Category Archives: Diseases

Sterilizing Potting Mix or “Die Pythium, Die”

(After two years, I’m starting to realize that this blog is a ten month blog – I practically forget about it in November and December. Oh well, it’s January so it’s time to start things up once again!)

Last year I posted African Violet Disaster – Pythium in which I described how my attempts to propagate african violets all failed because some potting mixed that I’d purchased must have contained pythium spores. This soil-born fungus is fatal to african violets. While most potting mixes that you purchase are sterilized, this experience made me a little wary of what might be in the potting mix that I bring home from the store.

003This fall I ordered a couple of violets from Lyndon Lyon Greenhouses in Dolgeville, NY. The plants were packed well and arrived in great condition. But when I saw them, I knew that sooner rather than later I’d have to move them into larger pots since they arrived in small 2″ pots. That got me thinking about my last african violet disaster and hoping I could find a way to not repeat it.

While I’ve never had a problem with potting mix purchased at a particular garden center, I wanted to take an extra step to sterilize the soil just in case and to calm my concerns! I went online and found a number of different ways to sterilize potting mix, the majority of them involving baking the mix in the oven.

As I looked closer into this approach, I noted a few warnings that made me think twice about trying it. The first is that if you over-heat the mix, chemicals that are toxic to plants can develop. Talk about defeating the purpose of sterilizing. The other thing that put me off to the oven technique is that all of the sites say that baking potting mix smells terrible! If it were spring or summer and the windows were open I wouldn’t have given it much thought. But in the cold of winter with everything sealed up tightly, I wasn’t too excited about smelling up the house.

So I tried a different approach – the hot water method of potting mix sterilization. With this technique, you put the mix into a pot, pour boiling water over it and let it drain and then repeat the process two more times. This should be enough heat to kill any pathogens without overheating the mix. Also it limits the smell of sterilization. The only downside is that you can only sterilize small amounts of potting mix.

Since I was looking to transplant two african violets, a small amount of mix was fine with me. I took a clean 8″ clay pot, put a piece of paper towel over the drainage hole and then filled it about half full with potting mix. The first tea kettle of boiling water drained well but the second kettle didn’t. The paper towel had gotten clogged so I poked a small hole in it and the water drained easily. After allowing the third application of boiling water to drain, I had a clay pot about one-third full of potting mix that should be sterilized. The only problem was that it was too wet to use. So I loosened it up a little bit and put the pot aside to allow the mix to dry – the in the dry winter air of a heated home, it didn’t take long for the excess water to evaporate. In my next post I’ll show you how I used it.

While the process is a little tedious, if boiling water can limit pythium, I’ll be using this technique to sterilize all of the potting mix that I use with african violets.

The experiment has begun and only time will show how well it works…

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African Violet Disaster – Pythium

Last year I wrote a number of posts about african violets. I got some leaves from eBay and I was propagating some of the plants that I had. In the early summer, all was well.

But by the end of the summer, all of the plants had died. On larger plants, the petioles (the stalk that attached the leaf to the crown of the plant) of the leaves had turned mushy and the crown of the plant looked strange – it was as if the small leaves weren’t expanding. Instead they were growing in tight little clusters. Smaller plants lost their outer leaves and weren’t growing well.

I went to the Optimara site and used their plant doctor tab to try to diagnose this problem. After looking at a lot of different options, I came to the conclusion that my violets had fallen prey to the fungus pythium.

Pythium is a fungus that can occur in potting soil. Spores of the fungus stay dormant in the soil until the conditions are right and then they can infect a plant. While most reputable produces of potting mix sterilize their soil, last year I purchased a bag of potting soil from a local garden center that was their own mix. It looked like a good potting mix but all of the violets that I planted in it ended up getting a pythium. It must have been filled with pythium spores.

003What I’ve learned from this is that it’s best to sterilize potting mix if you have any questions about it. To sterilize a potting mix, it needs to be heated to 180 degrees for 30 minutes. This can easily be done in an oven by putting the soil in a pouch of foil and using a meat thermometer to monitor the temperature of the mix. If I had sterilized the mix from the garden center I probably wouldn’t be writing this article!

But I also realized that I had something to do with this pythium outbreak. African violets needs a constantly moist growing medium. Letting them dry out can open them up to pythium due to the stress that it causes in the plant. I’ll be the first to admit that in the summer, my houseplants face the ultimate Darwinian experiment – it’s survival of the fittest. Watering and plant care fall by the wayside in the summer.  During those months the violets dried out and then were watered and this back and forth between dry and wet helped the fungus to find a home in the violets’ tissue. While the unsterilized soil provided the pythium spores, my care – or lack there of – gave the fungus a perfect opportunity to grow.

When a violet gets pythium, there’s nothing to do but discard the plant. Fortunately none of the plants I had were very valuable.

All of this has been a great learning experience. From now on I’ll be much more selective in the potting mix that I use with violets and I’ll be sure to sterilize the mix and the pots before repotting or propagating. I’m also playing around with some different techniques for making self-watering pots which would keep violets from drying out. In addition, when I see a plant with petioles that are soft or mushy, I’ll be much more aggressive in culling the plant to prevent any spreading of the fungus.

I am a little sad that the eBay violets bit the dust – I was excited to see what the unique varieties would look like in bloom. Now that I know about pythium I may try growing some again. But for now I’ll just be content with the lesson that I learned and the new fungus that I discovered – pythium.

The Perfect Fungal Storm – Septoria Leaf Spot on Tomatoes

A few weeks ago the determinate tomatoes started looking very bad. More than half of the leaves had died on some of the plants; a couple of plants had lost all of their leaves. Tomatoes usually look a little beat up by late summer with some of the bottom leaves dying but it’s never been this bad.

