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Pathogen Biology

Monilinia fructicola, M. laxa, and M. fructigena

The first published description of a brown rot fungus on decaying fruit was in 1796. Different species of brown rot fungi were later discovered. The fruit-rotting fungi were placed in the genus Sclerotinia in the late 1800s and were transferred to the new genus Monilinia in 1928.

Of the three closely related fungal species causing brown rot, M. fructicola is the species most commonly found in North America, Australia, New Zealand, Japan, Brazil and other South American countries. Monilinia laxa is found in all major fruit producing countries where brown rot occurs. Monilinia laxa is relatively widespread in California and also occurs in the midwestern and northeastern states, but has not been found in the southeastern states. This species is especially common in Europe, South Africa, and Chile. The disease it causes is frequently called "European brown rot." Monilinia fructigena occurs on both stone and pome fruits in Europe, but does not cause the extensive crop loss caused by M. laxa and M. fructicola. The mycelial growth characteristics of the three Monilinia species vary when they are grown on 2% potato-dextrose agar (PDA) medium (Figure 15). Isolates of M. fructicola can have different fruit rotting and sporulation capabilities (Figure 16).

Figure 15	Figure 16
Figure 1	Figure 10

Asexual reproduction

Conidia (asexual spores) are produced on tufts of conidiophores called sporodochia (figures 1 and 10). The conidia are hyaline (colorless), lemon-shaped, and produced in a moniloid manner (resembling a string of beads with constricted ends) (figures 17 and 18). Under ideal conditions, conidia germinate within 3 to 5 hours (Figure 18). Extensive mycelial growth can occur within 24 hours (Figure 19).

Figure 17

Figure 18

Figure 19

Sexual reproduction

Brown rot fungi are ascomycetes. They produce ascospores (sexual spores) in tubular sacs termed asci that are produced on the upper surface of a cup- or disc-shaped structure, known as an apothecium. (figures 20 and 21). Apothecia can be 5-20 mm (up to nearly an inch) in diameter and are borne on mummified fruit that have fallen to the ground. Although commonly described in textbooks, apothecia are rarely observed in most areas.

Figure 20

Figure 21

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