Cultural Control of Cacao Witches’ Broom Disease (Moniliophthora perniciosa) in Colombia: Results of Two Cases

Introduction

Witches’ Broom is one of the diseases with the highest impact on the cacao crops in South America. In Brazil, despite all preventive plans, the cacao production fell from 400000 to 150000 metric tons in a decade, after the arrival of the pathogen [1]. Currently, Cacao Witches’ Broom has a wide range of distribution. It’s reported in South and Central America in countries such as Brazil, Bolivia, Colombia, Ecuador, Guyana, Peru, Venezuela, Panama and the Caribbean Islands [2]. Holliday cited by Evans (2007) [1], considers that “if the disease is discovered early enough it can be eradicated without too much difficulty”. Consequently, this paper presents two control cases: one about a cacao farm where Withes’ Broom disease appeared for the first time, and the other about a farm that suffered an epidemic episode of the disease.

Epidemiology

Garcés (1946) [5], affirms that the brooms stay green for three weeks, they get dry for one more, and then they start to produce basidiocarps even 14 weeks after their formation. According to Aranzazu [4], in the Urabá region of Colombia, a broom takes four months from its initial appearance until it becomes a source of inoculum. Once diseased tissue begins to produce fruiting bodies (basidiocarps, Figure 2), it can continue to do so for two years or more under wet conditions. At the beginning, a large number of basidiocarps are produced, but the numbers normally decline after the first year. Significantly fewer fruiting bodies are produced on detached brooms discarded on the ground, than on brooms attached to the cocoa trees [4]. Basidiospores released from basidiocarps are carried by the wind, although they are susceptible to dehydration and survive only one hour in direct sunlight [4].

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Although Baker et al. [6] affirmed that infection occurs within 100 meters from the source, Evans (2016) [6] found out 20% of cacao trap plants had become infected although they were around 800-1500 meters from the nearest cacao plantations. “A high relative humidity combined with a fall in temperature favors basidiospore release; the greater the difference between relative humidity and temperature, the greater the rate and duration of spore liberation” [6]. The spores remain viable under normal conditions for nearly 12 hours and the median range of dispersion is 70- 80 km. They are released from late evening to early morning avoiding the effects of sunlight and low humidity, which rapidly kill the spores. There is risk in leaving pruned brooms on the ground so is recommendable to remove and destroy them, Evans (2016) [6].

Cultural Control

Phytosanitary pruning of the infected organs reduces the amount of inoculum, and thus the infection level, but it will never be a suitable pathogen eradication procedure [4]. Rocha and Wheeler (1985) [7] confirmed daily moisture drought regime between 8-16 hours generates the fructification of 95% of the brooms that are in this phase. In Colombia, the regimes of moisture–drought that favor the production of basidiocarps mostly occur in the seasons of April–May and October–November. Consequently, to get an effective control of the disease in those seasons, the cacao trees should stay free of brooms during the pruning processes of February-March and July- August.

Case 1. When the Disease is Detected for the First Time

Cacaoteras del Dique was a cacao farm located in the low region of the Cauca river in the North of Colombia (150 meters above the sea level) with a humid-tropical weather. Its crops comprised 150000 thirteen-year-old hybrid cacao trees. In January 1980, the first shoots infected by the Witches’ Broom were found in trees located near pathways. A rigorous inspection of the farm trees was immediately carried out and all diseased organs were cut and discarded on the ground (cultural control). The first inspection was made for a period of three months. The information collected from January 1980 to June 1983 (three and half years) is displayed in Table 1 [8].

The number of diseased trees throughout the observation period evidently decreases. The highest number was in 1980 (549 trees), it descended to 168 in 1981, 81 in 1982 and 7 in 1983. The increment of affected trees from 1980 to 1983 was 259, which only represents 47% of the first year. The disease didn’t have any impact on the crop the following five years. The highest number of contaminated trees was found in the first three months of each year (Figure 2). From June 1983 and ahead, the control of Witches’ Broom was reduced to trimming the affected organs during the pruning of February-March and July-August. Cultural control (cutting and discarding on the ground contaminated organs) can be considered the best measure so far. The organs discarded do not have epidemiological importance.

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Case 2. When the Disease is Found during Endemic Stage

España, a cacao farm in the municipality of Dabeiba, Northwest of Colombia, 800 meters above the sea level, average temperature of 25°C, relative humidity of 75% and 2200 millimeters of rain per year. Its crops comprised 3000 hybrid cacao trees (twenty-five years or older) in total abandonment because of Witches’ Broom disease [9]. 150 trees were treated using the method of canopy renovation that consisted in cutting off the main shoots from 30, 40 or 50 centimeters from the fork. In some trees, it was possible to keep the branches that were still in good conditions. All cut-off branches were discarded on the ground. 20 trees were chosen for the purpose of keeping a record of newly formed brooms. Two months after canopy renovation, each tree was pruned leaving the branches that would form the new canopy. Every four months after canopy renovation, the number of brooms detached from the trees was recorded for the following eighteen months. Table 2 shows the newly formed brooms that were cut off from the twenty selected trees during the period of January 84 to August 85.

It should be considered that the new brooms in the canopy building process are caused by near affected trees, but cutting them off help canopies to stay in perfect conditions. The monthly average of 0.29 brooms per tree is a relatively low figure taking into account the epidemic stage of trees surrounding the area. 20 out of 150 trees (13.3%) submitted to canopy renovation died; the remaining entered into a normal production phase 2 years later.

Discussion and Conclusions

Effectively, Witches’ Broom is one of the most impactant diseases of cacao crops because it can reach levels that force the total abandonment of the plantations. However, when the disease is detected early, after three and half years the level of the infection comes lowing to its minimum degree through cultural control which consists in the removal of all organs with symptoms of the disease and their free disposal on the ground. But, the cultural process should be carried out twice a year (first round immediately after the main harvest is finished and the second five-six months later). It can be concluded that it is easier to control Witches’ Broom when is detected before an epidemic stage is reached. Nevertheless, also it is possible to recover severely affected crops through the canopy renovation method. Although the brooms were discarded on the ground, this practice didn’t have significant impact like infection sources on the trees submitted to cultural control. Other aspects that should be considered inside the cultural control are the pruning and tree height (no more than 3.5 meters), because they help to facilitate the detection and removal of the diseased organs and keep labor-cost low, respectively.