Slicko, citrus is more sensitive than most crops to biuret, an impurity present in urea fertilizer created by excessive heating of chemical reactants during urea manufacture.  Toxicity can result from soil or foliar spray applications of urea applications with elevated biuret concentration. If urea is used as a nitrogen source for citrus, it should contain not more than 0.25 - 0.50% biuret by weight.  The leaf symptoms of biuret toxicity appear as an irregular butter yellow chlorosis that starts at the leaf tip.  Young leaves are more vulnerable than old ones, and affected leaves do not regreen.  On very young, expanding leaves, urea with a biuret concentration in excess of 1.0% may cause distortion as well as typical yellowing.  Succeeding growth flushes appear normal, provided applications of High biuret urea is not repeated.

Low Biuret urea is defined as 0.5% biuret or lower.

Phil, that same storm came through here (Colorado) two days ago.  We had a low of -4F.  Today it was sunny with a high of 31-F, and tomorrow should be around 55F. Sorry about your crop.  I have a Seville sour orange that has quite a few fruit on it.  Could you post once again your recipe how to make it.

Winter prebloom foliar spray application of low Biuret urea is known to greatly increase flower number, thus greater crop yield.  Proper timing is important to achieve the desired outcome.  Winter prebloom sprays are designed to increase flower number and fruit yield without reducing fruit size. Winter prebloom foliar sprays with low biuret urea (46-0-0 >0.5% biuret) is applied at the rate of .44-lbs. (200-grams) in 2 gallon of water plus one teaspoon of a good surfactant per gallon.. For large area sprays (acres) 50-lbs. per 225 gallons water. NOTE: a second low biuret urea foliar spray should also be done after early fruit drop and June drop periods to insure an  increase in fruit size.  In most areas  this would be around July 1 to July 20.

Citradia,, I thought  you were using heat cubes also.

Citradia, when the going gets tough, the tough get going.

Loisport, i do not know the answer to your questions.  If Oscar is selling it, it must be a good variety.   You can contact Tintorri and ask them for information.

It is called Frutto Rosa.  You can see it on Oscar Tintorri's home page.

https://www.oscartintori.it/categoria-prodotto/serra-degli-agrumi/serra-degli-agrumi-pummeli-citrus-grandis/

From the symptoms shown in the photos by Mtlgirl and Snek the problem does not look like Greasy Spot.    I'm not certain what it is, but also I don't think it is edema, as edema damaged leaves normally do not fall from the tree.  If it is a fungus, applying an effective fungicide normally stops the infection, but of course does not remove the leaf damage already done.                                             

Adrino, water with temperatures between  70 to 90F (21 to 30C)

Congratulations, good things come to those who wait.  Kishu is a favorite of many people.  The best to you and this tree.

With low night temperatures, and higher day temperatures your trees will still get a bloom, however the tree will have both a smaller number of floral shoots and flowers per shoot.   In areas such as Florida and other semitropical and tropical citrus growing areas, winter water deficit stress is imposed on citrus trees of all cultivars to compensate or inadequate exposure to low temperatures during their mild winters.   One last note: citrus are day neutral, meaning that day length does not induce flowering as it does in many annual and biennial plants.