Oidium combat strategies

In a previous article we informed you about the different types of oidium that can negatively affect your crop. In this article the focus will be on how to combat and/or prevent oidium in your crop.

In preparing a customised prevention strategy, one possible strategy you could adopt is as follows:

In outdoor crops

Growth phase

Until flowering begins, you can carry out preventative treatments using sulphur, repeating the treatment every 20 days or after rainfall (which washes the sulphur off the leaves). There are many ways of applying the sulphur. Ask in your usual grow shop. In any case, you should always follow the manufacturer’s instructions and take great care with small plants.

There are some drawbacks to using sulphur, such as problems of toxicity at high temperatures. If you prefer not to use it, you should spray the plants with some biological fungicide (see table 2) whenever two or more of the following factors occur:

• The average daily temperature is higher than 15º C (except if it has risen to 32ºC or more at some point in the day)
• Low average relative humidity (below 70%, approx.)
• Sudden drop in relative humidity
• Prolonged drought

The more of these factors that occur at any one time, the greater the chance of infection.

Table 1. Environmental factors related to ambient spore concentration of S. macularis

The greater the number of factors occurring simultaneously, the greater the risk of infection.

If daytime temperatures rise above 32º C, the risk of infection is reduced.

If the adverse conditions continue for a number of days, you should repeat the treatment every 2-3 days approximately (or as per the manufacturer's instructions). Always spray the plants in the evening at around 9 o’clock.

If you have performed all the right treatments and are sure that your plant is free from oidium, you can dispense with spraying on days when daytime temperatures are above 32º C. Note that in plants already affected by oidium (or which have had it in the past), and especially in larger plants, the ambient temperature may be considerably lower in inner and lower areas of the plant, where there is a greater concentration of leaves, lots of shade and poor ventilation. For this reason it is important to keep these parts of the plant clean, pruned and well ventilated since they can be an important reservoir of oidium. This might be one of the reasons why the first leaves to be affected can be found in the dense, shaded and under-ventilated areas.

Flowering phase

Do not use sulphur when the plant is in flower as it leaves a residue. You should treat your plants with bio-fungicides such as bee glue (propolis), specific enzymes, horsetail, etc. (See table 2). Potassium soap also tends to leave residue, and is therefore not recommended in advanced flowering. Like before, spray your plants when several of the environmental factors described above coincide (also shown in table 1), repeating the treatment (frequency as per manufacturer's instructions) if the conditions remain unchanged and always around 9 o'clock in the evening.

Table 2. Some products of biological origin with anti oidium action, available on the market.

Indoor crops

Infections in indoor plants are a result of conidia floating in from outdoors into the growing room, where they germinate and develop. Starting with a clean room, plants that are free from oidium (cuttings are silent carriers) and a new growing medium, your strategy should be to prevent spores from getting to your plants, and if any do manage to get through, to make sure they can't take hold.

Physical means of preventing spores from getting into the room. The number one way fungi get into the room is through the extractor fan. As you remove the air from your plants, it is replaced by the same volume of air from outside — which enters through the grille in the room and other apertures (e.g. beneath the door or through the bottom hole in the case of a cabinet), creating a current of air that goes all the way from outdoors to the interior of your growing room. The air that gets in through these grilles is not filtered, so that if the atmosphere is full of spores, you will be spreading them fast onto your plants. So it is very good practise to add air to your crops instead of removing it, and to pre-filter the incoming air, e.g. with a carbon filter or HEPA filter.

The same volume of filtered air that you allow into the room will be released and forced out through the grilles or openings, creating a current of clean air going from inside the room to the exterior, further preventing the entry of any spores (and insects), which would have to go against the flow to get in. This is the same principle used by laminar flow chambers and other recipients that need good asepsis.

Clearly, spores can also get into the room on your clothes. It is therefore a good idea to get into the habit of having a coat or dressing gown to hand, which you can put on before you go in.

As if these physical methods were not enough, you can also apply preventative treatments, especially at the most difficult times of year. Remember that you can use sulphur throughout growth until the day before the plants start flowering. However, when in flower, the plant will begin to grow new leaves 15 days after the last treatment. These new leaves are no longer protected by the sulphur, so if you want to be sure of having good protection (especially in plants with a long-flowering period or at times of year when the risk of oidium is highest) you will have to use other products.

