Effect of fruit load on the water balance of melon plants infected with Monosporascus cannonballus

The effect of Monosporascus cannonballus infection and fruit load on water balance in melon was studied under field conditions in naturally infested soil. Plant water uptake started to decrease shortly before plant wilting and death. Xylem hydraulic conductance of excised infected roots, measured at the onset of wilting, was five to seven times lower than that of healthy plants. An extensive rise in tylose formation was observed in xylem vessels and most vessels were plugged. Plant collapse and death usually occurred during the fruit maturation period. Fruit removal from infected plants prevented wilting, but did not prevent tylose formation and the associated increase in the root resistance to water flow. Continued removal of fruits increased the root: plant dry weight ratio in both infected and non-infected plants, 3.5- and 5.8-fold, respectively. Infected plants, showing reversible wilt symptoms and from which fruits were removed, regained leaf turgor and remained alive. Fruit removal caused an immediate and sharp drop in leaf stomatal conductance in both healthy and infected melon plants. The largest reduction was observed after complete fruit removal, compared to plants with one or four to five fruits. Fruit removal also reduced the rate of leaf CO₂ assimilation and leaf internal CO₂ concentration. Removal of 70-85% of the leaf surface in infected plants did not alter disease progress, relative to control infected plants. The presence of fruits in M. cannonballus-infected plants apparently subjects the plants to progressive water stress till they die. Fruit removal reduced leaf stomatal conductance and increased root growth, thus enabling the plants to survive the constraint to water uptake and translocation imposed by the pathogen, through root destruction, tylose formation and root function.