English abstract
One of Oman's most cultivated fruit crops, lime, is suffering severely from a devastating disease
called witches’ broom. Climatic conditions of the growing regions further aggravate the presence,
severity and spread of the deleterious disease. Huge efforts have been made by both farmers and
the government to alleviate the economic impact of the disease infection and improve the net
benefit from lime production. Pruning of the symptomatic branches is considered an effective
method of controlling Witches’ Broom Disease of Lime (WBDL). This study develops a dynamic
bioeconomic model using the Bellman equation to derive the optimal management strategy for
WBDL and the optimal replanting age of citrus trees after the first attack under multiple scenarios
of disease management options and climate conditions. In addition, the study investigates different
scenarios of disease presence and spread given different climatic conditions of arid, semi-arid and
tropical zones in Oman. The first novelty of the study is that it is the first to model and address the
economic loss associated with the disease risk or economic gain resulting from disease
management. Moreover, it is the first to incorporate climate conditions into bioeconomic modeling
for managing agricultural diseases or evaluate the effects of climate change and the interaction
with WBDL using a bioeconomic model. To the best of our knowledge, the study is the first
attempt to provide a detailed and thorough economic analysis of possible climatic factors that may
influence the presence and spread of the WBDL. In addition, the utilized function that describes
the total damage caused by the disease is created by incorporating the disease's behavior in the
field and the characteristics of the fruit. This study is among the few that employ a dynamic
bioeconomic model and analyze agricultural resources under risk in Arabian countries. The study
aims to help citrus growers make the best disease-mitigating decisions and policymakers in planning and implementing effective treatment programs for the alignment of both farmer and
government interests of profit maximization and social value.
The results of the study show that the farmer’s disease treatment decisions are significantly
influenced by the disease density and insect vector level on the farm, and adverse climate
conditions have a significant negative impact on the economic value of citrus cultivation. Overall,
the net benefit in the arid region is significantly lower than that of the other two zones for all the
considered scenarios. There is a 55.6% increment to the expected farm value due to the disease
management in the arid zone, while the increments are relatively lower in the semi-arid and tropical
zones, 33.5% and 36.5%, respectively. Introducing climatic factorsinto the model further increases
the benefit of implementing management action. While the increment is nearly 100% for the arid
zone, the value in semi-arid and tropical zones increase by 68.2% and 72.6%, respectively. There
is a massive reduction in farm value, 29%, due to climate conditions in arid zones if the disease is
left unmanaged (from 20,813 OMR to 14,774 OMR). However, management of the disease has
led to an improvement in farm value by 8.8% (from 29,538 OMR to 32,375 OMR). The adverse
effect of climate conditions on the expected net benefits is almost the same for semi-arid and
tropical zones, 24.5% and 4.8%, and 24.1% and 3.4%, for the considered scenarios. A similar
pattern is observed in the optimal replanting age, with the arid zone having the earliest replanting
age of 11 years in the absence of climate conditions. The rotation age further decreases to 9 years
in the presence of severe climate conditions. Hence, it is optimal for citrus growers to consider
WBDL treatment and climate conditions before any management decision for better outcomes.
The result further indicates that farmers should undertake an increasing level and frequency of
disease treatment to maximize the net benefit from lime cultivation as opposed to the current
management practice. It is recommended that the government raise awareness of the cost-sharing
strategy among neighboring farmers as a sort of cost-efficient collaborative effort to relieve it of
the financial burden. Furthermore, large-scale lime production should be done in the middle or
southern region to avoid the high risk of disease and climate in the northern region, thus
maximizing profits. The findings of this study imply that the government (regulator and subsidy
provider) as well as citrus growers should realize the importance of a well-managed and effective disease control program and the consequences of such a program on increasing the production of
citrus.
