
amendments on blast of dormant flower buds of pear: a field evaluation for disease 
control. European Journal of Plant Pathology 107:787–794. 
Pryor, B.M. and Michailides, T.J. 2002. Morphological, pathogenic, and molecular 
characterization of Alternaria isolates with Alternaria Late Blight of Pistachio. 
Phytopathology 92:406–416. 
Reuveni, M., Sheglov, D., Sheglov, N., Ben-Arie, R. and Prusky, D. 2002. Sensitivity of 
 843
Red Delicious apple fruit at various phonologic stages to infection by Alternaria 
alternata and control of Moldy-Core. European J. of Plant Pathology 108:421–427. 
Reuveni, M. 2006. Inhibition of germination and growth of Alternaria alternata and 
mouldy-core development in Red Delicious apple fruit by Bromuconazole and 
Sygnum. Crop Protection 25:253–258. 
Thomma, B.P.H.J. 2003. Alternaria spp.: from general saprophyte to specific parasite. 
Molecular Plant Patholoy 4(4):225–236. 
Timmer, L.W., Solel, Z., Gottwald, T.R., Ibanez, A.M. and Zitko, S.E. 1998. 
Environmental factors affecting production, release, and field populations of conidia 
of Alternaria alternata, the cause of brown spot of citrus. Phytopathology 
88(11):1218–1223. 
Timmer, L.W., Darhower, H.M., Zitko, S.E., Peever, T.L., Ibanez, A.M. and Bushong, 
P.M. 2000. Environmental factors affecting the severity of Alternaria brown spot of 
citrus and their potential use in timing fungicide applications. Plant Disease 84:638–
643. 
Wenneker, M., Heijne, B., Tjou-Tam-Sin, L.T., Van Bruggen, A.S. and Vink, P. 2004. 
Dead flower buds of pear: effect of tree growth control, and Alternaria alternata as 
causal agent. Commun. Agric. Appl. Biol. Sci. 4(69): 415–420. 
Wenneker, M., Tjou-Tam-Sin, L.T., Van Bruggen, A.S. and Vink, P. 2006. Alternaria 
alternata, causal agent of dead (dormant) flower bud disease of pear. IOBC/WPRS 
Bulletin Vol. 29 (1) 2006. Working Group “Integrated Protection of Fruit Crops”, 
subgroup “Pome Fruit Diseases”, Proceedings of the meeting at Lindau (Germany), 31 
August – 5 September, 2002 and Piacenza (Italy), 31 August–3 September, 2005:265–
270. 
 
 
 
 
 
 
 
 
Figures 
 
 
R2 = 0.902
0
10
20
30
40
50
60
0 20 40 60 80 100
percentage of buds infected with A. alternaria
pe
rc
en
ta
ge
 o
f b
ud
s 
w
ith
 v
is
ua
l 
sy
m
pt
om
s
 
 
Fig. 1. Relation between visual symptoms and infection of A. alternata. 
 844 
 
 
 
0
10
20
30
40
50
60
70
1 2 3 4 5 6 7 8
treatment
pe
rc
en
ta
ge
 o
f f
lo
w
er
 b
ud
s 
in
fe
ct
ed
 w
ith
 A
lte
rn
ar
ia
 
 
Fig. 2. Efficacy of treatments against A. alternata infections. 
 
 
 
 
 
 
 
 
 
 
f
b
a
cd d
c
ef
e
0
10
20
30
40
50
60
1 2 3 4 5 6 7 8
treatment
pe
rc
en
ta
ge
 o
f d
ea
d 
flo
w
er
 b
ud
s
 
 
Fig. 3. Efficacy of treatments against dead dormant flower bud disease. 
 
 
 
 845
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
R2 = 0.7527
0
10
20
30
40
50
60
0 10 20 30 40 50 60 70
percentage of flower buds infected with Alternaria
pe
rc
en
ta
ge
 o
f d
ea
d 
flo
w
er
 b
ud
s
 
 
Fig. 4. Correlation between infection rate with Alternaria and dead flower buds. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 846 
 
 
 
 


Control of Alternaria alternata, Causal Agent of Dead (Dormant) Flower Bud Disease of Pear

Abstract

Similar works

Full text

Available Versions

Wageningen Yield