The dry rot of citrus, scientific name Phoma tracheiphila, is a severe fungal disease that affects, precisely, citrus trees, especially lemon trees. It’s a cryptogamic disease that shows its symptoms on plants in winter and spring. Therefore, it’s essential to understand it well and prevent it with appropriate agronomic techniques.
In this article, we’ll see the damage caused by dry rot on citrus plants. We’ll also discover its biological cycle and how to act to prevent and treat the disease.
What are the damages of citrus dry rot?
This fungal disease can develop in two different ways, the main difference being the timing of action. It can develop slowly, attacking the tree’s canopy. Or it can act more quickly, affecting the plant’s root. This second mode is called lightning rot, and as the name suggests, the plant suddenly can no longer react. The damages caused by citrus dry rot are thus varied:
- In the aerial part of the plant, it immediately manifests with a yellowing of the leaves. This starts from the apical part, followed by a subsequent fall of the leaves themselves.
- As the disease progresses, there’s a drying out of the branches, starting from the younger ones. The inside of the affected branches has a characteristic yellow-orange color. This then turns reddish and black in the final stages of the disease. This is due to the plant’s reaction to the invasion of the woody vessels and the production of metabolites by the pathogen.
- In the case of infections of only the aerial part, the roots remain healthy. Affected plants react by producing new leaves, but in most cases, they survive only a few years.
- The infection affecting the roots has a much faster development. The plant withers, and the entire canopy dries up. The disease leads to the tree’s death within a few days.
What are the causes of this fungal disease?
What makes dry rot difficult to counter is that it’s spread by various asexual multiplication elements. These differentiate on the bark of the infected parts, and differentiation is favored by high humidity conditions and temperatures between 14 and 25 °C. These environmental conditions usually occur in citrus groves starting from late autumn. In particular, the mycelium differentiates into:
- pycnospores, produced by pycnidia;
- macroconidia, produced by specific fertile hyphae.
The infecting elements are usually carried by the wind, and once they reach the plant, they cause the primary infection. This usually occurs between October-November and continues until January-February. The plant, during this period, has reduced activity, so it doesn’t offer much resistance to the attacks of citrus dry rot. Greater susceptibility to the disease occurs in the case of root injuries, caused by too deep soil work. These are usually done in the autumn-winter period. The presence of injuries in the plant’s canopy also increases the risk of getting sick. These are often due to heavy hail or cold damage. The pathogen localizes in the blood vessels of the host plant’s wood (tracheomycosis). The infection spreads further through specific asexual multiplication elements of the mycelium. These are the so-called “thallus conidia”, which spread with the lymphatic flow.
How to prevent citrus dry rot
When we talk about fighting dry rot, we’re talking about a process that is primarily agronomic. Prevention consists of trying to reduce the possibility of infection during the plant’s highest risk moments. The good agronomic practices to follow to prevent citrus dry rot are:
- Limit or avoid injuries to the bark;
- Postpone pruning;
- Prefer planting resistant cultivars;
- Limit soil work and prefer the grassing technique;
- Transplant plants in late winter or spring;
- Immediately eliminate, using fire, the infected vegetation.
Treating citrus dry rot
An adult citrus grove at risk of dry rot can be saved with prevention based on products allowed in organic farming. This is done by spraying monthly with copper-based products, such as Bordeaux mixture, hydroxides, and copper oxychlorides. Here you can find these organic fungicides.