The terms oidium and mildew refer to a group of phytopathogenic fungi that cause diseases in plants and have similar symptoms. Oidium is also known as 'powdery mildew' and mildew as 'downy mildew'.
Fungi of this kind appear on the back of the leaves (abaxially). These fungi can usually be recognised because some whitish, grey or pink-hued marks appear on the leaves as if they had had ash sprinkled on them. Initially they look like traces of salt left on the leaves after the water has evaporated or like remains of powder, and they can therefore go unnoticed by the grower. However, as the disease advances, the leaves can end up being completely covered in this white layer and it can even colonise the buds, with subsequent losses in crop size and quality.
Although the symptoms of these fungi are apparently quite similar, there are differences between them that will help you to tell which particular fungus is attacking your plants. The best treatment against this type of fungi is prevention; once they have set in and developed they are very difficult to eradicate, sometimes even with chemical fungicides. Knowing which fungus you're fighting against will allow you to be certain of picking the best treatment and the one with the fewest side-effects, as well as knowing the best time to apply them.
If you want to use biological products to stop the fungus you will need to bear in mind that the effect is not very long-lasting; so unless you get the timing right, all you’ll be doing is wasting time and money. And a product that is effective against one type of oidium may not work against another similar-looking mildewy fungus.
In this article we will focus on the different types of oidium. In the next issue we will provide you with combat strategies.
Sphaerotheca macularis
This is an oidium that attacks a broad range of hosts, so it is very likely to have a reservoir near your grow area. It is a serious problem on strawberry and hop plants.
This fungus spends the winter in the form of asci (an ascus is a kind of bag containing a type of spore called ascospores) or mycelium on other wild or cultivated host plants (in parks and gardens, on verandas and terraces, etc.). Infected indoor and greenhouse crops also act as a good reservoir for the disease. This mycelium, which is harboured on other plants, releases conidia (a type of spore). These conidia, mostly carried by the wind, are what end up on the surface of your plant leaves; if the conditions are right, they then germinate and develop, forming new conidia, starting the cycle all over again and causing the fungus to spread to the whole plant.
The higher the concentration of conidia in the atmosphere, the greater your chances of suffering a serious attack by this fungus, and any preventative measures you take should therefore be targeted at creating conditions and applying treatments that prevent the conidia from taking hold on your plants.
This fungus develops on the surface (it does not penetrate deeper layers of the leaf), and another way of identifying it, is therefore by rubbing your finger across the leaf: the powder leaves a mark on your finger. That might fool you into thinking that it is easy to eradicate with fungicides. However, if you use biological products with a low systemic effect, the parts you haven't reached with the fungicide (or where it has not penetrated far enough), will continue to contain small parts of the fungus, which can grow back quickly under favourable conditions. That is why you must keep up the treatment even when you can no longer detect any oidium.
According to the studies we have consulted, the environmental factors involved in the establishment and development of the fungus on different hosts are as follows:
Average daily temperature
Temperatures of over 15º C favour the development of the fungus and the spread of the conidia. In general, production of conidia is reduced at temperatures of below 15-20° C or above 26° C. According to studies conducted on hops, exposure of around two hours to temperatures over 32º C reduces the incidence of the disease.
Relative humidity
The optimum range for germination of the conidia is between 75% and 98% humidity. At relatively low humidity rates, the fungus reacts by releasing a greater number of spores. This dispersion is also favoured by sudden fluctuations in humidity. The more the ambient humidity falls and the more abruptly it does so, the greater the number of spores released into the air. Although the environmental humidity is low, due to plant transpiration the leaf surface may be quite damp, facilitating germination of the conidia.
In such cases, a digital temperature and humidity gauge with maximums and minimums is a must, since it will give you a precise idea of when these high-risk conditions arise.
Rain
The rain washes away any spores floating in the air and the likelihood of infection on a rainy day is therefore low. A layer of water on the leaves can also prevent the spores from germinating and the conidia from developing and spreading.
The conidia need light to ripen, so the spores are scattered by day, roughly between 1 o’clock and 3 o’clock in the afternoon. The most critical time of day tends to come between 5 pm and 9 pm. Studies with hops show that spores that germinate in this time range are more likely to cause greater harm.
