Searching for citrus rootstocks resistant to mal secco disease: A review

Mal secco disease (MSD) is a severe tracheomycotic disease of citrus caused by the mitosporic fungus Phoma tracheiphila. Besides lemon, the most sensitive species, MSD also affects other citrus species, such as citron, lime, bergamot, chinotto, sour orange, rough lemon or volkamer lemon. When the infection is through rootlets of a susceptible species, more frequently in the nursery but also in bearing plants in the grove, the pathogen may be restricted for many years to the inner layers of wood, leading to a form of the disease known as 'mal nero', a syndrome characterized by blackish discoloration of the hard wood and by the absence of almost any external symptom. Eventually, after several years, the pathogen reaches the sapwood, causing a sudden collapse of the canopy, even on resistant scions. Host resistance remains the most desirable goal among the measures to control MSD, and the search for new resistant rootstocks to replace the susceptible ones, has long been pursued by Italian researchers. This paper reviews the conclusive data collected over almost three decades of research, conducted both in the field and in a controlled environment, on the behaviour of putative lemon rootstocks towards MSD. The rootstocks tested include: alemow, sour orange ('S. Marina' and undetermined selections), 'Ichang' lemon, 'Yuzu' orange, 'Nansho Daidai' sour orange, 'Siamelo', 'Yuma' citrange, 'Sacaton', 'Swingle 4475', and 'Swingle FF9' citrumelos, and the hybrids 'Cleopatra' mandarin × Poncirus, and 'Christian' Poncirus ×'Cleopatra' mandarin.