Effect of Meloidogyne incognita on Selected Forest Tree Species.

Four or five growth stages of 14 forest tree species were tested for susceptibility to Meloidogyne incognita at five inoculum levels. Responses ranged from the highly susceptible 'China fir' to immune 'Taiwania'. Even highly susceptible species became increasingly tolerant at later growth stages, thus root-knot appears to be a greater problem in nurseries than in established forests. Heavily suberized cells which restricted nematode development was the predominant host response in Norway spruce, and in the jack, scotch, and Virginia pines. Adult females in jack and scotch pine, which elicited a minimum of suberized tissue, were found adjacent to infection sites showing maximum suberization which indicates that resistance can be highly localized and variable within an individual host. A few gravid females, but no giant cells, were observed in these two species.

Meloidogyne incognita-Induced Changes in Cell Permeability of Galled Roots.

Electrolyte leakage of Meloidogyne incognita-infected and healthy tomato roots was compared by conductivity measurements, and by compartmental analysis using Rb. Conductivity measurements suggested difference in electrolyte loss from healthy and galled roots. On a percentage basis, a greater rate of efflux occurred for healthy plants, but galled roots contain more electrolytes and may show a larger net loss. Compartmental analysis indicated that: (i) the longer half-time for Rb loss from vacuoles of galled root cells could indicate either a greater vacuolar content or decreased tonoplast permeability, (ii) the shorter half-time for loss Rb from the cytoplasm of galled root cells could reflect either a reduced cytoplasmic content or an increased plasma membrane permeability, and (iii) in split-root plants, the permeability of the tonoplast and the plasma membrane of cells in nongalled roots is increased by nematode infection on the other half of the root system. Thus, a mechanism for mobilizing minerals to the infection site is proposed.

Biochemical Changes in Root Exudate and Xylem Sap of Tomato Plants Infected with Meloidogyne incognita.

Under two monoxenic culture techniques of growing plants (filter paper and silica sand cultures), sugar in root exudate from Meloidogyne incognita-infected tomato increased 133 to 836% over controls. In contrast, amino acids were moderately reduced 52 to 56%. Chromatographic analysis showed that galled root exudate contained three sugars, twelve amino acids, and three organic acids, whereas healthy root exudate contained four sugars, fifteen amino acids, and four organic acids. Polysaccharide was responsible for the large increase of sugars in galled root exudates. The concn and the absolute amount of total sugars in the infected plant xylem sap were greater than in healthy plant xylem sap up to 6 wk after inoculation, whereas amino acids were moderately lower than in controls throughout the test period. Chromatographic analysis showed that xylem sap from both healthy and infected plants at 4 wk after inoculation contained four sugars and five organic acids. We identified 18 and 17 amino acids in the healthy and infected plant xylem sap, respectively. The concn of sugar increased as the nematode inoculum increased at 2, 4 and 6 wk after inoculation. The amino acids in all samples from the infected plant moderately decreased with an increase of nematode inoculum. We suggest that changes in total sugars and amino acids, of infected plant xylem sap and root exudate are a probable mechanism by which tomato plants are predisposed to Fusarium wilt.

The Role of Secondary Invaders in Meloidogyne incognita Infection.

Secondary microbial invasion of tomato plants inoculated with 6,000 Meloidogyne incognita larvae caused 75 and 48% weight reduction of foliage and roots, respectively. Under aseptic conditions the same number of larvae caused only 37% reduction of foliage and increased root weight by 50%.