Your tree is pulling nutrients from the leaves to grow your fruits. My tree is doing the same.

It’s never done that before though when it was fruiting. Is there a reason why it’s doing it this year? Tree was weakened to begin with? Also should I be cutting off the fruit then

 That only happens when the trees aren't getting enough nutrition otherwise

I fertilize with foliage pro 1 teaspoon per 2 gallon water. Unless I need to be giving it a higher dose

Trees and herbaceous plants prioritize actions that require food/energy, but perhaps it's better to say that the strength of energy sinks has a distinct order: Energy is first allocated to respiratory function, i.e. to maintenance of living tissues, then, to production of fine roots, followed by flower and seed/fruit production, then primary growth (extension of both roots and shoots), then secondary growth (thickening), and finally, the synthesis of defensive chemicals. Allowing trees in containers to retain all fruit they set can be very stressful. You're seeing manifestation of nutritional deficiencies because reproductive parts come before primary and secondary growth; and, as you can see from the pecking order, a bumper crop of fruit can affect the plant's ability to defend itself against insect herbivory and/or disease pathogens.

It's very common for bearing branches to show chlorosis as fruit is developing. It might not be that there is truly a deficiency as it is fairly common during periods of rapid growth to show up even when appropriate levels of nutrients are in the soil solution and available for uptake. Blossom end rot is often blamed on a Ca deficiency, but in fact it results even when the Ca supply is appropriate from a combination of fertilizing at nitrogen levels too high and rapid early season growth. The plant simply cannot take on enough Ca to ensure cell growth is normal during periods of rampant early season growth. I guess the main point to take away here is deficiencies made manifest in foliage are not always due to an actual paucity of nutrients in the soil or soil solution. Some deficiencies will be of the physiological variety, others can be antagonistic deficiencies where an over-abundance (redundant?) of nutrient A can make it very difficult for the plant to absorb an adequate supply of nutrient B. Say you used a high phosphorous bloom-booster fertilizer because you felt it would produce more blooms and thus, more fruit. Your plant might easily show interveinal chlorosis because phosphorous is a known antagonist of Fe (iron) and several other nutrients (calcium, potassium, copper, zinc).

You can try upping the 9-3-6 as you're well below the rate I use regularly for all my trees (4 tsp/2.5 gallons, which is a shade over 1.5 tsp/gal during active growth), but the foliage on bearing branches will return to the same color as non-bearing branches when the fruit ripens.

Al

  That  happens  when the trees aren't getting enough potassium   and open

   I'm just  guessing it could  be a magnesium and zinc deficiencies

A magnesium (Mg) deficiency would present across the entire foliage mass because Mg is a nutrient that is considered mobile in the plant. As such, it can be borrowed from other organs to supply what is needed for new growth. A zinc (Zn) deficiency, because it is considered immobile in the plant, must be present in the nutrient stream at adequate levels at all times. If it isn't, deficiencies show only in new leaves that grew after the Zn became deficient.

Generally speaking, when a deficiency of the mobile nutrients (N, P, K, Mg) occurs, it is seen in all foliage, even though roots are first affected. The rest of the nutrients which are absorbed via the root pathway are considered to be immobile or mostly immobile (can't be 'borrowed' to provide the building blocks for new growth).

A simple analogy would be, leaves and fruit are having a tug of war to see which one gets the lion's share of food and nutrients. Since we know the pull of unripened fruit is always stronger than foliage (fruit is a more powerful energy sink than leaves), fruit will win every time. When fruit ripens, it's loses it's pulling power, which frees up food and nutrients that allow the leaves to recover to their normal color.

I'm not sure what you meant by, "That happens when the trees aren't getting enough potassium and open".

FWIW, most hobby growers end up shooting themselves in the foot when they think they have identified 'a nutritional deficiency' and move to treat it by adding an element or compound aimed specifically at correcting a deficiency of individual nutrients. The reasons for the shot in the foot are several. 1) Deficiencies are very often misidentified; or, 2) there might not actually be a physical deficiency of the nutrient the grower thinks (s)he identified as there could be another antagonistic nutrient, available in excess, in play, example: too much Ca limits Mg uptake - too much P limits uptake of K, Ca, Fe, Cu, Z, ....... ; or, 3) a nutrient could be acting as a synergist, example: an excess of K can cause an increase in the uptake of Mn and Fe, causing a toxicity of all three nutrients which would be read as a deficiency of who knows what; or 4) the deficiency could be culturally caused, example: in saturated soils, low oxygen levels cause Fe (iron) and Mn (manganese) to become electron acceptors. This leads to the precipitation of both, even though these elements might be present in soils at normally adequate levels, and the plant’s inability to assimilate them. Soggy soils also make uptake of Ca difficult or impossible.

Al

 Like I said I was  just guessing for I don't really know