Despite all our modern technology, information sources and even some considerable experience with hydroponics, plant plagues and pestilence can still get the better of us. Although some problems are relatively easy to identify and cure, others can besiege us for weeks or, even worse, some pathogens just keep coming back year after year.
Newer growers are often confused with disease symptoms that look very much alike (and even some that don’t look so similar) and those don’t live up to their names—downy mildew, for example, often first shows as round yellow spots on the upper leaf surface. On the other hand, powdery mildew appears just as the name suggests: a sprinkling of white spores over the upper leaf surfaces.
Nonetheless, it is sometimes confused with downy mildew, which is a completely separate disease with different control methods and conditions under which it develops. To complicate matters further, furry spores from a number of pathogens can be various shades of grey, cream, brown and black, making identification from furry growths alone a hit and miss process. And it’s easy to miss the sporulation when it occurs on the undersides of leaves.
Luckily, in the controlled environment of a well-run indoor garden, fungal and bacterial disease attack is less common than in outdoor or field crops, which are exposed to the elements and (in particular) the drenching effects of rain. Wet leaves and high humidity—which can’t be controlled outdoors—create a high disease pressure, as most fungal and bacterial pathogens need moisture to infect plant tissue.
By modifying the environment effectively, we can significantly reduce the likelihood of any opportunistic disease spores being able to attack plants. While prevention of disease with correct climate control goes a long way towards a healthy crop, it unfortunately doesn’t always guarantee a problem will never occur; so, regular inspection of all plants in an indoor garden needs to be carried out. Most disease issues can be fairly easily controlled if found and dealt with early before any major damage is done.
Powdery mildew prevention in the indoor garden
Powdery mildew is perhaps the most common and frustrating of all the diseases encountered in an indoor garden, especially since commercial growers have noticed that powdery mildew—which was once relatively easy to eradicate with a few quick sprays—has become much more resilient to chemical control options and outbreaks can reoccur in rapid succession. Indoor gardeners might already be noticing the same problem.
Powdery mildew is easily recognized, although the first signs of a new infection might go unnoticed as they often occur inside dense leaf canopies. Powdery mildew creates the development of fine, whitish powdery deposits that look almost like a sprinkling of talcum powder over the upper leaf surface. As the disease progresses, entire leaves can be completely covered in this white mycelium growth—at which point the leaf will begin to yellow; dry, dead brown spots develop; and, eventually, the leaf will abscise from the plant. Stems and fruit can also become infected in severe cases.
The term powdery mildew is not just one disease, however; the name applies to the symptoms that develop. There are a number of different fungi genera that cause this powdery problem. Some powdery mildew fungi are specific to certain plants, while others have a much wider host range. Under protected cultivation in greenhouses and with indoor gardens, the most common species of fungi that cause powdery mildew symptoms are erysiphe, leveillula, microsphaera, podosphaera, odium and sphaerotheca, and possibly a few others.
Erysiphe is common on lettuce and other salad green plants, and often the leaves show characteristic yellow patches after the appearance of the white mycelium growth. Leveillua occurs mostly on tomatoes and peppers, and might also produce fungal spores on the lower leaf surfaces and stems, as well as the upper surface. Leveillua on tomatoes is a little different from other powdery mildew species as it grows unseen within the leaf tissue for a latency period of up to three weeks from first infection.
Conditions for development of powdery mildew vary depending on the species of fungi. Most of the common powdery mildew species need high humidity (greater than 90%) for infection to occur, while leveillula infection on tomatoes and peppers can occur across a wide range of relatively humidity levels.
Generally moderate to warm temperatures (68 to 86°F) favor infection. While high humidity does favor the development of many powdery mildew species, the greatest rate of infection and spread within a growing area occurs when humidity levels climb at night (thus allowing the spores to germinate and infect leaves) and then the less humid daytime air allows the newly produced spores to dry and be released into the air for further spread.
So, control of humidity, particularly nighttime relative humidity levels (which are more difficult to deal with), is seen as one of the best tools for prevention of powdery mildew. Increasing air movement up, under and through the canopy often gives a good degree of mildew control, as does preventing of overcrowding and selective pruning to let air flow through the crop.
As with many diseases, genetic resistance is also one of the best forms of powdery mildew prevention; however, few crops have a wide selection of resistant cultivars to choose from. Many cucurbit plants (such as cucumbers and melons) have hybrid cultivars with a high degree of powdery mildew resistance and these should be selected wherever possible, as mildew is a very common and devastating disease on these crops. Some tomato cultivars have resistance to oidium species of mildew, but not to other forms, and certain ornamentals, such as zinnia, might also have resistant cultivars.
Since powdery mildew has its fungal hyphae and spores exposed on the leaf surface, it should—in theory—be easy to control with sprays (provided there is good and thorough leaf coverage). There are a few natural remedies, including spraying the foliage with milk and other weakly alkaline compounds to change the pH of the leaf surface, making it less desirable for the fungal spores to germinate.
While these have been proven to have only a short-term and limited range of effect, sodium and potassium bicarbonates have been scientifically proven to be more effective for prevention of a number of common mildew-causing fungi species.
Potassium bicarbonate has proven to be more effective than sodium bicarbonate and does not compound the problem with unwanted sodium runoff. Sodium and potassium bicarbonate do need to be used with care though, as overdosing plants with baking soda will burn the foliage severely. So, the general recommendation of 0.72 oz. per gallon or less should be followed.
Another highly effective compound is sulfur, either applied as a protectant fungicide (micronized sulfur) product or in a sulfur vaporizer. Sulfur works by a process of selective toxicity—that is, the sulfur is more toxic to the disease than to the host. However, sulfur needs to used with care.
