Picking fruits from under a tree on Shabbos:
All fruits which have fallen off a tree on Shabbos are forbidden to be eaten until after Shabbos and are therefore Muktzah. Furthermore, even if there is doubt as to if the fruits fell on Shabbos or beforehand it is forbidden. [Accordingly, it is forbidden for one to pick up any fruit under a tree on Shabbos unless one knows for certain that it fell before Shabbos, such as if they are very dried out. Even in such a case, one must beware not to transgress the prohibition of Miameir which is a Biblical prohibition against one who gathers fruits and joins them together in the area where they fell off the tree. Hence, one may not pick up more than one fruit at a time.]
Are vegetables which have become detached from the ground on Shabbos forbidden?
 Michaber 322:3; Admur 310:3 “The above [cases in which it is allowed to move and eat the fruits] only refer to [fruits] that have been removed from the tree [from before Shabbos], however if it was attached [to the tree on Shabbos] and then [later] got removed on Shabbos, then even if it belongs to a gentile which removed it for himself, and [furthermore] even if the gentile had intention from before Shabbos to pick these fruits on Shabbos, such as in a case that one heard the gentile say on Erev Shabbos that “tomorrow I will pick these fruits”, and even [furthermore, even] if the fruits were fully ripe, in which case they are considered prepared [to be eaten] even when they are still attached [to the tree] as will be explained in Chapter 318 [Halacha 6], nevertheless [despite all the above] they are forbidden to be eaten and moved. The reason for this is because of a decree of fruits that have fallen off [from a tree on Shabbos], as will be explained in chapters 322 and 325 [Halacha 8].”; Admur 325:8 “As well, a gentile that picked fruit which was attached [to the ground] for his own self use, then even if it is a scenario where there is no prohibition of Muktzah involved, such as for example when one heard the gentile saying from before Shabbos that “tomorrow (Shabbos) I will pick these fruits” and they are fruits which have fully ripened of which case preparing them [before Shabbos] helps [remove their Muktzah status] from them even if they are attached, as explained in chapter 318 [Halacha 6], nevertheless it is forbidden for a Jew [to eat] until after Shabbos. The reason for this is because [these fruits] are included in the decree against fruits that fell off a tree on their own [on Shabbos] as explained in chapter 322 [Halacha 3 in the Michaber] being that also these fruits have arrived to the Jew on their own just like those which have fallen off [the tree] on their own.” Rambam Shabbos 21:7; Beitza 2b; Kaf Hachaim 322:7; Piskeiy Teshuvos 322:2
 The reason: The reason for this is due to a decree one may come to pluck a fruit from the tree on Shabbos. [Taz 322:2; M”A 322:3; Beis Yosef 322; Bach 322; Levush 322; 1st explanation in M”B 322:7] Alternatively, it is because the fruit is Muktzah as one did not have it in mind from before Shabbos. [M”B 322:7]
 Ketzos Hashulchan 146:23 from Peri Megadim, Upashut.
 P”M 322 M”Z 2; M”B 322:5; Kaf Hachaim 322:7; Ketzos Hashulchan 146:23
 Ritva; Pischeiy Olam 322:4; M”B 325:24 based on Michaber 515:3; Kaf Hachaim 322:7; Piskeiy Teshuvos ibid footnote 10
 See Admur 340:15; Michaber 340:9; M”B 322:6; Kaf Hachaim 322:7; Piskeiy Teshuvos ibid and 340:31
 M”B 322:6 in name of Peri Megadim