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An Organic Solution for Plant Disease and Harmful Insects

A rich fungal smell is in the air. In my area during late May and June, plentiful rain and increasing humidity promote all sorts of plant diseases and harmful insects. Blights, black spot, anthracnose and powdery mildew afflict previously pristine leaves. Colonies of aphids and whiteflies feed voraciously on succulent plant juices. If I want to have a healthy garden, I have to spray.

Most diseases in the garden are caused by fungus. As an organic gardener, I try to prevent the problems by spraying with neem oil, which reduces disease and kills harmful bugs such as mites, scales, aphids and whiteflies. Oils have limited or no affect on beneficial insects. Neem oil is a vegetable oil pressed from the fruits and seeds of the Neem tree. I immediately remove any diseased leaves when I see them, and I start spraying with a neem oil solution toward the end of May and continue to spray weekly through June, or until I don't see new disease or colonies of whiteflies and aphids. I also use neem oil in late winter as a preventative and to kill fire blight, a bacterial disease that causes the leaves of plants to wilt and to appear as if they have been burned. Fire blight overwinters on branches, twigs, and trunks.

Sometimes I use a horticultural mineral oil, which is effective for similar problems (although neem oil appears to have better fungicidal properties, probably because neem contains sulfur-like compounds, which have their own fungicidal properties). There are two types of horticultural oils. Dormant oils are used as a preventative in late winter or early spring when the weather is cool, while all-season oil can be used during the growing season also. Always read the labels. Most horticultural oils cannot be used during very hot weather. I use an all-season oil. I spray during the cooler parts of the day, and I avoid spraying any good guys such as bees.

I don't spray everything in my garden, but I always spray vegetables and fruits.Unfortunately, I must spray to keep my tomato plants healthy. I also have a ligustrum hedge that has chronic disease problems, and it has done much better since I started spraying it a couple years ago. Otherwise, I monitor all my plants and spray particular plants at the first sight of trouble. 

More organic products for garden management are becoming available because of increased public demand. All products should be used according to directions. Misused, even organic products can be harmful.

The best defense against disease and harmful insects is to plant healthy plants that are less susceptible to begin with and to plant them where they can thrive. Right plant, right place! Good horticultural practices, such as appropriate watering and mulching, are fundamental. Using compost and other organic products that feed the soil will promote a healthy underground ecosystem and long term good health. Artificial fertilizers give a boost to the plant but do nothing for the soil. In fact, artificial chemical fertilizers can build up salts in the soil that drive away earthworms and other helpful soil dwellers.  

Artificial chemicals can ultimately harm the garden. Artificial chemicals aimed at killing harmful insects or treating disease will also kill beneficial insects. This is becoming a huge problem. We need the good guys, and over 90% of insects in the garden are good guys. This insect made a tasty meal for a wren in my garden.Plants have immune systems; if you use artificial chemicals to treat your plants, you run the risk of damaging the plant's natural ability to fight pests and disease. The plant becomes weaker and more prone to damage.

It is also important to clean garden tools with isopropyl alcohol to prevent spreading disease from one plant to another. I keep a spray bottle of isopropyl alcohol in my shed along with other garden products.


For more information on keeping the garden healthy, check out these additional posts: Steps to a Healthy Garden.

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Reader Comments (22)

I usually don't use anything other than insecticidal soap on aphids and even resort to that only if repeatedly hosing down affected plants with water doesn't work but of course neither of those approaches addresses fungal issues. I didn't realize that neem oil could be helpful with powdery mildew. Thanks for the tip - I'll have to try it.

June 11, 2017 | Unregistered CommenterKris P

Great post full of important information. Let's hope more gardeners will take the dangers of using chemicals seriously. I will have to keep Neem oil in mind. Always on the look out for anything that is environmentally friendly.

June 11, 2017 | Unregistered CommenterSally

Good practice to use organic as much as possible. About to do some organic pest control here too, nematodes against vine weevil.

June 12, 2017 | Unregistered CommenterMark and Gaz

Thank you for posting explaining the use of Neem oil. I did not know to use it in late winter, thanks for the many tips. oh, and I gave up on growing tomatoes, I tend to forget the spraying schedule so it is no use I have to buy my tomatoes :-)

June 12, 2017 | Unregistered CommenterGone Tropical

Have you tried it for blackspot on roses? I need to try this.

June 13, 2017 | Unregistered CommenterPhillip

Hi everyone, thank you for your comments! I appreciate you stopping by to read my blog and to take time to comment! Phillip, Yes I use neem oil to treat black spot on roses. I try to pick off all affected leaves when I see them and spray weekly with the neem oil as described in this post. All of my roses are tough ones that are not as susceptible as others, and the neem oil works as well as anything in this climate. Deb

June 13, 2017 | Registered CommenterDeborah Elliott

Haven't had to use it in years but NEEM oil is the best. Wonder if vinegar would work instead of alcohol for cleaning tools. The reason for vinegar is because I already use it for killing weeds and ants...I think it's pet friendly.

June 13, 2017 | Unregistered CommenterPatsi

Your humidity must make fungal deceases a real problem. I noticed the sweet peas I grew in the greenhouse and have to water very regularly are now covered with white fungus. I imagine it is because they are so stressed by the heat. Good advice, thank you.

June 13, 2017 | Unregistered CommenterChristina

Excellent advice! I don't spray much, but when I do, I use Neem oil mixed with a little detergent, and mainly on the Lemon tree. I don't have a large veggie garden like you do, but I use companion planting--placing pest-repelling plants and flowers near the veggies. Good luck with the growing season!

