Question: Luther and Zwingli disagreed on which doctrine and why?
About the article "Protestant Reformation", section "Zwingli".

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Ulrich Zwingli was a Swiss scholar and parish priest who was likewise influential in the beginnings of the Protestant Reformation.
Zwingli claimed that his theology owed nothing to Luther and that he had developed it in 1516, before Luther's famous protest, though his doctrine of justification was remarkably similar to that of Luther.
In 1518, Zwingli was given a post at the wealthy collegiate church of the Grossmünster in Zürich, where he remained until his death.
Soon he had risen to prominence in the city.
Zwingli began preaching his version of reform, with certain points as the aforementioned doctrine of justification, but others (with which Luther vehemently disagreed) such as the position that veneration of icons was actually idolatry and thus a violation of the first commandment, and the denial of the real presence in the Eucharist.
Soon the city council had accepted Zwingli's doctrines, and Zürich became a focal point of more radical reforming movements.
Followers of Zwingli pushed his message and reforms far further than even he had intended, such as rejecting infant baptism.
This split between Luther and Zwingli formed the essence of the Protestant division between Lutheran and Reformed theology.
Meanwhile, political tensions increased; Zwingli and the Zürich leadership imposed an economic blockade on the inner Catholic states of Switzerland, which led to a battle in which Zwingli, in full armor, was slain along with his troops. 
Ulrich Zwingli was a Swiss scholar and parish priest who was likewise influential in the beginnings of the Protestant Reformation.
Zwingli claimed that his theology owed nothing to Luther, and that he had developed it in 1516, before Luther's famous protest, though his doctrine of justification was remarkably similar to that of the German friar.
In 1518, Zwingli was given a post at the wealthy collegiate church of the Grossmünster in Zurich, where he would remain until his death at a relatively young age.
Soon he had risen to prominence in the city, and when political tension developed between most of Switzerland and the Catholic Habsburg Emperor Charles V. In this environment, Zwingli began preaching his version of reform, with certain points as the aforementioned doctrine of justification, but others (with which Luther vehemently disagreed) such as the position that veneration of icons was actually idolatry and thus a violation of the first commandment, and the denial of the real presence in the Eucharist.
Soon the city council had accepted Zwingli's doctrines and Zurich became a focal point of more radical reforming movements, and certain admirers and followers of Zwingli pushed his message and reforms far further than even he had intended, such as rejecting infant baptism.
This split between Luther and Zwingli formed the essence of the Protestant division between Lutheran and Reformed theology.
Meanwhile, political tensions increased; Zwingli and the Zurich leadership imposed an economic blockade on the inner Catholic states of Switzerland, which led to a battle in which Zwingli, in full armor, was slain along with his troops. 
Beginning in 1524, Bucer concentrated on the main issue dividing leading reformers, the eucharist.
In this dispute, he attempted to mediate between Martin Luther and Huldrych Zwingli.
The two theologians disagreed on whether the body and blood of Christ were truly present within the elements of bread and wine during the celebration of the Lord's Supper.
Luther believed in the real presence of Christ; and Zwingli believed in a symbolic presence.
By late 1524, Bucer had abandoned the idea of the real presence and, after some exegetical studies, accepted Zwingli's interpretation.
However, he did not believe the Reformation depended on either position but on faith in Christ, other matters being secondary.
In this respect he differed from Zwingli.
In March 1526, Bucer published "Apologia", defending his views.
He proposed a formula that he hoped would satisfy both sides: different understandings of scripture were acceptable, and church unity was assured so long as both sides had a "child-like faith in God".
Bucer stated that his and Zwingli's interpretation on the eucharist was the correct one, but while he considered the Wittenberg theologians to be in error, he accepted them as brethren as they agreed on the fundamentals of faith.
He also published two translations of works by Luther and Johannes Bugenhagen, interpolating his own interpretation of the Lord's Supper into the text.
This outraged the Wittenberg theologians and damaged their relations with Bucer.
In 1528, when Luther published , detailing Luther's concept of the sacramental union, Bucer responded with a treatise of his own, .
It took the form of a dialogue between two merchants, one from Nuremberg who supported Luther and the other from Strasbourg who supported Bucer, with the latter winning over his opponent.
Bucer noted that as Luther had rejected impanation, the idea that Christ was "made into bread", there was no disagreement between Luther and Zwingli; both believed in a spiritual presence of Christ in the eucharist.
Luther harshly rejected Bucer's interpretation.
During this time, Bucer and Zwingli remained in close touch, discussing other aspects of theology and practice such as the use of religious images and the liturgy.
Bucer did not hesitate to disagree with Zwingli on occasion, although unity between Strasbourg and the Swiss churches took priority over such differences.
In 1527, Bucer and Capito attended a disputation in Bern to decide whether the city should accept reformed doctrines and practices.
Bucer provided strong support for Zwingli's leading role in the disputation, which finally brought the Reformation to Bern.
The last meeting between Zwingli and Luther was at the Marburg Colloquy of October 1529, organised by Philip of Hesse and attended by various leading reformers, including Bucer.
Luther and Zwingli agreed on 13 of the 14 topics discussed, but Zwingli did not accept the doctrine of the real presence, on which Luther would not compromise.