In order for the treatment to be as effective as possible, you must know when the concentration of spores outdoors is at its height. Following the same rules as for outdoor crops, the greatest concentration of spores will occur on days when several of the environmental risk factors described above coincide (see table 1). When these outdoor conditions occur, you should take preventative action on your indoor plants. Always spray the plants for a few minutes before turning off the lights.

You should also keep the humidity levels constant and unfluctuating we recommend keeping humidity levels low (below 60%). Note that this humidity is good in cases in which there is no fungus on your plants and you therefore want to prevent any spores that have got in from germinating. Using fans to keep the leaves in motion can prevent the spores from taking hold on the leaf.

Many growers make the mistake of dramatically lowering the humidity when they discover that their plants have oidium. However, this sends out a signal that stimulates the fungus already on the crops to scatter its spores with the result that the disease is propagated very quickly. If you have found this type of oidium in your grow area and it is not at a very advanced stage, you should remove the most infected leaves, keep humidity constant at around 70% and treat your plants every 2-3 days with a bio-fungicide (or as per the manufacturer's instructions). Likewise, it is not advisable to keep the leaves moving with a fan if the oidium has already taken hold; the only thing you will be doing is scattering the spores even further. Once the oidium has been eradicated, you can lower the humidity and turn the fans on again to prevent it from growing back, but you should always repeat the fungicidal treatment whenever the atmospheric conditions described above arise.

To keep the relative humidity constant you can use a hygrostat connected to an intensity regulator (a power meter). The system connects the extractor at maximum power (the extractor operates at top speed) when the humidity rises above the programmed rate. When the humidity is at the right rate or lower, the hygrostat connects the extractor fan but through the regulator, which means that the extractor operates at a lower speed, which can be regulated.

In all cases, stop applying the preventative or curative treatment one week before harvesting.

There are practically no references on the damage caused by this fungus or on its biology as a pathogen. Until more information is available, you can take the following precautions:

• Spray the growing plants regularly with potassium soap to clear off of possible remains of dust, pollen or sticky remains that might have been deposited there.
• Keep the plants free from pests that might leave remains such as plant lice, whitefly, wood lice, etc.
• Avoid using plants which are genetically viable for fungus
• This fungus prefers humidity to spread its spores in, and also warmth, so you should take preventative action in these conditions.

Location of the mycelium	Microscopic diagnosis	Sporulation	Other characteristics	Fungus
On the upper and underside of leaves. Yellow spots can appear on the upper leaf surface	- The conidiophores arise from the stomata (underside of the leaf), they are wire-shaped with a conidium at the end	Image	- When you brush your finger lightly over the mycelium, it leaves a stain behind	Leveillula taurica Common name: powdery mildew
On the upper side of the leaf	- The conidiophores arise from the mycelium (on the upper side of the leaf), they are wire-shaped with the conidia forming a chain, like prayer beads	Image	- When you brush your finger lightly over the mycelium, it disappears	Sphaerotheca macularis Common name: powdery mildew
On the upper side of the leaf and on stems	- The conidiophores cannot be distinguished	Image	- In later stages of development, presents a pink colour - Associated with secretions from other pests (aphids, whiteflies, etc.)	Trichothecium roseum Common name: pink rot
Grey mycelium underside of leaves	- Sticking out of the stomata (underside of leaves), branching threads with some dark areas (sporangia) on the ends	Image	- Yellow spots occur between the veins that can turn brown in colour - Causes curling of the leaves	Pseudoperono-spora cannabina Common name: downy mildew

Finally, note that the strategies outlined here have been drawn up on the basis of information from studies conducted with these fungi on tomato, but also on other plants (strawberry and hops among others), and although they generally act similarly on all crops, research is needed on their specific behaviour in tomato in order to perfect these strategies or create new ones.

When deciding on the best prevention strategy to combat the fungi that might attack your particular plants, you should consider the weak points of the attacking fungus during each phase of the infection process.

As a grower, you want to keep your plants clean and free from any parasitic fungus at all times. It is therefore essential to prevent the inoculum from reaching your plant (the inoculum is any part of the fungus that can cause an infection). We will therefore analyse the factors that cause the fungus (and its spores in particular) to visit our crops.

