Evaluation of the Incidence and Severity of Olive Leaf Spot Caused by <i>Spilocaea oleagina</i> on Olive Trees in Palestine

doi:10.4236/ajps.2011.23053

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American Journal of Plant Sciences, 2011, 2, 457-460

doi:10.4236/ajps.2011.23053 Published Online September 2011 (http://www.SciRP.org/journal/ajps)

457

Evaluation of the Incidence and Severity of Olive

Leaf Spot Caused by Spilocaea oleagina on Olive

Trees in Palestine

Mazen Salman1*, Abd-Almonem Hawamda1, Ahmad Al-Ashqar Amarni1, Mahmoud Rahil1,

Hajaj Hajjeh1, Basel Natsheh1, Ruba Abuamsha2

1Technical and Applied Research Center (TARC), Palestine Technical University-Kadoorie (PTUK), Tulkarm, West Bank, Palestine;

2National Agriculture Research Center (NARC), Jenin, West Bank, Palestine.

Email: *[email protected]

Received June 18th, 2011; revised July 14th, 2011; accepted August 22nd, 2011.

ABSTRACT

Olive Leaf Spot (OLS) is found in many parts of the world and cause readuced growth and yield in olive trees. In this

study, investigations were carried out to measure the incidence (% infected leaves) and severity (number of lesions/leaf)

of OLS in olive growing regions in Palestine, including the regions of Hebron, Bethlehem, Tulkarm, Salfit, Jenin,

Nablus, Ramallah and Qalqilyah. OLS was found in all study areas with significa ntly higher percent incidences in Jenin

and Nablus (67.16% ± 18.16% and 46.06% ± 23.70%, respectively). OLS was more severe in Nablus, Qalqilyah, Jenin

and Tulkarm (severity grade 3.0 - 3.7). Analysis indicate that there is a positive correlation (R2 = 0.597) between dis-

ease incidence and severity. OLS appeared to be particularly severe on trees that were growing in regions with higher

annual rainfall. The correlation between severity and rainfall was positive (R2 = 0.543).

Keywords: Palestine, Spil ocaea oleagina, Peacock Spot, Disease Severity, Olea Europaea

1. Introduction

Olive tree (Olea eu ropaea L.) is the most important fruit

tree in Palestine covering approximately 45% of the Pal-

estinian agricultural land [1]. The tree is distinctly im-

portant in the economic and social life of the people,

comprising one of the main sources of income, reaching

its contribution to the good years to about 13% of the

annual agricultural production [1]. One of the most

widespread fungal diseases on olive trees is the Olive

Leaf Spot (OLS), also known as Peacock Spot disease

caused by the fungus Spilocaea oleagina (Castagne)

Hughes [2].

The disease is common worldwide and serious in

cooler olive-growing regions, with yield losses estimated

up to 20% [2]. The principal symptoms of OLS are dark

green to black spots surrounded by a yellow halo on

leaves. Petioles, fruit and stems are susceptib le but rarely

have lesions [3]. The disease causes severe premature

defoliation of olive, and sometimes leads to twig death

resulting in a subsequent loss in crop yield [4]. Infection

of fruit can delay in ripening and a decrease in oil yield,

causing unacceptable blemishes on table olives [2].

Chemical fungicides are the principal method used to

control OLS throughout olive-growing regions of the

world. In o live-growing regions, which are characterized

by long dry summers, OLS is controlled by application

of copper-containing fungicides prior to winter rains [3].

However, timing of the fungicide applications was re-

ported to be critical for effective control of the disease

[5]. In additoin to that, the use of fungicides against the

disease is limitted beacuase of their high costs and possi-

ble effects on human health and the environment. Re-

gardless of material, the application rate or number of

applications, copper containing fungicides will control

OLS when disease risk is low [3]. Moreover, regular an-

nual treatment is required to prevent disease build-up in

the grove, as high disease levels may be difficult to re-

duce [6].

Control of OLS disease in Palestine is limited due to

the lack of control methods, and due to the fact that the

costs of control will reduce the economic income of the

farmers. Many copper products are used to reduce infec-

tion of the disease. However, the Palestinian farmers

avoid spraying in order to keep the production of olive

oil devoid from chemicals advantage. Despite the high

Evaluation of the Incidence and Severity of Olive Leaf Spot Caused by Spilocaea oleagina on Olive Trees in Palestine

458

importance of olive tree in Palestine, little informations

are available about pests and diseases attaking the tree.

The aim of this work was to conduct a field survay to

assess the incidence (% infected leaves) and severity

(number of lesions/leaf) of OLS in different regions in

Palestine.

2. Materials and Methods

A field assessment was conducted during the summer

2010 on olive trees in different regions in Palestine in-

cluding Hebron, Bethlehem, Ramallah, Nablus, Qalqilyah,

Tulkarm, Salfit, Jenin and Tubas (Figure 1). About 20

trees (20 - 30 years old) from infested groove in each

district were sampled randomly. On each tree, 100 leaves

were randomly selected on the surrounding four sides of

the tree and taken to the laboratory for evaluatio n of dis-

ease incidence and severity. Leaves were dipped in 5%

NaOH for 5 min and the number of lesions was counted.

Disease incidence was determined by recoding the per-

cent of infected leaves per tree. For disease severity, the

number of lesions per leaf was counted and graded; 1 (1

lesion), 2 (2 lesions), 3 (3 - 5 lesions), 4 (6 - 10 lesions)

or 5 (> 11 lesions) [7].

