Collar rot is one of the most problematic diseases for tomato cultivation. This issue is caused by a highly persistent pathogen. Tomatoes are highly susceptible to diseases and pests; we have previously discussed blossom end rot, downy mildew, tomato viruses, cutworms, and tomato leafminer.
In this article, we will gain a better understanding of how to safeguard our crops from collar rot. We will explore the conditions that lead to disease proliferation and how to address it using organic methods, primarily through good agronomic practices.
Identification of Collar Rot
Collar rot, also known as basal rot, in tomato plants is a disease caused by the fungus Fusarium oxysporum f.sp. Radicis-lycopersici “Jarvis et Shoemaker 1978”.
It’s a plant pathology that can cause severe damage to production, even leading to the loss of the entire tomato crop.
The fungus responsible is a saprophytic parasite, meaning a pathogen that feeds on dead organic matter. It attacks the plant’s root system, initially causing decay in the primary root and then spreading to secondary roots. Affected plants exhibit stunted growth, withering relatively quickly, and ultimately succumbing to the disease.
Appearance of Collar Rot on Affected Plants
Let’s delve into more detail about the manifestation of this tomato disease and the visible signs on the plants.
Firstly, we might notice the disease attacking the leaves, causing initial wilting and subsequent chlorosis – a color change towards yellow.
The overall plant development is stunted, and a dark spot, more or less extensive, forms at the base where the stem connects to the roots. This spot usually affects only one side of the stem base, resembling a flame.
Another clear indication of the disease is the formation of reddish-colored mold around the stem’s base.
Finally, there’s progressive browning of the roots and stem, effectively blocking the plant’s vascular system.
Proliferation Factors and Spread Mechanisms
Initially, collar rot was predominantly observed in protected cultivation, such as greenhouses. However, over the years, open-field cultivations have also been affected.
The major problem lies in the fact that once the fungus is in the soil, eradicating it becomes extremely challenging.
Factors that encourage proliferation include water and nutritional stress, as well as temperature fluctuations with relatively low values around 15-18°C (59-64°F). The fungus typically enters the soil during the spring months. It can be easily transported by the wind or irrigation water, both of which carry the fungus’s conidia.
Another way the fungus can enter our garden is through infected nursery material, such as soil, pots, and polystyrene containers. This material might be recycled by plant producers.
Once present, the fungus can further spread through mechanical means. For instance, the tiller we use to work the soil can transfer the pathogen from one part of the soil to another. We might even inadvertently spread the fungus – a classic source of contamination is using the same scissors or gloves to handle both infected and healthy plants. Worse yet, leaving infected cultivation residues (leaves, roots, stems, etc.) in the field after a season is a sure way of propagation.
Preventive Measures and Damage Control
To combat the presence and spread of collar rot, implementing a series of good agronomic practices is essential.
If we notice that one of our plants has been infected, waiting for it to wither completely is futile. We need to moisten the soil and remove the plant, including all its roots. The plant should be sealed in a plastic bag and disposed of with general waste. Alternatively (if it can be done without risk), it should be incinerated.
The same applies to the fruits, which, by the way, are inedible and should be removed from the field.
This aspect is critical if we want to avoid the recurrence of the disease in subsequent seasons.
As a preventive measure, meticulous care should be given to the hygiene and maintenance of the irrigation system. At the end of each cycle, before transitioning to planting a new garden, it’s absolutely necessary to empty the system, remove all residues, disinfect it with hydrated lime (which you can conveniently find here), and clean it thoroughly. If this diligence is neglected, the risk, obviously, is spreading additional pathogen spores in the subsequent garden. Naturally, if our soil has been affected by the fungus, it must be regenerated. Initially, this involves deep tilling of the soil, preferably using a plow to turn over the soil clods and expose the fungus to atmospheric agents. This operation should be done around the first winter freezes. Subsequently, generous fertilization should be performed by applying manure, following a maximum dosage of 100 quintals per 1000 square meters (0.25 acres). Another precaution, especially if we suspect that the pathogen’s spread originates from nursery material, is to change suppliers or at the very least verify the hygiene standards followed by the plant producers.
Allowed Preventive Treatments in Organic Agriculture to Counter Tomato Collar Rot
In addition to the aforementioned agronomic measures, to prevent tomato collar rot, you can use certain products allowed in organic agriculture.
The first product we recommend is mycorrhizae, which are naturally occurring fungi that interact symbiotically with cultivated plants. A field treated with mycorrhizae will reap benefits such as increased root system expansion and natural resistance to fungal pathogens like collar rot. Mycorrhizae create a sort of “barrier” that shields plants from harmful fungi. The most commonly used strain of mycorrhizae in organic gardens is Trichoderma; you can find a wide selection here. Mycorrhizae are administered before transplantation, concurrently with it, and subsequently as a cover.
Another product with excellent preventive action against tomato collar rot and other fungal diseases is pythium oligandrum. This is a naturally occurring antagonist fungus in the soil and can be effectively used as a bio-fungicide. Tomato plants treated with pythium oligandrum exhibit greater resistance to pathogens like Fusarium. This product also acts preventively and should be applied at the beginning of cultivation. It is only available in highly specialized stores.
For both of the aforementioned products, we advise you to carefully read the usage instructions (dosages and application timings) provided on the labels.