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How to prevent plant fungal diseases

Experiencing poor crop yields due to plant fungal diseases such as downy mildew, rose black spots, or rust? Do not see this as a necessity of reality. Plant fungal diseases are becoming more and more prevalent as the climate changes. They cause significant economic losses and are challenging to deal with, but with the right approach, remarkable results can be achieved.

Did you find yourself again in the situation in which the plants show symptoms? You probably ask yourself what this disease is and how I should treat it. Unfortunately, many growers stop there. However, the most critical question to ask should be — how should I prevent it from happening next time.

In the ideal situation, growers should be able to apply fungicides before the disease starts to develop in the field. You might be tempted to spray when there is a slight doubt. However, excessive spraying is not economical and can cause unnecessary damage to plants. We have limited opportunities to apply preventative measures, so it’s better to be at the optimal time.

One condition that should be met before a decision to spray is made is the presence of spores in the field. However, the presence of the fungal spores on plants is not enough; the disease develops only when the weather conditions support that. 

Farmers find it difficult to monitor these conditions as it requires knowledge about the environmental conditions that can not be easily obtained. But fungicides applied when spores are not in the field or when weather conditions are not suitable for the disease development get wasted. In addition, such applications increase the risk of pesticide injury to the plants, are bad for the environment and growers’ health, and reduce the potential profits at the end of the season.

So how can you decide when it is time to spray? Let us dive a bit into the technicalities. By the end of this short article, you should be informed on how to time the preventative applications correctly.

How do plant fungal diseases develop?

Fungal spores are the first stage in the fungal life cycle and the mean by which fugal diseases spread. Their mobility helps them travel by riding other organisms or by traveling long distances carried by the wind. Once they land in a supportive environment, the spores germinate and form a mycelium. The mycelium provides nutrients to the spores and supports their growth.

As the mycelium grows, it may encounter another compatible fungus. The fungi cells merge into a single cell and then split into two cells, which results in mixed genetic information. This sexual reproduction ensures genetic diversity.

In some environmental conditions, most fungi can reproduce asexually. When it’s time to reproduce, instead of branching out and combining with another mycelium, they produce mitospores (Asexual spores) that look just like the parent. These then start a new cycle and grow a new mycelium.

Germination and reproduction of the spores initiate the disease. Pathogenic fungi can live inside plants or on their surfaces. They feed on the plant tissue and damage it.

In the stage where the spores were not germinated and infection was not started, protectants fungicides can be applied. Protectants can be used in healthy plants to prevent spores from growing or penetrating the host tissue. Some examples of protectants are mancozeb, chlorothalonil, and copper-based fungicides. After the disease is initiated, other fungicides should be applied to eradicate the fungi. These fungicides should have a different mechanism of action that disturb other processes that allow the fungus to develop and survive. Therefore, it is important to alternate between different fungicides. Utilizing different mechanisms of action help to prevent resistance development.

Philippa Uwins, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Fugal spores as can be seen in electronic microscope | Philippa Uwins, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons

Spore traps can provide early warning of the spread of plant fungal diseases

How can you know if the spores were carried with the wind and arrived in your region? The direct way is to install spores traps in the fields. Agricultural spore traps are used to detect the presence of fungal spores in the air. They help farmers to take preventive measures against plant fungal diseases. Farmers should install the traps in good distribution in fields and send the air samples from the traps to the lab at a high enough frequency. The lab does a microscope analysis to quantify the spores and classify them.

The trap costs, maintenance overhead, and the need to send frequent samples to the labs might make it a non-practical practice. There are attempts to bring costs down and do the analysis in the field, but in the near future, growers might want to consider alternative approaches.

Airborne spores cause the spread of plant fungal diseases. Courtesy of Dr. Sahay
Airborne spores cause the spread of plant fungal diseases. Courtesy of Dr. Sahay

How can we learn about spores spread without installing expensive hardware?

Considering the obstacles that were discussed above, we can think of other ways to monitor the spread of spores. Plants can be thought of as spore traps, in the fields and around them. Gardens are excellent examples of places in which fungal diseases can get discovered first.

When spores land on plants and disease develops, the symptoms make it possible to identify the exact pathogen and save the need to do the lab testing. The region in which the disease is initiated is going to benefit less from the early warning but the fields around can. But actually, we can do better than that by modeling long-distance spread.

How far from the hot spot can such an observation be used as a warning? Once an area with an infestation is discovered, weather-based models can be used to predict the routes of spread of the spores by analyzing the wind direction and speed. By combining such predictions with actual observations on the ground, as the migration progresses, we can have a good understanding of the spores spread in real-time.

What are the weather conditions that support fungal disease development?

Weather variables such as temperature, humidity, and rainfall can help model the risk of spores germination and reproduction. When the optimal conditions persist for several uninterrupted hours during the day, the disease initiation is expected to happen. Each fungus should be modeled differently. Scientists arrive at such models by experimenting with the environmental conditions in a controlled environment. Growers and crop advisors can utilize such models to track the development of the fungi of interest in their fields.

With the ability to track the weather constantly and apply such models, an early warning system became possible. As discussed above, if the weather conditions are suitable but the spores are not present, an alert will cause unnecessary fungicide application. Therefore it is essential to wait for these two conditions to be met. With the aid of technology, and the ability to collect data globally, we managed to bring the predictive capabilities of this approach to an all-new level, and we made it easily accessible for you.

Summary

By considering the presence of the spores in remote areas, the wind variables, the presence of potential hosts in the route between your fields and the monitored remote regions, and the risk of spores germination and reproducing; we can provide an accurate map of the risks of many diseases across the globe. By using this approach, we can optimize the decision-making in fungicide applications without the need to install expensive hardware in fields. In Agrio we built a large community of farmers and gardeners that are monitoring their crops. The information that is being uploaded to the system, together with the identification of the plant problems, help us to see the big picture. We invite you to take advantage of this progress and increase your planning capabilities for better plant protection.