Dogwood - is there any hope?

callousedknees(z5 IN)September 15, 2008

I planted several Cornus alba 'Elegantissima' after reading many positive things about them here. This is their second year in the ground. They are about 4'-5' tall now, but both years they have been covered with dark purplish red spots. It looks like anthracnose. The shrubs are now losing most of their leaves and they look terrible. Is there any cure for this disease? I keep finding treatment options, but I really don't want to have to deal with this every year. Any suggestions for a cure or a good substitute if there is none? Thanks!

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It is septoria leaf spot - very common with twig dogwoods, especially if they are under stress or grown in a climate with high summer humidity. A number of sources refer to this as "cosmetic" leaf spot, the assumption being that the effect is disfiguring but not necessarily harmful (unlike anthracnose). It has been my experience that the shrubs will eventually outgrow the problem, however my climate has low summer humidity and not much heat so your situation may prove different. This tends to be a mid- to late summer issue, so I'd recommend treating with an appropriate fungicide before you see any signs of leaf spot and repeat as necessary. At this time of the season, I'd do nothing, except to clean up and destroy any fallen foliage and the same as it drops through fall. A good mulch can reduce fungal reinfection as well as will avoiding any overhead irrigation.

    Bookmark   September 18, 2008 at 3:26PM
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callousedknees(z5 IN)

Thanks gardengal

    Bookmark   September 18, 2008 at 5:01PM
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prairiemoon2 z6 MA

Timely topic. I visited a nursery over the weekend and happened to notice that they had a row of Cornus 'Ivory Halo' with horrible looking foliage on them. I was very surprised, because I had just left my yard and I have a Cornus Ivory Halo that looked wonderful this year. I have noticed a few branches along one side, have some spottiness if you get up close to it, but that's all. The rest of the shrub has great foliage. I wondered what the difference is, because we have had the worst humidity and tons of rain in our garden this year. I assume not much could be different at the nursery, although it is not around the corner, I know people who live in that area who have been commiserating all summer about all the rain.

I have also noticed that our lilacs are almost mildew free this year and the Phlox paniculata hasn't got a drop on it. Yet during summers where we had drought and I was trying to keep up with it with sprinklers and soaker hoses, the lilacs were covered with mildew. I have wondered before if water from the hose and water from rain have a different effect on plants. There is so much chlorine in the public water now and perhaps that has some unknown negative effect. At the very least, it would seem that according to my experience at least, humidity seems to have much less of an influence in a year when along with the humidity, the plants have gotten tons of rain and therefore not stressed.

Just my observations... :-) Hoping my Ivory Halo will have a chance at a fresh start next year and I will also collect up all the fallen leaves and discard of them.


    Bookmark   September 18, 2008 at 7:16PM
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callousedknees(z5 IN)

I looked up septoria and that is definitely what it is. Thanks again gardengal. Interesting ideas prairiemoon2. I have four of the Elegantissima shrubs: three on the north side of the house on a fence row that runs north to south and one on the south side right next to the house. All on the north side have the disease but the one on the south side does not. The south location is very dry and does not get full sun. The north location is in full sun and gets all the rain plus whatever I water (which wasn't much this year). I use a soaker hose on the south bed so they are both getting a little city water, but not much. Hopefully, as they develop better root systems and are more established it will get better. Thanks for the input.

    Bookmark   September 18, 2008 at 9:28PM
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tishfromwis(z5 WI)

This is so odd! I was out by my mailbox a couple of weeks ago removing mildewed leaves off of my monarda 'Jacob Kline' when a couple of my neighbors came over to talk. I told them that the monarda looks so beautiful, but mildews so badly. I said "funny thing is that my phlox (paniculata 'david') didn't mildew this year as they have for the four years prior when I planted them. They both said "neither did mine". But one also has the same monarda and hers mildewed as well (my monarda 'pink delight' has never mildewed, but it is a short variety). We also got tons of rain this Spring and into early Summer and everything flurished. The only difference between me, prairiemoon2 and callousedknees is that we have well water, so I suppose the theory of city water versus rain water doesn't apply. I wish someone could figure it out. My monarda (Jacob Kline) was planted a year after the phlox, so maybe next year it won't have mildew either! I won't hold my breath...

    Bookmark   September 19, 2008 at 11:48AM
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Powdery mildew is primarily caused by environmental conditions - just the right combination of warm temperatures, dry soils and higher air humidity that seem to be present in specific areas beginning in late summer and continuing into fall. And some plants are very prone to the problem anyway - both phlox 'David' and monarda 'Jacob Kline' are supposed to be more resistant to PM, but again, how resistant may depend on both location and environmental conditions.

    Bookmark   September 19, 2008 at 4:27PM
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I had 3 var Dogwoods (maybe Cream Cracker?) in morning sun & moist mulched semi-clay. They always looked like garbage from the leaf spots. Tried fungicides for 2 years but didn't help much so I gave up. This past spring I composted 2 of them, but temporarily moved the 3rd into wet, sometimes flooded shade under a walnut tree, but no clay. It's now growing like crazy and has no spots - I'm using no chemicals.
I've got many other types of Cornus around my yard, & the ones w/ leaf spots are all in semi-clay. Cornelian Cherry is the only one who seems to like clay. Resources say Cornus tolerates clay, but I'm starting to doubt this.
And about powdery mildew - Years ago, my grandfather (a master gardener who grew up on a farm near humid St. Louis) told me PM is mostly encouraged by morning dew & how long it stays on leaves, and air circulation. Eastern exposure dries off dew quicker than western exposure, but most breezes come from the southwest so if a wall, fence or other plants block any air movement PM can occur easily.
And it's not how much it rains, but when (drizzle in evening is worse than buckets in the morning). It's just a combination of what Mother Nature serves up each year. There were also the many lectures about the evils of watering anything after noon-time...
I had an Amelanchier Laevis that always was grey by Aug, and he said to remove all the mulch around it - he believed that mulch can increase the humidity of the air around a plant. It seemed to help (but pulling all the weeds is a pain).

    Bookmark   September 25, 2008 at 2:12AM
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