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The Editor's Tutorial

.Pests, Diseases & Problems

Mildew.
Powdery mildew is a fungal disease which first appears like cigarette ash on leaves, if not treated promptly it will quickly spread, especially on yellow varieties.  Some years it is very prevalent, other years it is hardly seen.
Spray with a systemic fungicide.  Prevention is better than cure and a precautionary spray early in the season and before the buds are allowed to develop is advisable.  Always read the label, some precautions are not for begonias, this includes the re-launched Systhane.

Vine weevil.
The vine weevil beetle has become the arch enemy of the begonia grower, not that the beetle itself causes any damage, the problem is that it lays eggs in the compost.  The resulting grubs burrow into the tuber, munching away and causing irreparable damage.
There are a number of treatments available on the market, the current favourite in this area  is Provado.

Stem rot.
Usually caused by overcrowding and/or lack of air circulation.  Sometimes a result of top dressing.  If caught early the rot can be wiped away and the wound treated with a spray as used for mildew, this will stop the rot spreading, alternatively green sulphur will have a limiting effect.  Cases like the illustration  are terminal  and the best hope is for the tuber to be saved.  Treat as before having cut the stem right off and back to unaffected tissue.

Earwigs and caterpillars.
Attacks can spoil leaves and flowers.  Often the caterpillars are very small and difficult to trace especially as their colour blends with foliage.  Both especially enjoy the taste of the centre of blooms and often their feasting only becomes evident when the bloom opens. A precautionary spray with an insecticide is advisable, treat affected plants the same way.   

Tuber rot.
Some losses from rot must always be expected, especially during dormancy, and a loss rate of about 5% is quite common.  Regular inspection and removal of any brown areas will help to save affected tubers.  At the base of where the stem joined the tuber a scab will form, this must be removed.  It can easily be prized off with a potato peeler.

Eelworm.
Can attack the tuber or the leaf. The former, root knot eelworm causes swellings on the tuber and there is no cure, tubers should be burned.  Leaf eelworm (pictured) attacks between the veins and the leaves shrivel and die. Any suspect plant should be quarantined as the condition is contagious.  Luckily eelworm is not a common problem,
The hot water treatment is a worthwhile precaution.

Tarsonamid Mite.
These are particularly nasty infestations.  Too small to be seen by the naked eye their effects can rapidly become devastating if left unattended.  Symptoms are corky deposits on the stem and leaf, also discolourisation and withering of the growing tip which takes on a blackened knurled effect.
Any plant that is suspected of being affected should be isolated before the problem has a chance to spread.

A preventative  treatment practiced by some growers is to soak tubers in a solution containing 10% Domestos for 15 minutes followed by 15 minutes in fresh water. This treatment has not however been proven.
It would appear that this "begonia plague" is on the increase, with reports of outbreaks from growers throughout the country. Some enthusiasts are now keeping any tubers acquired from unproven sources in isolation for the first season as a precaution against contaminating their stock.

Foliar Petal.
This is not a disease just a problem associated with, mainly, yellow varieties.  Invariably it affects only early buds and subsequent ones are not affected.  One or more petal develops as a leaf.  The only treatment is to remove the malformed bud as soon as the problem is apparent.

Premature damping of blooms.
The length of time that a bloom will remain at it's best is governed by conditions and the weather.  Premature damping can be caused by too much humidity.  Although it is beneficial to damp down the greenhouse floor and staging (on warm mornings) whilst the plant is growing this must stop once the flowers open.

Sun scorch.
Not a disease just a problem caused by lack of greenhouse shading.

Warning.
 Before applying any previously untried product or treatment test it on a spare plant and allow plenty of time before committing all your stock to the treatment.
     A few years ago to combat an attack from caterpillars I applied a previously untried spray on one plant only, left it for a week as recommended on the container then treated about half of my collection .  A few days later leaves on the first plant started to shrivel and take on the appearance of ancient parchment.  The same thing happened to the other begonias that had been subjected to the spray.
Disclaimer.
  Many growers have their own recipes and cures, some of the above methods and products I have, thankfully, not had cause to try.  In all cases I believe that prevention is better than cure. Always read the instructions and safety advice on any product before using it .