Handbook of Vanilla Science and Technology

According to Dequaire (1976), this rotting can be caused by Fusarium oxysporum and Rhizoctonia solani (syn. Corticium solani), but the primary causal agent is not known. Days after planting, cuttings exhibit rotting of the underground section, which advances towards the upper part of the stem. In some cases, soft rotting is observed as well as the formation of white-cottony mycelium at the base of the stem (Figure 2.5).

Fig. 2.5 Rotting caused by Fusarium oxysporum and Rhizoctonia solani.

2.6.2 Damage

The percentage of damaged cuttings varies from 5 to 50%, depending on the quality and health of the cuttings, the season of planting, and the type of land. Thus, more damage is observed when cuttings are planted in rainy months. Poorly drained land presents the greater percentage of rotting. Also, the percentage of damage increases when very young cuttings are used, they are not disinfected, or the soil was already contaminated with fungi. Rotted cuttings will not produce roots or vegetative growth.

2.6.3 Control

Only healthy cuttings should be planted and they should be desinfected with carbendazim (2 g/L of water). Planting should be done during the less rainy months, but after an irrigation or rain. If the cuttings become infected by the fungus, they should be replaced by new healthy cuttings and the soil should be drenched with carbendazim (2 g/L in water).

2.7 YELLOWING AND SHEDDING OF YOUNG FRUITS

2.7.1 Description

In Mexico, the yellowing and shedding of young fruits happens 2 months after pollination and with greater intensity in June, after heavy rain. Intense sunlight with high temperatures (>32°C) and low relative humidity (<80%) are characteristic of May to June and favor infection. The fallen fruits (Figure 2.6) are of normal size, but of yellow color and smaller weight, without the floral remainder (corola). The color on the inside is coffee and with tender white seeds. After fruits fall, or even before, rotting appears in the apical part and continues throughout the fruit.

Fig. 2.6 Yellowing and falling of fruits: diseased fruits (left) and healthy fruits (right).

In Mexico, Fusarium incarnatum-equiseti species complex and Colletotrichum sp. have been isolated from yellowing fruits (Hernández-Hernández 2007). However, Fusarium is found most frequently, which is why it is considered the probable causal agent. It develops when environmental conditions are appropriate. In India, Fusarium sp. has been reported as causing the same problem (Vijayan and Kunhikannan 2007), although Colletotrichum vanillae has also been found (Anandaraj et al. 2005).

2.7.2 Damage

The damage is more severe in plantations exposed to high sunlight and with poor ventilation, for example, in plantations under plastic mesh (shade-house) with temperatures of 45°C. In these conditions, up to 90% of the fruits can fall. In plantations where the vanilla is grown on tutors of orange trees at a spacing of 5 × 5 m between trees and 7 × 7 m between rows, which results in intermediate shade and greater ventilation, losses have been quantified around 50% of fall of yellow fruits. On the other hand, in intensive systems with high densities of Erythrina sp. or Gliricidia sepium tutors (1.5 × 2.5 m) and therefore better shade, the fall of fruits has been lower than 15%. Also, the damage is more severe in the border plants, which are not protected from the sun, and in fruits without the remaining floral parts (corolla), since this favors dehydration and attack by pathogens

2.7.3 Control

During the flowering stage and development of the fruit, the conditions conducive for the development of the disease should be controled. It is important to provide the crop with greater than 50% shade and sufficient irrigation. Also, vanilla should not be cultivated in spaces with poor ventilation, since the temperature is increased, which can cause major damage. In India, it is also recommended to apply any of the following fungicides: Methylic Tiofanato (0.2%) or the mixture of carbendazim + mancozeb (0.25%), during the time of flowering and pollination, with intervals of 15 to 20 days to prevent the development of the mentioned fungi (Anandaraj et al. 2005).

2.8 VIRAL DISEASES