After the discussion broke down between the two, Bucer tried to salvage the situation, but Luther noted, "It is obvious that we do not have one and the same spirit."
The meeting ended in failure.
The following year, Bucer wrote of his disappointment at doctrinal inflexibility: 
Huldrych Zwingli, whose theology is considered the first expression of Reformed theology was appointed to ministry in Zürich, Switzerland in 1519.
He was influenced by Renaissance humanist Desiderius Erasmus, which lead him to study the New Testament and the early Church Fathers as well as to preach from the Bible.
Zwingli was also aware of and influenced by the Wittenberg reformer Martin Luther, but he developed his theology independently and differed with him in several ways.
Zwingli opposed any religious practice for which he could find no scriptural justification, such as the use of images, organs, and singing in worship while Luther actively opposed the destruction of images in churches.
Zwingli's emphasis on strict adherence to God's command as found in his word also lead to greater moral activism in Zwinglian Reformation movements than in Lutheran ones.
Martin Bucer, Wolfgang Capito, John Oecolampadius, and Guillaume Farel were also influential figures in the development of Reformed theology.
These reformers came from diverse academic backgrounds, but later distinctives of Reformed theology can already be detected in their thought, especially the priority of scripture as a source of authority.
Scripture was also viewed as a unified whole, which led to a covenantal view of the sacraments of baptism and the Lord's Supper as visible signs of the covenant of grace.
Another Reformed distinctive present in these theologians was their denial of the bodily presence of Christ in the Lord's supper.
Each of these theologians also understood salvation to be by grace alone, and affirmed a doctrine of particular election (the teaching that some people are chosen by God for salvation).
Martin Luther and his successor Philipp Melanchthon were undoubtedly significant influences on these theologians, and to a larger extent later Reformed theologians.
The doctrine of justification by faith alone was a direct inheritance from Luther.
At first Zwingli regarded Luther as an ally, and union between the followers of Zwingli and Luther may have been possible but for controversies over the Eucharist beginning in 1524.
Luther strongly believed it necessary to maintain that Christ is physically present in the Eucharist, while Zwingli along with his followers including John Oecolampadius in Basel favored a symbolic meaning to the rite which would be called sacramentarianism.
Martin Bucer in Strasbourg took a mediating position.
Luther labeled Zwingli, Oecolapadius, and others who took this view blasphemous, un-Christian, and fanatic.
In 1529, German prince Philip of Hesse arranged the Marburg Colloquy as an attempt to reconcile the warring Protestant factions and forge an alliance.
The two sides agreed on many points of doctrine but could find no common ground on the Eucharist, and Philip's hope for an alliance failed.
Throughout the 1520s, Zwingli's influence grew in Switzerland, though many Swiss, especially in rural and forest areas, remained loyal to the Catholic church and opposed Zwingli's reforms.
Before 1530, some southern German cities supported Zwingli, and some German Lutheran theologians in other areas of Germany developed sacramentarian views independently of Zwingli, but they were quickly silenced by Lutheran theologians in Wittenberg.
In 1530, Phillip of Hesse formed the Schmalkaldic League, solidifying the Lutherans' hold on Germany to the exclusion of Zwinglianism.
Meanwhile, Zwingli began to use force aggressively to solidify Protestant gains in Switzerland.
Following success against Catholics in the First War of Kappel of 1529, Zurich enforced a harsh blockade on Catholic cantons, leading to the Second War of Kappel in 1531.
The Zurichers were defeated, and Zwingli was killed in the battle.
Catholics continued to hold power in a majority of Swiss cantons, though three-fifths of the population was Protestant. 
Huldrych (or Ulrich/Ulricht) Zwingli (1 January 1484 – 11 October 1531) was a leader of the Reformation in Switzerland.
Born during a time of emerging Swiss patriotism and increasing criticism of the Swiss mercenary system, he attended the University of Vienna and the University of Basel, a scholarly centre of humanism.
He continued his studies while he served as a pastor in Glarus and later in Einsiedeln, where he was influenced by the writings of Erasmus.
In 1518, Zwingli became the pastor of the Grossmünster in Zurich where he began to preach ideas on reforming the Catholic Church.
In his first public controversy in 1522, he attacked the custom of fasting during Lent.
In his publications, he noted corruption in the ecclesiastical hierarchy, promoted clerical marriage, and attacked the use of images in places of worship.
In 1525, Zwingli introduced a new communion liturgy to replace the Mass.
Zwingli also clashed with the Anabaptists, which resulted in their persecution.
The Reformation spread to other parts of the Swiss Confederation, but several cantons resisted, preferring to remain Catholic.
Zwingli formed an alliance of Reformed cantons which divided the Confederation along religious lines.
In 1529, a war between the two sides was averted at the last moment.
Meanwhile, Zwingli's ideas came to the attention of Martin Luther and other reformers.
They met at the Marburg Colloquy and although they agreed on many points of doctrine, they could not reach an accord on the doctrine of the Real Presence of Christ in the Eucharist.
In 1531 Zwingli's alliance applied an unsuccessful food blockade on the Catholic cantons.
The cantons responded with an attack at a moment when Zurich was ill prepared.
Zwingli was killed in battle at the age of 47.
His legacy lives on in the confessions, liturgy, and church orders of the Reformed churches of today.