Foliar (Chrysanthemum) Nematode 545
few chloroplasts can b e found in either the spongy or the palisade parenchyma cells. Cells in the mesophyll b e g in to break down. T h e leaf cells b e c o me totally brown, chloroplasts disappear, and cell walls break down creating large cavities in the mesophyll. In the early stages of infection the collenchymatous cells of the vein sheath, which in large veins extend from the upper to the lower surface, do not allow penetration of the nematodes through their intercellular spaces and thus block extension of leaf necrosis across the veins. In advanced stages of infection even the collenchyma breaks down and the nema-todes and leaf necrosis spread across the veins over the entire leaf. In heavily infected leaves the disintegrated cells have a thick layer of brown substance on their walls, the epidermis is broken down in places, and the leaves shrink and after clinging to the stem for s o me time, fall to the ground.
Control
Several sanitary practices are very helpful and very important in controlling the foliar nematode. T h e leaves and stem should b e kept dry to prevent movement and spreading of the nematodes. Cuttings should b e taken only from the tops of long, vigorous branches, not from shoots near the b a se of the plant. Early in the spring the soil sur-face around chrysanthemums should b e m u l c h ed to cover old infested leaves and prevent nematodes overwintering in them from reaching the lower leaves. S u s p e c t ed dormant cuttings or stools may b e disin-fested by d i p p i ng in hot water (50°C) for 5 minutes or at 4 4 °C for 3 0 minutes. Excellent control of this nematode can b e obtained by spray-ing plants with parathion or malathion from July to early September.
Selected References e e l w o rm Aphelenchoides ritzema-hosi (Schwartz) Steiner in florist's chrysanthe-m u chrysanthe-m. I. S p r e ad of e e l w o rchrysanthe-m infestation. Ann. Appl. Biol. 49: 195-203.
H e s l i n g, J. J., a nd H. R. Wallace. 1 9 6 1 . Observations on the b i o l o gy of c h r y s a n t h e m um e e l w o rm Aphelenchoides ritzema-hosi (Schwartz) Steiner in florist's chrysanthe-m u chrysanthe-m. I I. S y chrysanthe-m p t o chrysanthe-ms of e e l w o rchrysanthe-m infestation. Ann. Appl. Biol. 49: 2 0 4 - 2 0 9 .
Stewart, F. H. 1 9 2 1 . T h e anatomy a nd b i o l o gy of the parasitic aphelenchi. Parasitology 13: 160-179.
Voss, W. 1930. B e i t r a ge zur Kenntnis d er Aelchenkrankheit der C h r y s a n t h e m u m. Z.
Parisitenk. 2: 3 1 0 - 3 5 6 .
Stubby-Root N e m a t o d es Occurrence and Importance
Stubby-root nematodes occur all over the world. T h e habits and kinds of symptoms c a u s ed by these nematodes make the collection and association of these parasites with their hosts rather difficult to prove. Therefore, only where nematodes have b e e n studied exten-sively have stubby-root nematodes b e e n found and shown to cause d a m a ge to plants.
Stubby-root nematodes attack a w i de variety of host plants includ-ing oats, cabbage, tomato, clover, corn, bean, grape, peach, and many others. T h e y affect plants by devitalizing root tips and stopping their growth, resulting in reduction of the root system of plants. Numerous infections by stubby root nematodes result in severe stunting and chlorosis of the whole plant, r e d u c ed yields, and poor quality of pro-duce. Infected plants, however, are seldom, if ever, killed by these nematodes.
Symptoms
Susceptible plants infected with stubby-root nematodes appear stunted within 2 to 3 weeks from inoculation. T h e y have fewer and smaller leaves and branches than healthy plants, although at first their color appears normal. As the growing season progresses the difference in size b e t w e en healthy and stubby-root infected plants increases, healthy plants b e i ng 3 to 4 times as large as infected ones. T h e latter also begin to show changes in color, appearing chlorotic instead of normal green.
T h e root symptoms of infected plants appear several weeks later than the aboveground symptoms and vary considerably in appearance and severity. T h e r e is usually an abnormal growth of lateral roots and proliferation of branch roots. Parasitized root tips show no necrosis or other injury although they are usually darker than normal in color. In such root tips meristematic activity and root growth stops, but cells already formed may enlarge abnormally and cause swelling of the root tip. Frequently, affected roots produce numerous lateral roots, which are in turn attacked by nematodes. R e p e a t ed infections of lateral roots and their branches produce a generally smaller root system devoid of finer feeder roots and characterized instead by short, stubby, swollen protrusions of root branches the growth of which was stopped by the nematode infections (Figs. 130 and 13IB).
Stubby-Root Nematodes 547
F i g. 130. Roots of y o u ng corn plants injured by the stubby-root n e m a t o d e. (Photo by courtesy of U.S. D e p t. Agr.)
The Pathogen: Trichodorus spp.
N e m a t o d es of the genus Trichodorus vary in length from 0.5 to 1.5 mm and in thickness from 30 to 50 ì . T h e b o dy of both males and fe-males is cylindroid, rather p l u m p, and with blunt, rounded tails. Their cuticle is thick and smooth —without lines or punctuations. Although all Trichodorus species are called "stubby-root n e m a t o d e s ," only T.
christiei has b e e n shown to produce typical stubby root symptoms like those described above. This species is quite small, 0.60-0.67 m m long by 40 μ wide. It lives in the upper 30 cm of the soil. It is an ectoparas-ite, feeding on the epidermal cells at or near the root tip region, never entering the root tissue (Fig. 131A). It lays eggs in the soil which hatch to produce larvae and then adults. T h e life cycle of this nematode is completed within about 20 days (Fig. 132). T h e number of stubby-root nematodes in fields infested with them varies considerably depend-ing on the kind and age of host plants, factors determindepend-ing the number and availability of young root tips on which the nematodes feed.
Pop-F i g. 131 . (A) Trichodorus christii n e m a t o de f e e d i ng externally on b l u e b e r ry root. (B) B l u e b e r ry root s h o w i ng stubby-root s y m p t o m s. (Photos by courtesy of Â. M. Zuck-erman.)
ulations build up quickly around susceptible hosts but decline when host plants b e c o me old and do not produce ne w root tips or when good host plants are absent. E g g s, larvae, and adults are usually found in the soil throughout the year, although preadults and eggs s e em to b e the stages found mostly during winter.
Several species of Trichodorus, T. christiei included, are capable of transmitting plant viruses from one plant to another and thus serve as vectors of many virus diseases of plants. T h e external feeding habits of these nematodes and their tendency to fee d on a cell for a few seconds or minutes and then move on to another part of the plant or a ne w plant facilitates the spread of viruses by them. Tobacco-rattle virus and p ea early-browning virus, both rod-shaped viruses, are the only ones known to b e transmitted by Trichodorus nematodes at the pres-ent. Several other plant viruses, all polyhedral shaped, are transmitted by the only other nematode vectors: Longidorus and Xiphinema.
Fig. 132. Disease cycle of the stubby-root nematode Trichodorus christii.