Is this calcium deficiency ?

[Soh Joo Liang]

[Guanabanus]

Maybe.

I have not worked much with Durian, and certainly not with any fruiting ones.

Usually, with other fruits, such defects are usually blamed on Boron deficiency.  But one of the effects of Boron deficiency is the inability to properly use Calcium.

[KarenRei]

Any leaf symptoms (e.g. is a deficiency in a mobile nutrient?) or unusual growth habits (e.g. boron deficiency usually leads to weirder growth than calcium deficiency)?

[Soh Joo Liang]

Thanks
Long drought during fruit growing stage & insufficient irrigation. No leaf symptoms but old leaves fell & heavy flushing of new shoot when rain came. Some of the affected fruits can still be eaten, some are hard & tasteless.

[Lory]

Long drought during fruit growing stage & insufficient irrigation

You answered yourself, that's the reason.
Shortage of water impairs the uptake of all nutrients included also calcium....

[Guanabanus]

Also abundant new shoots while fruits are still on the tree causes Calcium deficiency in the fruits, as the Calcium flows in the sap, which is going mostly to the new growth, in part to replace the water being transpired from the tender growth.

[Soh Joo Liang]

Thanks Guanabanus. Never mind you have no experience with durian. Your experience in other crops can help. I am a retired man planting some tropical fruits mainly durian. Beside fungal diseases, this fruit has many other problems such as uneven ripening. Some fruits remain hard even after 2-3 days after dropping from tree. When the pulp is hard, not the normal tender soft,could it be due to over N or K. When seeing heavy leaf drop, I applied 1-2 kg of 15-5-20 per tree about 2 months after an thesis. The early clone like musang king takes only 90 days to mature.

[Guanabanus]

Fruit trees usually need other nutrients in addition to NPK.  You could have leaves and soil tested to know for sure what you need to add.

[Lory]

Thanks Guanabanus. Never mind you have no experience with durian. Your experience in other crops can help. I am a retired man planting some tropical fruits mainly durian. Beside fungal diseases, this fruit has many other problems such as uneven ripening. Some fruits remain hard even after 2-3 days after dropping from tree. When the pulp is hard, not the normal tender soft,could it be due to over N or K. When seeing heavy leaf drop, I applied 1-2 kg of 15-5-20 per tree about 2 months after an thesis. The early clone like musang king takes only 90 days to mature.

I completely agree.
Just back from a trip to Davao, Mindanao island.
Apart from the cultivated ones, there are thousands of Marang, Mangosteen, Soursop and Durian trees growing beautifully without any care.
Soil in that area is of vulcanic origin, naturally rich not only in N, P and K but especially in Iron, Manganese, Zinc, Magnesium, Sulphur, MOlibdenum, Boron.....all these micronutrients make a HUGE difference....together with a neutral-slightly acidic pH....