Handbook of Vanilla Science and Technology

This disease is caused by the fungus Fusarium oxysporum f. sp. vanillae, also known as Fusarium batatatis var. vanillae Tucker (Childers and Cibes 1948; Bouriquet 1954; Childers et al. 1959; Ben Yephet et al. 2003; Ranadive 2005; He 2007).

2.2.1 Description

F. oxysporum f. sp. vanillae is the most harmful fungal pathogen of vanilla, causing root and stem rotting and consequently the death of the plants. The fungus lives in the ground and is difficult to eliminate. Lesions in the roots are initially brown, which is followed by a blackening, and finally the infected tissue dries out (Figure 2.1). In general, a plant with root rotting, will also display apical rotting, stop producing new buds or shoots and, therefore, its growth pauses. A plant with root rotting does not die quickly, since it develops new roots from the aerial part of plant, which grow to the ground and, if they find sufficient moisture and organic matter, can survive for a considerable time. However, if there is not enough moisture, the stem dehydrates showing longitudinal cracking, the leaves wilt and become yellow, and the plant finally dries out and dies (Curti-Diaz 1995; Hernández-Hernández 2005). Stem rot disease begins with a dark lesion that extends longitudinally, eventually covering the stem, and the plant dries out (Loredo 1990).

Fig. 2.1 Stem rot (left), and a diseased plant exhibiting symptoms caused by Fusarium oxysporum f. sp. vanillae (right).

Heavy and prolonged precipitation, deficient soil drainage, excess shade, poor ventilation, drought stress, low nutrients, and high plant density are favorable conditions for root and stem rot disease. Plants that are not well rooted, with inadequate nutrition, overpollinated, under drought stress, or planted at high densities, are the most susceptible to the disease. The rotting of roots is observed when there is high moisture in the soil; however, the number of dead plants exhibiting the disease symptoms is higher under drought conditions.

2.2.2 Damage

The fungus causes varying degrees of damage and production losses in vanilla plantations throughout the world. In Mexico, it killed 67% of plants in a 4-year-old plantation and also infected 15% of the total fruit production (Hernaandez-Hernaandez 2005). This fungus has been the main limiting factor for some vanilla producing countries, such as Puerto Rico, Costa Rica, China, and currently Madagascar (Anonymous 2008a,b).

2.2.3 Control

When the fungus infects the plant, it is difficult to eradicate the disease. Therefore prevention of the disease through cultural methods is recommended. Some of the cultural practices that are recommended for disease prevention are:

• Use land with good drainage.

• Use healthy and vigorous cuttings.

• Maintain a 10 cm cover of mulch over the roots.

• Keep plants well-nourished.

• Avoid overcrowding plants by maintaining appropriate distance between plants (1.52.0 m × 2.0-2.5 m).

• Avoid excess shade and excess sunlight.

• Prune plants to remove infected parts.

• Avoid over pollination.

• Sterilize any new planting areas.

Some chemical agents (carbendazim fungicide) have some effectiveness against Fusarium. However, the treatments are very expensive and not practical. In addition, they contaminate the ground.

For these reasons, other strategies of control are the use of essential oils (i.e. clove and cinnamon oil), tolerant or resistant plants, and biological control micro-organisms (Tricho-derma harzianum Bacillus spp., and Pseudomonas fluorescens).

Although V. planifolia is susceptible to Fusarium, some other Vanilla spp. are resistant. Resistant hybrid plants, the product of crosses and backcrosses between (V. planifolia × V. pompona) x V. planifolia were developed in Madagascar. These plants were called “Tsy taitry”, which means “nonsusceptible” (Anonymous 1995; Grisoni et al. 1997). The hybrid plants are very vigorous and produce very heavy fruits larger than 10 mm in thickness and 20 to 30 cm in length, but were not commercially cultivated in Madagascar.

2.3 BLACK ROT (PHYTOPHTORA SP.)

2.3.1 Description