I AM constantly surprised by the way different varieties of roses respond to the most common disease, pests or disorders.
A few of the most beautiful roses are highly vulnerable to black spot disease, some becoming so badly infected that they drop all but the youngest leaves.
Other roses, even those growing alongside, appear to be immune or at least resistant to the disease.
Among the most spectacular roses in our garden is Dame Elisabeth Murdoch.
I planted a group of three together a couple of winters ago and the blessed things absolutely refuse to stop blooming. Same with Elina, another black spot resistant beauty.
Naturally I snip off all dead heads before the seed-heads get a chance to form.
Growing nearby, under the same conditions is Kardinal, with its vibrant, rich-red, fragrant blooms on an otherwise quite vigorous bush.
Yet every year this rose is devastated by black spot disease.
Let's make it clear that this is an appalling, frustrating disease.
It begins in spring, when some of the older leaves develop irregular black spots that quickly grow larger, often surround by an unhealthy yellow.
Then other leaves begin to become infected.
As the leaves change colour they die and drop to the ground.
This parasitic fungus keeps re-infecting the rose and adjacent roses from this deadly debris.
There are a couple of ways of dealing with the problem.
Spraying helps, but is a waste of time if diseased leaves are left still attached or lying around on the ground.
I go around all the rose plants carrying a bucket.
Every diseased leaf is pulled off (they practically fall off when touched).
And every fallen leaf is carefully collected and the whole lot carted away, placed in a plastic bag and carted off with the garbage. That keeps most roses clean, but not susceptible varieties.
I've got to the stage when the really vulnerable ones are just lifted, roots and all, and replaced with more resilient varieties.
There is no doubt that districts with wet summers have major problems with rose black spot.
Some of the worst rose gardens I've ever seen were in Sydney, despite frantic spraying by the owners.
This disease is spread by water splash but it still needs special conditions in order to penetrate and infect the leaves.
Watering roses by means of sprinklers is asking for trouble.
Black spot organisms need leaves that remain wet for several hours before they can get into the tissue.
This is why watering roses by sprinkler in the evening invites disaster.
It means the rose leaves remain wet all night and that's when fungal infections really begin to spread.
Some people have successfully used diluted milk (50-50 milk and water) sprays to combat black spot.
Others have found that a tablespoon of baking powder (bicarbonate of soda) dissolved in a litre of water makes an effective deterrent spray.
However, to be effective, these sprays can only be applied to leaves before the disease strikes; in other words in early spring or after every diseased leaf has been removed.
Perhaps these sprays work because they are mildly alkaline.
Black spot organisms - like the similar apple and pear scab fungi - need acidic conditions in order to thrive and alkaline surfaces are less attractive.
So the sprays work, but only as a deterrent.
In the final analysis, the real solution in wet-summer districts is to choose the most resistant roses and prepare to protect them. Luckily they happen to produce some of the most beautiful of all rose blooms.