If applied when temperatures are too warm, it can cause considerable plant injury. Foliar sprays of silica, salicylic acid or chitosan also might help provide protection by providing a barrier to infection or by inducing the plant’s natural defense response to attack by powdery mildew.
There are also some biological fungicides, including those using suppressive beneficial fungi like Bacillus subtilis (serenade); however, results with these can vary somewhat depending on the environmental conditions, which need to be just right for the beneficials to grow and multiply before they can start suppressing the pathogen.
Some growers have found that light sprays of horticultural oils or neem oil have assisted with powdery mildew control, but many research studies have found these are not very effective.
Many chemical fungicides are still highly effective if used correctly and rotated so that disease resistance does not build up. The effectiveness of different fungicides can vary significantly with the particular powdery mildew species and the crop being grown, so if one product does not appear to be working, another should be tried.
Keep in mind that eradication fungicides need to be used as soon as the first powdery signs are seen as early control is critical. It is also important to check the label of spray products; many are only registered for use on ornamental crops and should not be applied to food-producing plants, and others may have withholding periods which must be waited out before the plants can be harvested and consumed
As with most disease, successful control of powdery mildew doesn’t just involve one quick fix spray. To get complete control, the environment, plant density, humidity levels, air movement, genetic resistance, natural, biological and chemical control compounds all have something to offer—and often more than one approach will be needed to get the mildew monsters under control.
Downy mildew prevention in the indoor garden
Downy mildew is a completely different disease from powdery mildew, although the two are often mistaken. It can be a major issue, particularly on lettuce and some annuals and flowering ornamentals under certain conditions.
Downy mildew produces fine greyish-white powdery patches on the underside of the lowest leaves (it will then progress up the plant if the disease remains uncontrolled). Nonetheless, the first symptom of a downy mildew outbreak is the appearance of light green to yellow spots bordered by the leaf veins on the older foliage.
Often, the appearance of these yellow spots is not linked to a downy mildew infection until the furry spore-producing bodies pop up on the undersides of the leaf. Downy mildew is common in greenhouses in winter, as periods of cool temperatures and high humidity (where leaves remain damp) encourage the mildew to attack.
The spread of downy mildew spores is maximized when night temperatures are between 41 and 50°F, with day temperatures of 54 to 68°F). Under these favorable conditions, the disease can progress from infection to sporulation (and spread via spores) in less than five days.
Control of downy mildew involves selecting genetically resistant cultivars wherever possible, modification of the growing environment and the use of fungicide rotational spray programs (downy mildew, just like powdery mildew, is capable of developing fungicide resistance).
The main environmental control option that is usually successful in controlling downy mildew is to increase air temperature (above 77°F) and reduce humidity levels at night while also avoiding getting the foliage wet and increasing ventilation.
For those who don’t wish to spray fungicides in their indoor gardens, any plant that shows symptoms of downy mildew should be removed from the growing area and destroyed to prevent spores spreading to other plants and creating an epidemic.
Good hygiene practices, including wiping and washing all surfaces with a strong disinfectant and removing all plant debris, will also help prevent any carry-over of disease from one crop to the next.
Grey mold (botrytis) in the growroom
Another common fungal disease that also develops greyish furry patches on stems, leaves and fruit—and is sometimes mistaken by new growers for downy mildew or other rot pathogens—is grey mold (Botrytis cinerea). It has a wide host range, from lettuce, tomatoes and peppers to a wide range of annuals and ornamentals, herbs and strawberries (pretty much any plant we could choose to grow). Botrytis can infect plants right from the early seedling stage and might cause damping off through all stages of the plants development, and it can even cause rots in harvested produce during storage.
Botrytis spreads by air-borne conidia—spores that germinate on leaf surfaces when conditions are wet or humid. The conidia germinate and rapidly penetrate the leaf surface cells, especially where damage might have occurred through leaf trimming or other wounds. Botrytis can also infect any rotting or dying older tissue present on the plant.
After infection, the first visible signs of this disease appear as brown water-soaked areas, from which the grey or brown mycellial growth (which sometimes resembles fine ash) forms. Conditions that favor botrytis development are high humidity (above 95%) and cooler temperatures, and the disease becomes much more severe where damp stagnant air forms and where there is insufficient air movement and ventilation.
Botrytis is a disease that has, over the years, developed multiple resistances to a wide range of chemical fungicides, making many sprays ineffective for control. The first form of defense should be modifications to the growing environment, increasing air flow, venting out moist air rapidly, particularly at night when condensation would otherwise form.
Taking care with plant density is also important, as overcrowding often leads to botrytis problems, and any leaves that show the first signs of grey mold infection should be removed before the spores have a chance to mature, become air-borne and infect surrounding plants.
Preventing overfeeding with nitrogen, which creates soft succulent growth prone to fungal infection, and maintaining good levels of foliar calcium are important to produce plant tissue strong enough to prevent opportunistic infection.
Some beneficial fungi have been formulated into products that can be sprayed onto plants to act as antagonists to botrytis, including formulations containing Trichoderma harzianum spores.
Using silica, salicyclic acid and other plant strengthening agents as both foliar sprays and in the nutrient solution might also offer some production against botrytis and other fungal pathogens by inducing the plant’s natural defense mechanisms.
Furry sporulating diseases might seem like a formidable foe; however, many outbreaks are due to a combination of problems with environmental control (humidity and stagnation of the air flow) and a source of active infection from other plant material, crop debris or spores left over from a previous outbreak.
So, cleanliness, attention to growing environment, preventing overcrowding and knowing what the early signs of an outbreak look like are all vitally important for controlling these furry plant invaders.