We have already seen that the more spores there are in the air, the greater the chance that your plant will become infected. For indoor plants, therefore, it is a very good idea to feed filtered air to the plants, rather than extracting it. Meanwhile for outdoor plants, you will need to establish the temperature and humidity ranges which lead to the highest numbers of spores in the air. These spores may originate from various sources in the vicinity, such as horticultural and ornamental plants, weeds, waste material or your other plants. They will usually be carried on the wind, though you may be responsible for transporting some of the spores yourself, on your clothes or hair, for example.

Rainfall is another factor to take into account. As these two articles have shown, the spores of some fungi can be spread by water running off the leaves, whereas the same wet conditions can actively inhibit the spores of some other types of fungi from spreading. In other cases, rainfall promotes the growth of the fungi; and not just rain either, but water that is splashed or sprayed onto the leaves during irrigation, for example.

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You should also bear in mind that certain environmental factors can damage the inoculum or reduce its chances of survival (for example, a temperature of over 40ºC for a period of 6 hours reduces the viability of the l. taurica conidia). You also need to know exactly where the inoculum will germinate and develop the best. For example, we have seen that t. roseum begins its development on waste matter such as the sticky excretions from pests such as white fly, plant lice, or on accumulations of pollen or dust. Other fungi, on the other hand, germinate directly on the plant. So keeping your plant free of pests that secrete sticky substances and other waste will help prevent t. roseum but will not help against other fungi. You should also remove any plants that are not part of your crop and could host parasitic fungi.

If, despite your best efforts, the inoculum manage does come into contact with your plants, all is not lost. It may still be possible to prevent the germination of the spores by altering the environmental conditions. The most important factor is humidity. For any fungi, it is vital to know what level of humidity will promote germination. Some fungi prefer pools of water while others are harmed by them. The second important factor is the temperature at which the spores will germinate. As we have seen, in extreme temperatures (at certain times of the day in the summer or winter months), many of these fungi cannot germinate. However, in the case of indoor plants which you are trying to maintain at a mild temperature, limited action can be taken to adjust this temperature.

Finally, there are many substances of natural origin that will hinder the germination of fungi, so a basic part of your strategy should include the preventative application of organic fungicides. There are also other products that rather than preventing germination, stop the fungus’s germ tube from penetrating the plant. For example, some fungi take advantage of natural irregularities in the cuticle to penetrate the internal tissues. Others, meanwhile, force their way into the cuticle. In both cases, the use of oils can be helpful since they provide an extra barrier against which the fungi have to fight. There are also products that reinforce the plant’s cell walls, increasing the cell's mechanical resistance. Extract of Common Horsetail (equisetum arvense), for example, contains silicates that perform this strengthening function.

Of course, you won’t know that you have not done enough to prevent the fungus from invading and colonising your plants until the first symptoms appear. After the inoculum has penetrated the plant, it can be a long time before the appearance of the first symptoms (this is called the incubation period). During this time, the plants may appear to be perfectly healthy but the fungus is silently securing its position inside it. It is therefore important to keep using the fungicide treatments even if you can see no trace of fungus; the fact that you cannot see it doesn't mean it is not there.

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Once your plant is infected, you will have to radically change your strategy, and concentrate on deciding on the best products for eradicating the pathogen or at least slowing down its development. One thing you should bear in mind is that in general, these types of phytopathogenic fungi prefer different temperature and humidity levels for germination, dispersion and growth. This explains why most fungal attacks take place during months when the weather is mild but there are major variations in temperature and humidity, such as in the spring and autumn. If your plants are already infected, therefore, you need to determine the best environmental conditions for the growth of the pathogen, and in case of indoor plants, always try to keep the temperature and humidity of the plants’ environment constant, avoid sudden changes and if possible create conditions that will hinder the growth of the pathogen.

Of course, none of this will be of any use at all if you do not know which fungus you are dealing with. It is essential to identify its biological cycle and the optimal conditions for its development so that you can make life as difficult as possible for it. You can use other tactics to do this apart from fungicides (filtering the interior air, for example). If you do have to use fungicides, though (and these should preferably be natural ones), you can limit their use and save money by knowing your enemy’s Achilles’ heal.