I was told many years ago by a gentleman who has a garden here on LI with over a 1000 roses, "If you want to grow roses on LI you have to spray". At that time, I was bound and determined to prove him wrong and so I initially purchased many Kordes roses that were specifically touted to be disease resistant. It has been my experience after acquiring over 138 different roses, not counting duplicates, that the gentleman was right. I believe the the fungus is everywhere - it's in the air. I did notice that some roses as they matured (3 yrs.old) they did get the black spot to a lesser degree. Almost all of my roses recover quickly.
When you have high temps AND high humidity as humans, we have to watch for heat exhaustion which may lead to heat stroke basically because we can't perspire due to the fact that our body temps reach > 105 F. This can effect all your organs. The combination of high heat and humidity effects plants transpiration process = basically it can't take up nutrients through it's roots and can't sweat or perspire like it normally does through it's leaves. The plant becomes stressed out and is susceptible to blackspot, yellowing/brown leaves which eventually defoliate. Mature leaves will also drop. I know this isn't a very scientific way of describing transpiration but I think you get the idea.
Once you have black spot all you can do is try and control it. Ideally, it is best to apply a fungicide to act as a preventative so it doesn't start in the first place. It rained for days and days here and there was no opportunity to apply a fungicide without it being washed off followed by more rainfall. When it finally did stop raining, we had high temps in the 90's along with 90% humidity - and this weather pattern continued.
The first Spring flush was marvelous. The second flush was almost nonexistent - blackspot, powdery mildew, spider mites, plants defoliated, fewer & smaller flowers, little new growth, normally orange colored flowers appeared red. This past Sat. Sun. & Monday, we had 3 days of relief with dry low humidity & temps. Tomorrow we will be back to high humidity with no end in site for at least the next week. Praying the temps don't go into the 90's.
It is my understanding that there are 5 different strains of fungus in the US. The fungicide I am using is effective here.
I apply OxiDate every 2 weeks as a spray. If there is a breakout of black spot and/or powdery mildew, it is applied every 5 - 7 days.
http://www.biosafesystems.com/...
I apply this to the leaves, the stems and to the soil under the rose canopy and adjacent plants. When pruning, the pruners are dipped into a solution of 10% bleach to water or alcohol to water mixture with each cut. After pruning OxiDate is applied. I prune in the Spring when the forsythia begins to bloom. I would use the OxiDate on the wisteria.
Water - about 7 gals. per rose per week using slow drip method.
Mulch under the roses but not touching canes.
Spider Mites - blast with water as described by jerijen in previous post - thank you
Thrips - Blue sticky Traps -
https://www.planetnatural.com/... I hang these about 15 feet away from my roses. The idea is to lure the thrips away from the roses and towards the traps. I place them in the early Spring before the irises bloom.
For Japanese Beetles you might want to consider applying Milky Spore or nematodes "specifically" for Japanese Beetles.
Slugs - lids from jars filled with beer. They don't seem to show a preference for the type of ale.
Presently, I am trying out Actinovate SP -
https://www.amleo.com/actinova...
Read the reviews. I think it's a good idea to alternate products from one year to the next.
Please feel free to contact me.
