Gemara

103b (Mishnah): If Reuven accepted to work on Shimon's Beis ha'Shelachin (a field that is irrigated from a spring) or Beis ha'Ilan (a field with a tree, which is profitable for the worker), and the well dried up or the tree was cut down, he does not deduct from the agreed rental.

Question: If a big river dried up, why doesn't he deduct? This affects the entire region (we cannot say that it is due to his bad Mazal)!

Answer (Rav Papa): A small spring dried up. The worker could bring from the big river.

(Rav Papa): This Mishnah applies to sharecroppers (who get a fixed portion of the produce) and Chokerim (renters who pay a fixed amount of Peros each year). Every Mishnah later in our Perek applies to only one of them.

105b (Mishnah): If Reuven was a Choker on Shimon's field and the crop was eaten by locusts or withered (by the sun or wind), if it was a regional affliction, he deducts from the rental. If not, he pays the full rental.

R. Yehudah says, if the rent was a fixed amount of money, in either case he pays the full rental.

Question: What is a regional affliction?

Answer #1 (Rav Yehudah): The majority of the valley was stricken.

Answer #2 (Ula): Fields in all four directions were stricken.

Question (Mishnah): If the crops were stricken with withering, dryness, or it was Shemitah, or there was a drought like in the years of Eliyahu, the year does not count towards the two years (before redeeming a sold field).

Suggestion: The years of withering or dryness are like the years of Eliyahu, in which nothing grew at all, but if some things grow, this is not a region-wide affliction (for redemption and rental)!

Answer (Rav Nachman bar Yitzchak): It says about redemption "Shnei Sevu'os Yimkar", i.e. years in which there is grain (somewhere).

106b: Reuven rented a land on the bank of a certain river, to plant garlic on it, for a fixed amount of money. The river was dammed up (and therefore, the garlic did not grow well).

Rava: It is unusual to dam up that river. This is a regional affliction, so you deduct from the rental.

Question (Rabanan - Mishnah - R. Yehudah): If the rent was a fixed amount of money, in either case he pays the full rental.

Answer (Rava): No one says that the Halachah follows R. Yehudah.

Rishonim

Rif and Rosh (9:14): The Halachah follows Rav Yehudah, for the Sugya is like him.

Nimukei Yosef (DH Eino): R. Yehudah holds that if the rental was for money he does not deduct, for there was no decree against money, so we attribute it to the renter's bad Mazel.

Rosh: The Gemara brought many questions according to Ula. Ula himself asked them (so this does not show that the Halachah follows him)!

Rambam (Hilchos Sechirus 8:4): If Reuven was Choker or accepted to work on Shimon's Beis ha'Shelachin or Beis ha'Ilan, if the spring dried up but the river did not, and Reuven can bring from the river with a bucket, or the tree of the Beis ha'Ilan was cut down, Reuven does not deduct from the rental. If the entire region was stricken, e.g. the river dried up, he deducts from the rental.

Magid Mishneh: The Ramban and Rashba explain that a sharecropper does not deduct for regional afflictions. The owner gets the same fraction of the produce that they agreed upon.

Rambam (5): If Reuven rented or accepted to work Shimon's field and the crop was eaten by locusts or withered, if most fields of the city were stricken, he deducts from the rental according to the loss that occurred. If most fields were not stricken, he pays the full rental.

Magid Mishneh: Rashi says that this applies only to a Choker. What would a sharecropper deduct? They share whatever there is! This is obvious, for Rav Papa taught that each later Mishnah applies to a Choker or sharecropper, but not both. Why did the Rambam say 'accepted to work' (a sharecropper)? He adopted the text of the Mishnah, but he means rental or Chakirus. Still, he should have explicitly said so. The Rambam's text said 'most valleys', i.e. of the city. Some texts of the Rif say so.

Hagahos Ashri (6:6, b'Sof): If a one was hired to teach a child, and the mayor decreed to forbid this, this is a regional affliction and he receives his full wage. This requires investigation later regarding one who accepted to work a field.

Poskim

Shulchan Aruch (CM 321:1): If Reuven was Choker or accepted to work on Shimon's Beis ha'Shelachin or Beis ha'Ilan, if the spring dried up but the river did not, and Reuven can bring from the river with a bucket, or the tree of the Beis ha'Ilan was cut down, Reuven does not deduct from the rental. If the entire region was stricken, e.g. the river dried up, he deducts from the rental.

Beis Yosef (DH u'Mah she'Chosav Gabei): Talmidei ha'Rashba say that even if a small river dried up in the entire city, it is not a regional affliction. It merely causes more exertion; this is normal.

Rema: A sharecropper does not deduct when the entire region was stricken. The owner gets the same fraction of the produce that they agreed upon.

SMA (4): This is unlike one answer in Tosfos (104a DH d'Efshar) that we discuss when it is possible to draw from the river, but if he cannot, even a sharecropper need not toil to work the field.

SMA (5): The Rema is like the Nimukei Yosef. Tosfos, the Rosh and Tur hold that even a sharecropper deducts. The Tur holds that we do not deduct when all crops in the region were withered or eaten by locusts, but here he can say 'I did not accept to toil to bring water from afar.' When crops are withered or eaten by locusts, this is no reason to refrain from tasks.

Shulchan Aruch (322:1): If Reuven was Choker or rented Shimon's field and the crop was eaten by locusts or withered, if most fields of the city were stricken, he deducts from the rental, all according to the loss that occurred. If most fields were not stricken, he pays the full rental.

Beis Yosef (DH veha'Ra'avad): It seems that Rabbeinu (the Ra'avad) rules like Ula. Surely, we do not rule like him against all those who rule like Rav Yehudah (the Rif, Rambam and Rosh).

Shitah Mekubetzes (95b DH Aval, citing the Ra'avad): The questions according to Ula were not answered. Even though the other questions are according to Rav Yehudah, Ula is not rejected, for each of them agrees to the other. Alternatively, Ula agrees that when the majority was stricken, it is a regional affliction. He teaches a bigger Chidush (fields in all four directions are like a majority). Since in Eretz Yisrael they asked according to him, we say that also what he said is a regional affliction.

SMA (3): 'He deducts, all according to the loss that occurred' connotes that if this field was stricken more or less than others, he deducts according to the loss that occurred in this field.

Rema (334:1): If one rented out his house, he is like a worker (if Ones occurred). If the tenant fled due to bad air or another Ones, the worker (or owner) loses.

Maharam Padova (86): A student fled Venice due to a plague. This is Ones; the teacher is not paid. Hagahos Ashri says that when there is a regional affliction, he receives his full wage. Here it was not regional affliction, for only a minority fled. If a river dries up and the workers knew about this possibility as much as the owner, they are not paid. Loss of the river is Mevatel many people, but not all, from work. Hagahos Ashri was discussing illness; and he switched to discuss a decree, and not a regional illness, for a regional affliction is only when everyone, not just a majority, is affected.

Rebuttal (Shach 3): A regional affliction is when the majority of the valleys were stricken! All Poskim agree to this. Hagahos Ashri discussed a decree for this is clearcut. He did not discuss a plague, for then we must determine whether or not the majority were stricken. Indeed, if the majority were stricken or fled, it is a regional affliction.