June 13, 2017 | Unregistered CommenterBeth @ PlantPostings

Good post. I have never used neem oil. A few times I've used insecticidal soap. Generally I do nothing - either let the plant recover on its own or dispose of it.

June 13, 2017 | Unregistered CommenterJason

A great post Deb! Powdery mildew is a huge problem in my garden and it is so disfiguring and frustrating to deal with. I have been using Neem oil this spring too and find it is the most effective product to date. Like you I garden organically. I have had some problems with leaf burn however and I was wondering if you have had that too? Now that we have much cloudier, rainy days I am able to use it more and have less damage. I am considering getting a powered pump too because it takes me so long to get around with a manual pump. I am growing lots of zinnias and they are so prone to getting mildew.

June 14, 2017 | Unregistered CommenterKate R

Hi everyone, again! Patsi, I know vinegar is a good antibacterial. I don't know how effectively it will kill mold spores. I would have to do some research! Bleach will definitely work, but it can cause your tools to rust and pit if you don't wipe it off completely. Lysol and Pine-sol also work well. I like rubbing alcohol because I don't have to wash it off.

And Kate, yes any oil can cause leaf burn under hot sun. Instructions often say not to spray when temps are above 90, which eliminates a lot of our summer! I love zinnias but also have experienced powdery mildew with them. I don't have a lot of full sun, which they need, so I don't grow many. Sometimes I find some I can't resist!

Best wishes to you all! Deb

June 14, 2017 | Registered CommenterDeborah Elliott

Some good info! I've used neem oil before, though I'm not very good about keeping up with spraying to prevent problems. Thankfully we have less humidity up here, so the fungal problems aren't quite as bad as when I lived down south. I really try to space out my vegetable plants too, so that there is more air circulation. It's a hard thing to do when the plants are so little, though!

June 15, 2017 | Unregistered CommenterIndie

Deb, the timing of this post is perfect! For a while now I have managed to rely on beneficial insects appearing when they're needed, but now my climbing roses look revolting with sooty mould and aphids, and not a ladybird in sight. So I'm off to buy Neem Oil. Do you think meth. spirits would be OK too for sterilizing garden tools?

June 17, 2017 | Unregistered Commentercatmint

Hi Catmint, methylated spirits, called denatured alcohol here in the US, should work well. Best wishes on getting rid of the aphids and sooty mold! Deb

June 17, 2017 | Registered CommenterDeborah Elliott

Good idea about the spray bottle in the shed with the garden tools.

Bit concerned about neem oil and passing bees.
Must organise some alchohol to clean my secateurs!

June 27, 2017 | Unregistered CommenterDiana Studer

Hi Diana, thank you for your comment! Yes, I have to be careful not to spray neem oil directly on bees, so I avoid times of the day when they are active and watch carefully for them when I spray.
Happy gardening!

June 27, 2017 | Registered CommenterDeborah Elliott

I want to add that Neem is not so good if you have lizards, earthworms, and other beneficials. Simply because a product is certified organic and has "OMRI" symbol on the label does not mean the product is not harmful. There is a lot to read about OMRI and how that works, and it is not such a great symbol to use. Organic and in fact permaculture gardens do not use any of these in order to ensure that all beneficial beings can still participate in the eco system of the garden.

July 29, 2018 | Unregistered CommenterMarge

Hi Marg, Thank you for stopping by my blog, and I appreciate your input! What are the sources for your information? I would like to read any scientific based articles you have read.

I have done some searching on this and the research I have read states that Neem seems remarkably benign to spiders, butterflies, and insects such as bees that pollinate crops and trees, ladybugs that consume aphids, and wasps that act as parasites on various crop pests. According to the National Academies of Science, Engineering, and Medicine, this is because neem products must be ingested to be effective. Thus, insects that feed on plant tissues succumb, while those that feed on nectar or other insects rarely contact significant concentrations of neem products.

National Academies Press also states that a number of studies have found that neem products can, in fact, have a beneficial effect on earthworms. When neem leaves and seed kernels were incorporated into potting soil containing earthworms (Eiseniafoetida), the number of young worms produced increased 25 percent. In field trials there were no differences in the number of worms, but the average weight of each worm was highest in neem-treated plots. There is a fair amount of anecdotal evidence, mostly coming from organic orchardists who swear by a whole list of neem oil uses, that it’s actually helpful for the soil and arboreal food web.

I have seen little data about reptiles and neem oil. I did see one report that stated neem oil is slightly toxic to fish, but the fish have to consume plants treated with neem to be affected.
I hope this helps!

July 29, 2018 | Registered CommenterDeborah Elliott

Hi again, Here are a few places to check, concerning your response.

And please check on the fact sheets concerning toxic products. Sure, Neem is listed on the OMRI approved list, and so is Spinosad, and with the same wording for their uses. The main thing is to be very cautious. Often, tests are waived for lizards and the like. This only means they don't check, but they do check earthworms and these are toxic to them. Information often shows that they don't find toxins below 2'. Well, earthworms live above 2' in the soil. Just a tidbit more clarity.

August 1, 2018 | Unregistered CommenterMarge

Hi Marge, thank you for your links! After reading through each one, I still have not found anything that says Neem is detrimental to earthworms; in fact, the opposite. However, as you say, caution is always in order. A balanced ecosystem with integrated pest management is always best! However, I live in a climate where summer brings rampant disease and insect infestations, and the public often automatically reaches for the harmful chemical pesticides commonly found in the big box stores. I believe Neem is a much better, but certainly not perfect, alternative.
Thank you, Deb

August 1, 2018 | Registered CommenterDeborah Elliott

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