Native to the Mediterranean and Southwest Asia, the common fig tree (Ficus carica) is harvested for its succulent fruit. This small tree or large, deciduous shrub grows between 20 to 35 feet high and features long, lobed leaves and a smooth gray bark and fruit that turns brown or purple when ripe. The fig tree is susceptible to a number of diseases including rust fungus that causes defoliation.
Fig rust fungus is caused by Physopella fici. Infected leaves feature tiny yellow spots that enlarge in time as the growing season progresses and turn orange-brown or rust colored. The Florida Times-Union asserts that if left untreated, the infected leaves fall off while the fungus continues to spread to other leaves, causing the tree to become bare or ragged-looking in extreme conditions. Sometimes infected leaves that fall off too early are replaced by new growth that makes the tree susceptible to frost.
All fungi thrive in wet and cool conditions, and fig rust is no exception. This disease poses a serious threat to home and commercial fig growers in wet and rainy conditions and calls for careful inspection of the trees in such circumstances.
The rust spores form on the underside of the leaves, which is why rain water cannot wash them away. Gentle nighttime air currents carry these spores to the tops of the leaves.
According to North American Fruit Explorers, fig rust fungus usually attacks young leaves first and eventually causes defoliation of the entire tree. Fortunately for fig growers and consumers, this disease usually attacks after the fruit is harvested. However, the Texas A&M AgriLife Extension states that it damages fruits of some late-ripening varieties.
Good sanitation practices and timely applications of antifungal sprays prevent the disease. According to the Texas A&M AgriLife Extension, one or two applications spaced three to four weeks apart in May or early June provides sufficient protection until the fruit ripens. In extremely wet conditions the tree benefits from an additional application or two. Good leaf coverage is essential.
Control the disease from spreading to other parts of the same tree, or to other trees nearby. Collect all dropped leaves immediately and discard them in a knotted bag or burn to prevent spread. The fungus is prevalent in dropped leaves and spreads through spores. Mulch the tree heavily with a 4- to 5-inch thick layer of compost, leaves or pine needles spread 3 to 4 inches away from the trunk.