How to Deal with Common Plant Pathogens: Mold

Time to learn how to deal with common plant pathogens! Cultivating a garden can be incredibly satisfying, but it takes a lot of work to grow healthy, productive plants. Much of the sweat equity associated with gardening gets put in at the beginning of the season as people prepare the soil, remove unwanted plants, and learn how to provide their beloved flowers, vegetables, or hemp with everything they need to thrive.

Unfortunately, all that hard work can wind up going to waste if it’s an especially bad year for plant pathogens. There are dozens of infectious bacteria, fungi, and viruses that can affect plants, and the number of know plant pathogens is increasing all the time [1]. It would take a lifetime of study to learn about every pathogen that affects plant growth cycles or crop yields, so this article will focus on three common plant pathogens that plague novice gardeners: mold, spider mites, and mildew.

Sooty and Snow Mold

Mold plays an essential role in the ecosystem by facilitating biodegradation to enable rot and decay [2]. Unfortunately, some types of mold can also spread to otherwise healthy, living plants. The two most common culprits are sooty mold and snow mold, both of which can impact a wide range of species.

These two types of mold can be a huge problem for cannabis growers, in particular. Many gardeners start out thinking they’d like to grow hemp for personal use, then wind up opting for more mold-resistant garden plants and buy high CBD, low THC cannabis instead. That’s a perfectly fine solution, but not for those who are determined to make backyard hemp growing work for them.

Sooty Mold

Sooty mold, also referred to as gray mold, is a fungus that grows on honeydew, a sticky deposit left by certain types of plant-sucking garden pests. It forms primarily on leaves, impeding photosynthesis and stunting plant growth, often causing heavily affected leaves to drop off completely. It can also affect certain types of produce, starting out as spots and eventually turning the whole fruits or flowers to rot on the stems.

Dealing with sooty mold requires addressing the underlying problem: the honeydew produced by aphids, mealybugs, leafhoppers, and other plant-sucking insects. The best way to deal with these pests is to apply insecticidal soap or organic neem oil. Since ants often domesticate aphids, protecting them from predators to ensure access to their delicious honeydew [3], it’s also wise to keep ant populations in the garden low by setting traps around the garden.

Snow Mold

Snow mold flourishes in cold, moist conditions, which helps to explain its name. It acts on grass and other ground cover plants in the winter, leaving them looking wilted and brown come spring. The best way to deal with snow mold is to rake the matted plant matter after the snow melts to break up mold colonies and facilitate new growth. In the fall, cut back perennial plants to stop them from creating a mat beneath the snow.

Spider Mites

Spider mites are garden pests that feed on over 200 different species of plants [4]. Severe infestations can stunt the plants’ growth or even kill them outright, so it’s important to identify spider mite infestations early and deal with them quickly.

Since spider mites are tiny, the easiest way to identify an infestation isn’t to look for the insects themselves. It’s to check the plants often for signs of damage associated with an active infestation. Common signs of spider mite damage include:

  • Tiny white or yellow spots on leaves
  • Leaves with a mottled or striped appearance
  • Plants that look bleached or bronzed
  • Leaf drop
  • Distorted leaf or flower shapes
  • Webbing
  • Plant dieback, in extreme cases

Spider mites are most active during drought conditions, and they’re most likely to attack plants that are already stressed. The best way to prevent a spider mite infestation is to keep plants well-watered and check them every three to five days for signs of infestation. If they start taking over, try introducing predatory mites like Phytoseiulus persimilis [5]. or the right species of lady beetle to the garden before the infestation gets too bad. If the situation is already out of control, try insecticidal soap or horticultural oil that will kill the spider mites without reducing populations of beneficial insects.

Powdery and Downy Mildew

It’s common to hear people use the term mildew generically to refer to any kind of mold growth, but gardeners know better. Mildew is a certain type of mold with a flat growth habit. There are two types that are notorious for damaging plants: powdery mildew and downy mildew.

Powdery Mildew

Powdery mildew is a scourge to gardeners and farmers across the country. It leaves a white, dusty coating on leaves, stems, and flowers alike and affects everything from hemp to roses and apples to cucumbers. Like most fungi, powdery mildew spreads by spores and it is especially bad in humid environments [6], so the keys to keeping it at bay are to reduce the spread by giving the plants plenty of air circulation and ensuring that the foliage stays dry by watering in the morning instead of at night and ensuring proper drainage. Some gardeners also recommend spraying a baking soda and water solution or, in extreme cases, commercial fungicides.

Downy Mildew

Downy mildew also thrives in wet conditions, discoloring the upper portions of leaves and creating a spore mat of white or gray mold on their bottoms. Downy mildew affects annuals and perennials, flowers, and food crops alike and unlike powdery mildew, it does not respond to fungicides. The best way to control downy mildew is to remove and destroy infected foliage and to buy resistant cultivars whenever possible.

The Bottom Line

Growing a beautiful flower garden or producing a high yield of hemp or food crops takes a lot of work. While Mother Nature can be kind, offering gardeners much of the light, water, and nutrients their plants need to thrive, she can also be cruel. It’s impossible to prevent every pest or pathogen from entering the garden, but experienced growers know that having early control strategies in place is key. Follow the advice above to reduce the impact of mold, spider mites, and mildew in any garden.



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