What to do when cactus is rotting???

The before picture. 
when my mammillaria was still healthy in the big pot

Thailand is in the rain season now and some cacti in my garden were also rotten.  At this time, it happened with my favorite mammillaria which became rotten and I just noted it recently.  So I think it’s important to fight with rotting seriously although before that, I led the plant rot without do anything. Here is the way to do that and hope it’ll be helpful when you have same trouble like me.

• How do we know or notice that the cactus is rotting?

The brown base of can mean that the plant is aging (become woody) or rotting. To tell the difference, touch the base and see if it is hard or flabby/soft. Hard means everything is OK. But, it’s soft, it means that plant is rotting and it must be treated with care.  

Here is after picture.  It was so soft and I had to cut some rotten part out and dried the rest for 3-4 days together with the rotting aloe

• Can I still save the rotting cactus? 

The second way to save a rotting cactus is to simply cut of its top.  Using a sharp knife cut off the top of the plant.  After that, let’s it dry for a 2-7 days. Once dry, set about 1/2 the plant into potting mix/perlite or cactus mix/perlite.

Do not water too much. Just enough to make the soil to become a little humid. Over watering at this point could cause rot again. With luck and the right conditions it will start to root. Once it develops its own roots, the watering can be increased.  Establishing the new roots may take several days or weeks depending on the condition of plant and care.

I put them on top of pot with 100% perlite

After several days with humid from rain, the new root came out finally! 

Another option is to take a cutting or two for grafting which need skill & experience to do that.

• What if my cactus is rotting from the top? What shall I do?

If a cactus is rotting from the top you need to act immediately before more of the cactus rots. Just like as if it is rotting from the base, carefully cut off the top of the cactus that is rotting. Dispose of the cactus top.

As for the bottom, clean it with a spray to make sure it does not catch the top's disease. Change the soil unless it looks as if the cactus can't take a repot. The remaining cactus should now be healthy. After a few weeks of extra care, start caring for it as for a normal cactus. It will not grow a new top or increase in height but it will grow branches which will grow.

Source: http://www.botanicalgarden.ubc.ca,  http://www.alexandgregory.com/

Disease of Cactus and Succulent

Rotting

The main diseases of cacti and succulent plants are due to various fungal and bacterial rots.

In general healthy, growing plants are quite able to resist infections with these. However they commonly occur as a secondary effect of other problems such as attack by insects pests, physical damage leaving exposed plant tissue or incorrect growing conditions. The single biggest cause is probably fungal rots entering via dead roots caused by poor root aeration.

The best way of avoiding these problems is to provide conditions which prevent their development, i.e., a healthy pest free environment. If they occur and are spotted early it may be possible to save the plant by cutting away all the diseased tissue with a clean knife.

In particular look for discoloured vascular tissue which may be red or brown and penetrate some way into otherwise healthy tissue. The knife should be cleaned with alcohol to prevent spreading fungal spores. It may be beneficial to dust the cut surfaces with flowers of sulphur.

Fungicidal chemicals can be used to give some protection but this should only be tried as a last result since they are not effective against the whole range of different rot producing fungi. Young cactus and succulent seedlings are particularly prone to 'damping off' which is a fungal attack. This can be partially controlled by copper fungicides. The other frequently mentioned chemical for this purpose, "chinosol", is suspected of causing some damage to the seedlings and is not recommended.

Deficiency diseases

The other group of diseases affecting succulent plants are caused by soil deficiencies of various minerals. This may not simply be that the soil does not contain the required elements but that they are not available due to the soil become too alkaline from build-up of salts from minerals contained in the water which is used for the plants. It is useful to know how alkaline your local water is, and if necessary take steps to correct this by adding suitable acid.

Unless you are a chemist and have the necessary knowledge it is inadvisable to use strong mineral acids for this purpose. It is possible to use citric acid or acetic acid (vinegar) but the favourite remedy is potassium dihydrogen phosphate which also supplies useful elements. If your local rainwater is sufficiently clean then this is the best way to avoid this problem.

It is more likely that deficiency diseases will shown up in peat based rather than soil based composts. If a plants looks chlorotic or refuses to grow properly it is worth trying fresh compost to see if this solves the problem. If locally available soils are know to have particular deficiency problems it may be worth adding appropriate supplements.

source: www.cactus-mall.com