Pink rot
This is the fungus Trichothecium Roseum. Its symptoms are similar to those described above, but usually occur on the upper side of the leaf. What distinguishes this fungus from S. macularis is that during certain phases of its development it can take on a pink hue. However, in another of its stages, it has the same white or greyish colour as S. macularis, and is therefore easily mistaken for it. According to McPartland et al., another difference with S. macularis is that this fungus is usually limited to the leaves, whereas pink rot can colonise even the stems.
Unlike our previous case, this fungus is a saprophyte — i.e. it also develops on dead matter — which means that there is always a reservoir of this fungus nearby. As a result, the fungus begins to develop on the plants helped by sticky remains of pests such as white fly, plant lice, wood lice, etc.) or on remains of dust or pollen that may have been deposited on the leaves. Once it has developed and built up its strength on these remains it is in a better position to infect the living tissue.
Leveillula taurica
This oidium differs from the others in two main respects: while s. macularis and t. roseum only penetrate the surface layers of the plants, l. taurica penetrates further and colonizes deeper areas. The result is that when you run your finger over the characteristic surface ‘powder’, the residue does not come off altogether (as it does with other types of oidia) or leaves a mark on the leaf. The second difference is that, whereas the conidiophores (the parts of the mycelium in which the reproductive spores, or conidia, are located) of the fungi s. macularis and t. roseum grow on the surface of the mycelium produced, the conidiophores of l. taurica emerge through the stomata of the plant, which are located mainly on the back of the leaves.
The symptoms of l. taurica vary greatly, depending on the species on which it is growing. Under the microscope, l. taurica can be distinguished from s. macularis by the morphology of the conidia and conidiophores, and by the emergence of the conidiophores through the stomata. Remember that the stomata are mostly located on the back of the leaves, so you might notice a whitish felt running round the leaf. Very little information is available on the factors that affect this species in hop plants. Although this phytopathogenic affects around 700 different species, the principal studies carried out have been on hop plants.
These studies of l. taurica in hop plants have observed that the conidia germinate at temperatures between 10ºC and 35ºC, with an optimum temperature of 20ºC. More extreme conditions (6 hours at 40ºC) significantly reduce the viability of the spores (meaning that there is a reduced chance of infection in very warm areas in summer). It has previously been noted that the development cycle of the fungus depends on climatic variations and this is also the case here. While the optimum temperature for germination is 20ºC, no new spores form in infected plants at higher temperatures; however the optimum temperature for the growth of the mycelium is between 15º and 25ºC.
As for relative humidity levels, the most favourable conditions for germination are a day-time figure of between 85% and 95%, combined with very high night-time humidity. But although high humidity levels favour germination, they also hinder the development of the mycelium. The obvious way of ensuring that plants do not become infected is therefore by using a preventative treatment when the average temperature is between 10º and 35º and when relative humidity remains high during both day and night (such as during long periods of rain or frequent daytime showers).
If your indoor plant has already become infected (whether the symptoms are visible or not), you should ensure that you keep the temperature and humidity constant and without sudden variations. By keeping the temperature as high as possible (while taking care not to impair the development of the plant), you will help stop the formation of new spores and hinder the growth of the mycelium, thus preventing the disease from spreading. You should also use natural fungicide treatments regularly. This will prevent the release of new spores, or make them less viable, stopping the spread of the disease. At the same time, it will hinder the growth of the fungus.
Toxicity of the oidium
Turning to the intrinsic toxicity of the various oidium fungi that can attack hop plants, l. taurica and s. macularis produce no toxins that might be considered dangerous to human health. However, it is important not to forget about false powdery mildew or pink oidium, which even experienced growers find difficult to distinguish from the real thing.
Indeed, scientific studies have been carried out to try to establish the difference between the two fungi on hops, so that they can be safely identified. This pink oidium, trichothecium roseum, produces a number of micotoxins which are highly toxic in mammals. Studies carried out in 1969 showed that extracts of this fungus at various levels of concentration were fatal to mice, rabbits and even 19-day-old pigs, as well as causing other adverse effects.
Given how dangerous this fungus is, it is obviously critical to prevent the development of oidium on your plants and minimize the risk of consumption of contaminated plants.
