The atmospheric chemistry would be upset by cutting out sunlight. It would be like a nuclear winter and could last for tens of thousands of years. These comets are 200-300km (186 miles) in diameter they are sheer monsters and could sterilise the earth if we are hit by one.
Heat stress â Another reason for rose leaves turning yellow is the rose bush is under some sort of stress. A common cause of stress is heat stress. The rose bush turns yellow and drops its foliage in an effort to cool down in the case of heat stress.
Alice Aks - CA, Zone 9. After reading all of the above, I was going to mention temperature changes. Here in my colder climate, I get some red rose petals that have that black edge on them. Most of the time it is due to a light frost that gets on the roses on some cold evenings. The edges of the petals get frost bitten and look black and disfigured a bit. How heavy a frost and the timing can cause more of the black edges too. Stan the Roseman Bearrose1954 - Colorado, Zone 5.