Today we go a bit out of how to grow, or talking about garden plants to talk about crop diseases.

Diseases that cause real disasters and economic costs to avoid them, which in many cases account for a large part of the total cost of cultivation. We are talking today about Sclerotinia in lettuce.

It is no longer necessary to outline the importance of knowing how to identify a pest or disease in any crop in time. The damages can be very serious and the economic losses even more serious if it is a crop for commercial purposes.

Hence, many human and economic efforts are diverted to fighting pests and diseases.

In this case we are talking about white rot in lettuce. Two types of rot are mainly known: white and gray, which can be perfectly associated and appear at the same time. Gray rot is well known and we have already talked about it on occasion.

It is the dreaded Botrytis cinerea . White rot is produced by two fungi of the same genus, Sclerotinia . One is Sclerotinia scleroriorum and the other is Sclerotinia minor .

It is very common for them to appear associated and that is why both are included as white rot. Also, it would not be difficult to see white rot combined with gray in many cases.

WHEN AND UNDER WHAT CONDITIONS DOES SCLEROTINIA APPEAR ?

It is to be imagined that when we talk about a well-known and common disease, the conditions and stages of development of the crop in which it can develop are very broad.

It is a very polyphagous fungus , that is, it can develop apart from lettuce, in hundreds of others, (yes, you read that right) hundreds of other plants that serve them simply and simply as hosts to continue spreading without major problems. Even the remains of previous crops can be your means of survival. It doesn’t just affect lettuce.

It can also attack crops such as peppers, tomatoes or cucumbers and zucchini and even wild species that have nothing to do with crops. Such is their polyphagia, that cultural methods such as rotations are not of much use, unfortunately. Later we will see how to combat it.

In the cultivation of lettuce it can appear at the time of planting but it is usually very common from the moment it begins to nest until the moment of harvesting.

This is due to the fact that both the mycelium and the ascospores penetrate better into the old tissues (base and outer leaves) that are closer to the ground, and often in contact with it. Old leaves are susceptible to deficiencies and imbalances that weaken the tissues, allowing fungi to enter easily. From that moment on, the invasion into younger and internal areas of the lettuce is more than assured, being able to wilt and rot the plant in a couple of days if the conditions are optimal.

The optimal environmental conditions for the development of Sclerotinia sclerotiorum and Sclerotinia minor are around 20ºC, and they can develop from 4-6 ºC to 30 ºC.

If, in addition to the mild temperatures, we provide it with moisture in the form of rain, dew or very humid soil and leaves in contact with it, we have the infection assured.

Remember that just before harvest, when the lettuce is large, we have a higher risk of infection. It is the most critical moment.

SCLEROTINIA SCLEROTIA SURVIVAL

For those of you who have never heard of what a sclerotia is , we give you a brief brushstroke.

Some fungi have the ability to form dry, compact structures of latent mycelium with enough reserves to remain inactive for years, until the mycelium finds the conditions to regenerate and continue its development.

These sclerotia remain in the soil and can be perfectly transferred by mechanical action of man to other plots.

Sclerotia are the ones that make Sclerotinia difficult to control year after year , due to their ability to remain in the ground for long periods of time (up to 10 years).

In addition to the sclerotia, the Sclerotinia develops spores in its apothecia that are spread by the wind hundreds of meters.

IMPORTANT POINTS FOR YOUR CONTROL

If symptoms of white rot are observed in a lettuce crop, it is very possible that a control with an approved fungicide for lettuce neck rot is necessary.

The active ingredients at this time (July 2014) include metalaxyl or thiram in the formulations established for the cultivation of lettuce according to the Ministry of Agriculture.

Always remember that the active materials allowed are modified regularly. It is necessary to make a check of which are allowed or not at all times.

Remember that in this link you can check the registered and authorized products at all times.

NON-CHEMICAL CONTROL MEASURES

• In the case of greenhouses and seedbeds, control humidity a lot , aerating as much as possible so as not to favor the humid environments that you like so much.
• The manual control is expensive and often inefficient but emerging infections may remove debris to prevent the spread affected to some extent.
• Soil disinfection using chemical methods is contemplated but it seems to us too aggressive and sometimes inefficient. However, solarization , already commented on this page a long time ago, is an effective way to reduce, at least in the most superficial part of the soil (which is not little).
• Not long ago we talked about the advantages of tillage the soil and also those of no tillage . In the case of soils infected with Sclerotinia (usually in the form of sclerotia), deep workings enter said sclerotia to deep layers where they are degraded by other microorganisms. Here is a tilling advantage for this very specific situation.
• A practice that has been used for centuries, if not millennia, is ridge cultivation. Especially in the cultivation of lettuce it is very beneficial for the control of neck rot. The reasons are obvious. The basal part of the lettuce has much less contact with the ground and that makes it difficult for the fungus to enter.

The padding on the ridges also allows to reduce contact, ground sheets, reducing the risk of infection.

Since humidity plays an important role in the development of Sclerotinia , it is necessary to manage the risks well. The localization is obviously better to avoid the wetting of the leaves.

Likewise, it is recommended to do the watering during the morning or around noon, being the moments when the environment will dry out before the soil plant.

It is true that irrigation efficiency decreases as there is more evaporation, but in the face of a fungal problem like this, it is preferable to keep Sclerotinia at bay.