1st, baking soda sprays are essentially ineffective against black spot (the disease). They can be effective against powdery mildew and other fungal diseases, just not black spot.

High temperature and high humidity are two variables that can greatly increase phytotoxicity (damage to plants). The general rule for sprays of any kind is to avoid application when the temperature is above 85.

Leaf temperature is the real issue, meaning sunny days are worse than cloudy days. Leaf temperature can be up to 15 degrees higher than air temperature during an average sunny summer day.

High humidity causes two problems. First, it slows the evaporation of active chemicals, meaning the stick around longer and have more opportunity to cause damage. This is probably not an issue with baking soda and soap, since neither evaporate appreciably. High humidity also decreases the ability of a plant to lower leaf temperature through evaporative cooling, which increases phototoxicity.

Never spray when your plants are under moisture stress, as this is guaranteed to result in damage.

Does anyone in Tennessee or close areas have major blackspot problems other than me?

everyone in the south has blackspot problems lol

if you are really desperate get a willow tree or some willow tree cuttings... aspirin also basically has the same active ingredient but people will flip because its technically not organic (even though it works a little different) .... basically Salicylic acid and artificial forms of it stimulate plant defense systems ... every couple of weeks might help some ....

what varieties of roses are you growing? bs resistance varies widely... even amongst cultivars with respect to where they are grown.

Hi Tnsw: There are other organic ways ... check out the link below;

Thank you, mimosamonster,, for that great tip!

Here is a link that might be useful: Preventive measure for blackspots

anytime :)

i've selected really resistant varieties to avoid black spot... i got tired of dealing with it and subsequently found ogr in my area that are AMAZING. this question did get me remembering back to some of my experiments with it though and I remembered one that might be useful for you and your plants!

i'm a huge health nut and i drink something called water kefir. its a fermented fruit juice beverage that is basically a DIY home probiotic. as i recall i had some good results spraying the stuff on the roses.

some things about kefir. unlike yogurt which you buy in the store, kefir has strains that can actually colonize your gut... and the soil. in particular lactobacillus plantarum i believe is common to most kefir concoctions. this is a strain that is also associated with activating plant defense systems... some lab experiments have been able to raise overall defense molecules and even flavonals in select plants by inoculating them with this...

it has amazing properties on humans. seemed to help the plants.