Native Range and Climate

White mulberries (Morus alba) are native to China.  I have not found any other details about their range, but based on their cold hardiness and ability to thrive in a semi-arid climate, I would guess that its predominately in the north.  Its range may seep into the southeastern section of China as well, where precipitation is quite heavy, since a few varieties are known to have enough disease resistance to thrive in the humid subtropical climate (Cfa).  However, hybridization could, at the very least, be partially responsible for this.

Red mulberries (Morus rubra) are native to the eastern part of the United States.  The area primarily experiences the humid subtropical (Cfa) and hot summer continental climate (Dfa).  Rainfall ranges between 0.5" to 5.5" per month throughout the year, but it generally reside around 3" to 4" during the grow season.  Red mulberries are often seen growing as an understory tree.

Black mulberries (Morus nigra) are believed to be native to the mountainous region of Mesopotamia and Persia (Iran).  Other areas of interest include the Aegean region and Transcaucasia, but the exact location is unknown due to extensive cultivation in the fairly distant past.^[1] They thrive in the hot summer mediterranean climate (Csa) and appear to tolerate the hot desert climate (BWh) quite well (once established), but in the humid subtropical climate (Cfa), they are highly susceptible to leaf disease and will fail to survive.

Shahtoot mulberries (Morus macroura) are native to the eastern Himalayas, the rainforests of Indo-China, and the islands of Sumatra and Java.  Most of the area experiences a tropical rainforest (Af), tropical savannah (Aw/As), or the humid subtropical climate (Cwa).  In Islamabad, where the variety 'Pakistan' was selected, the climate is humid subtropical with dry winters (Cwa).  The city also experiences fairly hot temperatures, with the average high in June rising above 100F/37.8C.  Morus macroura has also been grown in the hot desert climate (BWh) and other dry environments.  I am not sure how often they are irrigated, but it is implied to be infrequent (in comparison to most other species).  However, excessive water deprivation will cause their fruit to drop.

Fruit Color

The white mulberry species (Morus alba) does not necessarily produce white mulberries.  Most trees produce black fruit, but with the exception of Morus macroura 'Saharanpur', the only varieties I know of that produce white or lavender mulberries are from the Morus alba species.  The red mulberry (Morus rubra) produces black fruit as well, and the black mulberry (Morus nigra), which also produces black fruit, is named after its large dark buds that the other species do not share.

Disease

Popcorn disease, caused by the fungus Ciboria carunculoides, is one of the few mulberry diseases regularly mentioned in the US. While it will not harm the tree, it can ruin a large percentage of the crop.  It appears to only affect some Morus alba trees and a few of its hybrids, including the popular 'Illinois Everbearing' variety, but the problem may not be too common north of the humid subtropical climate (Cfa).

Leaf spot from bacterial blight, or various fungi, can be fairly common as well, but most varieties appear to be quite resistant.  Mulberry trees from the species Morus nigra are the primary exception.  They are highly susceptible to one or more leaf defoliating pathogens in climates that are fairly wet, and it will eventually kill the tree.

Pests

The spotted wing drosophila (SWD) is the primary mulberry pest, but I have only seen a few complaints about them.  The earliest ripening varieties, such as Gerardi Dwarf, Kokuso, and a few others, should ripen early enough to avoid them, but results may vary by region or year.  I have seen a few reports about 'Illinois Everbearing' (IE) getting hit by SWD, but at what point in the season, I am not sure.  IE appears to ripen at least slightly later than most varieties.  In addition, mulberries tend to ripen over a long period of time, and IE is known for extending much further into the season than many others, which makes them more vulnerable to SWD infestation.

Root knot nematode can also be a problem, particularly in sandy soil.

Fruit Quality

Out of the various mulberry species, Morus nigra produces the best flavored berries (when harvested at their peak).  The difference between the few varieties that are available is said to be mostly unnoticeable.

Morus alba has the least complex flavor, but some varieties, such as 'Sweet Lavender', can be very sweet.  Some believe the flavor of certain varieties, including rubra x alba hybrids, are highly reliant on the climate they are grown in, with wetter climates being less favorable.  A lack of flavor, or a grassy flavor, may also be due to harvesting the berries too early.

Nutritional Values

The ascorbic acid content of mulberries, from various studies, ranged between 1 to 42 mg/100g FW, but it was generally around 20mg.^{[2][3][4][5][6][7][8]} Selections with a low level of acidity tend to have very little ascorbic acid, but a few exceptions have supposedly been observed.^[3][8]

Their total phenolic content ranged between 80 to 2575 GAE mg/100g FW.^{[2][3][4][5][6][7]} The mean low and high (from every study cited with multiple selections tested) was 553.3 to 1075.1mg/100g, but the actual results were generally quite distant from this range.  They were either well above or well below these values, and it did not seem to correlate with climate.  When the phenolic content of Morus nigra was compared to alba or rubra, Morus nigra seemed to have the edge, but the sample size was quite small and all three had multiple selections with results in the upper and lower range.

In one study that compared Morus nigra (232 g/100g FW) to a few other berries known for having a relatively high phenolic content, the mulberry had a similar amount as the blackcurrant, but it had ~78% of the amount of the honeyberry, ~50% of blueberry and blackberry, and ~44% of the european juneberry.^[9]

Elsewhere, Morus alba (1516 mg/100g FW) had a significantly higher phenolic content than loquat (199.4mg), strawberry (363.7mg), and asian plum (668mg) – three species that aren't known for regularly having a high phenolic content.^[10]

In another study, Morus nigra was compared to 24 other fruit species.  The mulberry (174.9 mg/100g FW) had a higher phenolic content than 13 of them.^[11] White currant (40.8mg) had the lowest amount, and chokeberry (1013mg) had roughly double the amount of the second highest.