In plant pathology, root diseases are problems arising from the infection and colonization of the roots and collar (trunk base) of a given plant by a pathogen (causal agent). With the internal colonization of the roots, external darkening and internal rotting occur, preventing the absorption of water and essential nutrients for the proper functioning of the plant. So another technical term is “root rot”.

In addition to this local symptom in the root, symptoms also occur in the aerial part, such as yellowing of the crown, drying of leaves and tips, leaf fall and even the death of the plant. Sometimes, the tree falls due to this rot. In forests, there are several types of root diseases, which can be observed both in native and planted forests and in urban afforestation. The main impact is the mortality of the affected trees and their fall. Here are some of the main diseases:

Armillariasis or Root Rot by Armillaria:

Armillariasis is caused by fungi of the genus Armillaria, affecting a large number of woody plants and occurs, in Brazil, mainly in fruit trees (loquat, peach, apple and vine) and in pine plantations. There is no report in eucalyptus. The disease often manifests itself in pine plantations aged between one and eight years, but it can be seen in trees over 20 years old. Mortality percentages between 20 and 25% were estimated in pine areas with high pathogen infestation, in southern Brazil, at the end of 25 years of rotation. The most susceptible species is Pinus elliottii variety elliottii, and on a smaller scale of susceptibility are Pinus caribaea, Pinus patula, Pinus radiata and Pinus taeda.

Symptoms begin with a general yellowing of the needles, then tanning and drying of the canopy, which precedes the death of the trees. Death results from the destruction of the root system or from the internal girdling of the plant collar and can be accelerated in periods of lack of rain. The signs are observed in the form of intense exudation of resin, which accumulates in the soil, around the roots, or the trunk, forming a crust of soil and solidified resin. Mycelial plaques of the fungus, whitish in color, are formed below the bark, from the base of the tree up to 4 meters in height. This mycelial growth is the most important sign for the diagnosis of the disease. The fungus can also form rhizomorphs, filamentous structures similar to shoelaces, dark brown in color, visible to the naked eye, measuring 1 to 2 millimeters in diameter, which can be on or inside the bark.

The pathogen has a wide geographical distribution in the world, classified in Brazil as Armillaria mellea, but this taxonomic identification was incorrect in pine plantations in Brazil. Recent studies indicate that it is another similar species. The disease occurs in recently deforested areas, where plant residues (roots and stumps) are left that function as a source of energy for the fungus and as a source of inoculum. The spread of the fungus in the soil is by mycelium and rhizomorphs from woody plant remains, such as stumps, branches or roots. Tree mortality tends to decrease as the plantation ages, when the original source of inoculum is exhausted, and trees become more resistant.

For control, it is recommended that the recently deforested areas have plant residues removed and incinerated during the preparation of the land for planting. Little is known about the resistance of pines against Armillaria. The planting of susceptible species should be done in areas free of the pathogen or that have already been cultivated with non-host plants of the pathogen (agricultural crops, pasture and forest species). Another recommendation is that planting be done with good quality seedlings, with a well-developed root system, in deep soil, without the presence of an impediment layer. The plantations must be correctly managed, with proper pruning and thinning, to avoid stressing the trees and predisposing them to Armillaria attack.

Phytophthora Association with forests:

the oomycete Phytophthora is a pathogen with a wide geographic distribution, with economic and environmental importance for agricultural crops and planted and native forests. It was a landmark for plant pathology when, in the mid-19th century, it decimated European potato plantations and, indirectly, caused the death of thousands of people on that continent. In the literature, oomycetes are known as false fungi; however, in culture medium, they resemble them. At present, the oomycetes are classified in a group called the Straminipila.

There are seven species reported to cause disease in forest species in Brazil: Phytophthora boehmeriae, Phytophthora capsici, Phytophthora cinnamomi, Phytophthora citrophthora, Phytophthora heveae, Phytophthora nicotianae, Phytophthora palmivora, Phytophthora frigida and Phytophthora acaciae (species described in black wattle, in 2019). In this article, we will focus on black wattle gum disease caused by Phytophthora.

The black wattle is a forest species native to Australia, the main source of vegetable tannins in the world. In Brazil, black wattle was introduced in the state of Rio Grande do Sul in the 1930s. In 2019, there were around 75,900 hectares planted, involving approximately 10,000 small rural producers. Gummosis is one of its main phytosanitary problems and is distributed in all producing areas of Rio Grande do Sul.

The Phytophthora species nicotianae, Phytophthora frigida and Phytophthora acaciae have been found to cause gummosis and collar rot (base of the trunk) of black wattle trees. Soil is considered the main source of primary inoculum . This disease causes significant damage to the black wattle crop (with up to 23% of the trees attacked), by damaging the bark, mainly in the basal and middle portions of the trunk. In addition to Brazil, it also occurs in South Africa and Asian countries.

Gummosis is difficult to control, and the most viable strategy, in the long term, is the planting of resistant trees. There are recommended complementary measures, such as: a) not establishing a new planting in an area where the severity of the disease in the previous planting was high; b) avoid injuries to the plants, caused by agricultural equipment; c) avoid shallow soil, poorly drained and subject to waterlogging; d) avoid burying part of the stem of the seedlings during planting, or grounding them in the field, for subsequent cultural practices; e) use seedlings free of Phytophthora in new planting.

Such pathologies reduce the productivity of forests, both due to tree mortality and the lower development of affected individuals. In addition, there is an environmental impact due to the reduction in the capture of carbon dioxide by the forest, as well as the emission of greenhouse gases due to the decomposition of dead plants.