For statistical analysis, data on the percent of infected

leaves were Log-transformed. All data were analyzed for

variance by Analysis of Variance (ANOVA). Significant

differences among treatments were computed after

Tukey HSD test at P < 0.05.

Figure 1. Map of the West Bank and the regions surveyed.

3. Results

It was found that all regions are infected with the Olive

Leaf Spot disease (OLS). Investigations showed that

three (Hebron, Bethlehem and Tubas) of the nine regions

surveyed have lowest percentage of leaves infected with

OLS (F = 8.73, df = 8, P < 0.05) (Figure 2). The highest

disease incidence was recorded in Jenin (67.2%) fol-

lowed by Nablus (46.06%). The lowest incidence (11.3%)

is measured in Tubas (Figure 2).

The disease severity (Figure 3) was also calculated for

OLS in all regions. The severity is significantly different

between regions (F =9.17, df = 8, P < 0.05). Higher se-

verity (grade 4) was recorded in Nablus and Qalqilyah

followed by Jenin, Tulkarm, Salfit and Ramallah (grade

3). The lowest disease severity was recorded in Tubas

and Bethlehem (grade 2) followed by Hebron (grade 1).

Figure 2. Disease incidence of Olive Leaf Spot (OLS) in

different regions in Palestine. Data with different letters are

significantly different after Tukeys HSD test using ANOVA

at P < 0.05.

Figure 3. Severity of Olive Leaf Spot (OLS) in different

regions in Palestine. Data with different letters are signifi-

cantly different after Tukeys HSD te st using ANOVA at P <

0.05.

Evaluation of the Incidence and Severity of Olive Leaf Spot Caused by Spilocaea oleagina on Olive Trees in Palestine459

The relationship between disease incidence and dis-

ease severity is described by a linear regression (Figure

4). A positively significant correlation (R2 = 0.597) was

recorded between incidence and severity.

Interestingly, the prevalence of OLS significantly in-

creases with the mean annual rainfall (Figure 5). The

relationship (R2 = 0.543) between rainfall and disease

incidence was positively correlated (Figure 5).

4. Discussion

Olive Leaf Spot (OLS) disease is found in many parts of

the world and was recognized in the Mediterranean areas

for over a century [8]. The assessment of OLS levels is

Figure 4. Relationship between disease incidence (% of in-

fected leaves and severity (number of lesions per leaf) across

all trees and assessment regions.

Figure 5. Relationship between mean annual rainfall and

disease incidence. Annual rainfall data were obtained from

the Palestinian M eteorological Dep a r tme nt website.

usually indicated by incidence or severity [9]. For many

plant diseases, severity is considered to give an accurate

indication of disease effect on plants [9]. However, vis-

ual estimation of disease severity can vary between as-

sessors whereas assessment of disease incidence is faster

and more objective [10]. In this study disease incidence

(percent of infected leaves) and disease severity (number

of lesions per leaf) were investigated in different regions

of Palestine. The results showed a positive correlation

between disease incidence and severity.

According to this study, higher disease incidences and

severities were recorded in Jenin, Nablus, Tulakrm,

Qalqilyah and Salfit. The higher incidence in these re-

gions could be attribu ted to the wet and humid cond ition s.

The annual rainfall in these regions ranges, approxi-

mately, from 494 to 558 mm per year [11]. Cool and

moist environmental conditions are favored for epedimic

development of the fungus in cooler olive-growing re-

gions [2,12]. Moist weather conditions favor S. oleagina

sporulation, conidium germination and infection [2,12].

It was reported by Obanor et al. (2008) that conidium

production was optimal at 15˚C under high humidity

(100%), whereas conidium germination and infection

required continuous free moisture for 12 - 24 h and tem-

peratures ranging from 5˚C to 25˚C [13]. The results of

this study are in good agreement with the findings of

Obanor et al. (2008) who reported abundant conidium

production during spring and autumn, but limited co-

nidium production during the summer months.

To the Authors’ knowledge, the present study was the

first of its kind, in regard to surveying of disease inci-

dence and severity of OLS in Palestine. Epidemiological

studies are needed to determine the latent infection of

olive trees by the disease and to assess in more details the

climatic conditions for disease development under the

Palestinian environmental conditions. This will then en-

able to plan a control strategy of disease using chemical

fungicides and/or tree management strategies and devel-

opment of a disease forecasting system [14]. The effect

of OLS on tree health, fruit yield and olive oil quality

also requires further evaluation. This survey was con-

ducted during autumn season and only provides a pre-

liminary assessment of prevalence of OLS during this

period.

This study showed that OLS is widely spread in Pales-

tine with all regions being infected. However, further

monitoring is needed to better understand the disease

incidence and spread in Palestine.

5. Acknowledgements

The Authors would like to thank Dr. Azmi Awad, Pales-

tine Technical University—Kadoorie (PTUK)) for facili-

tating this work by providing the university car. Thanks

Evaluation of the Incidence and Severity of Olive Leaf Spot Caused by Spilocaea oleagina on Olive Trees in Palestine

460

also are due to Dr. Hilmi S. Salem, PTUK for his con-

structive and objective comments on the manuscript. We

would like to thank Mr. Eng. Fares Jabi and the director-

ates of the Palestinian Ministry of Agriculture for Guid-

ance and help during the project.

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