Fusarium

ARTICLE

For many years avocado orchards have worked in fear of a terrible pathogen, the famous Phytophthora cinnamomi that causes Avocado Tristeza. The first step is to ensure soil drainage by digging with an excavator or bulldozer and then building raised beds, especially in clay loam soil with high rainfall. In semi-arid areas, currently only dragging the soil plus a table instead of a raised bed is sufficient, especially if the soil is light and rainfall is scarce. Then it is essential to water well without causing waterlogging and to stimulate constant root growth. But even so, in the field there are weak or uneven plants that begin to decline not in the first year, but as they begin to produce, when yield is demanded from the chosen genetics. If these plants are moved by hand, they are very likely to tilt easily, showing a deficiency in anchorage. These are plants with multiple roots that never formed a vigorous taproot in the nursery and then in the third-fourth year when production is required they begin to show weakness, iron chlorosis and reduced growth. They must be removed immediately since they will never be good producers, and may even develop pathogens that can contaminate neighboring plants. If they are taken to a laboratory, it is very likely that several species of pathogenic fungi will be found, but one is very common in terminal plants, the famous Fusarium spp. which in many vegetables is lethal in a few days, while in fruit trees it does a silent work by attacking the weakest plants first. This endemic fungus can originate from the nursery, since preventive treatments or analyses are only carried out with Phytophthora cinnamomi in mind, and then it is transferred to the final site, causing a slow decline that ends up eliminating the productive potential.

We are currently intervening in nurseries to detect the presence of Fusarium spp. and determine its population level or CFU in order to then inoculate with complete Microbial Consortia that inhibit the development of various pathogens, stimulate root growth and also provide secondary metabolites that strengthen the plants, delivering a preventive treatment before sending the plants to the field. Then, during the first year of training, this inoculation is repeated to ensure a healthy start to the project.

This problem is not only observed in plants with seed rootstocks but also in clonal plants since the tolerance developed to Fusarium spp. is low. The first visible symptom is finding reddish-colored tissue in the neck or main roots.

Weeds are also carriers of Fusarium spp., with some species such as Johnson grass (Sorghum halepense) showing greater tolerance, and the dominant presence of this grass being characteristic of a soil with a high degree of infection.

In semi-arid areas with water restriction, the presence of Phytophthora as a limiting pathogen is practically not observed, but other pathogens that develop under high stress conditions are. Not only is Fusarium spp. found in isolation, but also associated with the Wood Fungus complex that attacks from an early age.

The best recipe is to work on the physiology of stress, keeping plants with high immunity throughout the season. In the nursery, inoculating the container and painting the graft preventively helps a lot to ensure rapid growth. But then the management depends on the producer, especially if the land where the project starts has already been cultivated with vegetables or fruit trees, increasing the pathogenic load of the profile. Specific pathogens from other species can attack the avocado if there is no other substrate to survive on, modifying its feeding habits to adapt to this new condition. Even in other countries there are findings of nematode attacks in soil that were cultivated by very sensitive species, leaving a very high population in the soil, affecting the development of the avocado tree. The nematode does not live inside the avocado root, but it can cause wounds in new roots, facilitating the entry of other fungi and ultimately weakening the new planting.

In all situations where a new avocado project is developed, it is recommended to carry out preventive treatments from the nursery, post-planting, and even to perform a prior planting of oats or other ancestrally used crops in rotation to lower the pathogenic load of the profile, incorporating organic matter to restore the balance to the soil.