The original manuscript is extant here and reads in the past tense for the clause “behold the field was ripe”. When copying from 𝓞 into 𝓟, Oliver Cowdery initially replaced the past-tense was with the present-tense is, but later (perhaps when proofing 𝓟 against 𝓞) he corrected the is to was (the level of ink flow is heavier for the supralinearly inserted was). Clearly, was is correct: 𝓞 reads that way. Moreover, the rest of passage refers to the missionary harvest in the past tense (“for ye did thrust in the sickle and did reap with your mights / yea all the day long did ye labor”); thus “the field was ripe” is wholly appropriate.
Oliver Cowdery’s error may have been influenced by the immediately following present-tense verb form are in “and blessed are ye”. Another possible influence is the present-tense use of is in the sentence “(for) behold the field is white already to harvest”. This sentence appears in a number of revelations that Joseph Smith received for various individuals, including Oliver Cowdery, during the translation period (in the first half of 1829):
date recipient book of commandments
(doctrine & covenants)
February 1829 Joseph Smith Senior 3:1 (4:4)
April 1829 Oliver Cowdery 5:2 (6:3)
May 1829 Hyrum Smith 10:2 (11:3)
May 1829 Joseph Knight Senior 11:2 (12:3)
June 1829 David Whitmer 12:2 (14:3)
The book of Alma was copied from 𝓞 into 𝓟 during the fall of 1829. Oliver was undoubtedly quite familiar with the language of his own personal revelation, which may therefore explain why he initially wrote “behold the field is ripe” in 𝓟 rather than the correct “behold the field was ripe”. David Calabro also points out (personal communication) that the present tense is also found in the parallel language of John 4:35: “and look on the fields / for they are white already to harvest”, although here the present-tense form is the plural are rather than the singular is.
Summary: Accept in Alma 26:5 the past-tense was in “behold the field was ripe”, the extant reading in 𝓞 and the corrected reading in 𝓟.