There’s been a lot of talk about late blight but when I looked at the tomatoes I found that only the leaves were affected. I’ve seen late blight and when plants have it there are lesions on the fruit and stems. The stems and fruit of these tomatoes were fine. The only problem was that the leaves were dying.

Tomato Plants with Septoria Leaf Spot

With a little research I found out that the problem was septoria leaf spot, a fungal disease caused by the fungus Septoria lycopersici. The initial symptoms of this disease are small spots on the lower leaves of tomatoes. If the conditions are wet, the fungus can form fruiting bodies which produce spores. The spores are spread by rain and can defoliate the plant if the conditions are right. It’s obvious that the conditions were right for a bad case of septoria this year so what were those conditions?

The first condition was that the tomato plants this year were huge. I planted Pony Express, which is a determinate tomato, and the plants were the biggest I’ve ever seen and each was full of fruit. As an habitual “underfertilizer,” I had made the decision to fertilize a little more this year. It’s obvious that the tomatoes liked the additional fertilizing.

Septoria Leaf Spot in All Stages – unaffected leaves, leaves beginning to show spots and leaves that have died from the fungus

The second problem was that the plants were so heavy with green tomatoes that the tomato cages I got at the local big box store weren’t able to stand up to the weight of the plants. A couple of wind storms right before the fruit started to ripen knocked over most of the cages. As a result, the plants were jumbled together and no longer elevated above the ground.

The final issue was that after a dry June and early July, the rain started to fall. During the middle of July it rained a little every few days and when it wasn’t raining, it was hot and humid.

Put these three conditions together and you have the perfect storm for an outbreak of septoria. The large, collapsed plants no longer had good air circulation. Also, the septoria spores, which are found in the soil and on the infected leaves, could now splash on most of the leaves of the plants. Add to these conditions a little rain, heat and humidity and it’s no wonder that the tomatoes look so bad.

While I doubt I can prevent septoria completely, I am already making some plans for next year to limit its damage. While I have no control over the weather, I can make sure that the plants remain above the ground and have good air circulation. I plan to make sure that the tomato plants are spaced properly and have some sturdy support. I read about a way of supporting determinate tomatoes called the Florida weave that’s used by professional growers – it could work well. I might also make my own cages out of some sturdier material. I have the winter to think about this and plan for the coming year.

While the septoria has done a good job of damaging the tomatoes, I’ve still had a good harvest. There have been more than enough tomatoes for freezing and canning. So while no one wants diseases in the garden, septoria is one that isn’t too bad. It only affects the leaves and, with proper growing conditions, it can be managed.

So here’s to next year with tomatoes more widely spaced, more strongly supported and, hopefully, less affected by Septoria lycopersici.

Zucchini – Dead, but a Different Cause!

I’m a gardener who remember when zucchini was a new vegetable. On the farm we planted a pretty long row of this new squash and learned very quickly that it’s easy to be overrun with zucchini. They wouldn’t stop producing in upstate NY.

But in southern PA it’s a different story. My experience is that the zucchini plants only produce fruit for about 2-3 weeks and then they die. The problem has always been bacterial wilt. This disease is carried by cucumber beetles. When these beetles chew the leaves of zucchini, they leave behind droppings that have the bacteria Erwinia tracheiphila in them. This bacteria get into the plant, multiplies, clog up the xylem (the plant tissue that carries water from the roots to the stems and leaves) and the next thing you know, the zucchini plants are wilted and then dead. If you break the stem of a zucchini with bacterial wilt, you’ll see a sticky mucus-like substance in it – that’s the bacteria.

I’ve been dealing with this for years… until this year. For the first time the zucchini plants didn’t die from bacterial wilt. I’m not sure why, though I have two possible reasons. This year I planted a different variety – Zucchini Elite – which I purchased from Harris Seeds. It might be that this variety is somewhat resistant to bacterial wilt. The other possible reason is that last year I had hung a cucumber beetle trap in the garden. I know a lot of beetles were caught on the trap and, while I saw beetles this year, I didn’t see as many. Like I said, I’m not sure what made the difference this year.

While the zucchini didn’t die of bacterial wilt, they still died but they produced for about twice as long as other years. The killer this year wasn’t bacteria wilt but the squash vine borer (Melitta  curcurbitae.)

Squash Vine Borer Larva

The adult squash vine borer is a moth that looks like a wasp. It has two clear wings and two that are bright metallic green. The body is orange and black. The pupae of this insect overwinter in the soil and, here in PA, the adults emerge in mid to late June. The females lay eggs at the base of squash and in about 10 days, the eggs hatch and the larvae  tunnel into the stem of the plant. The larvae then feed on the inside of the stem and disrupt the nutrient and water transfer within the plant. After feeding from 4-6 weeks, the larvae leave the stems and pupate in the soil, waiting to emerge ten months later.

The borer damage causes the squash to wilt and then die. When I pulled the dead zucchini out of the ground this year I noticed that there wasn’t any mucus-like bacteria so I knew they didn’t have bacterial wilt. But what I did see was a white grub-like caterpillar inside of each stem – the squash vine borer larva.

Squash Vine Borer Damage

Pesticides don’t work very well on squash vine borers because, unless you’re able to kill the adults, once the larvae are in the stem, insecticides won’t touch them. Row covering is suggested to exclude the adults but I’ve never had very good luck with row covers. I also read that you can try planting a second crop later in the season. In northern areas this moth only produces one generation of offspring.  Zucchini planted in July will mature after the borers have finished laying their eggs.

I destroyed the plants that had squash vine borers to make sure the larvae didn’t get a chance to pupate. I also recently planted some more zucchini to see if a late planting will make a difference.

While the borers killed the zucchini, I harvested more squash than I ever have in the current garden. If I had to make a choice, I’ll take squash vine borers over bacterial wilt any day!