  A subset of vertices is a {\it maximum independent set} if no two of the
vertices are adjacent and the subset has maximum cardinality. A subset of
vertices is called a {\it maximum dissociation set} if it induces a subgraph
with vertex degree at most 1, and the subset has maximum cardinality. Zito [J.
Graph Theory {\bf 15} (1991) 207--221] proved that the maximum number of
maximum independent sets of a tree of order $n$ is $2^{\frac{n-3}{2}}$ if $n$
is odd, and $2^{\frac{n-2}{2}}+1$ if $n$ is even and also characterized all
extremal trees with the most maximum independent sets, which solved a question
posed by Wilf. Inspired by the results of Zito, in this paper, by establishing
four structure theorems and a result of $k$-K\"{o}nig-Egerv\'{a}ry graph, we
show that the maximum number of maximum dissociation sets in a tree of order
$n$ is
  \begin{center} $\left\{
  \begin{array}{ll}
  3^{\frac{n}{3}-1}+\frac{n}{3}+1, & \hbox{if $n\equiv0\pmod{3}$;}
  3^{\frac{n-1}{3}-1}+1, & \hbox{if $n\equiv1\pmod{3}$;}
  3^{\frac{n-2}{3}-1}, & \hbox{if $n\equiv2\pmod{3}$,}
  \end{array} \right.$ \end{center} and also give complete structural
descriptions of all extremal trees on which these maxima are achieved.

Combinatorics