Fusarium graminearum Schwabe , a maize root and stalk rot pathogen isolated from lodged plants in northwest Spain

On September 2001 a survey of lodged maize (Zea mays L.) plants to identify the main root rot pathogens related to maize lodging was carried out in 23 cornfields of Abegondo (A Coruña, Spain). From 328 maize plants showing lodging, 33 isolates of the following potential maize pathogens were collected: Fusarium semitectum Berk & Rav., F. graminearum Schwabe, F. culmorum (W. Smith) Sacc., F. solani Appel & Wall and F. moniliforme Sheldon. The inoculation of four strains of F. moniliforme, three of F. graminearum, one of F. culmorum and one of F. semitectum on two maize inbreds with different susceptibility to lodging —EC23D and EC136 as susceptible and resistant inbreds respectively— showed F. graminearum Schwabe as the most pathogenic fungus considering either root rot symptom or seedling growth reduction. The importance of considering the susceptibility to root rot pathogens in maize breeding programs for resistance to lodging is also discussed.

Palabras clave: Zea mays, podredumbre radicular, encamado raíz, Galicia.(White, 2000).When symptoms occur, they range from a very slight brownredish colour to dark black discoloration of completely rotted roots.Roots infected by F. graminearum are often red or pink.Fusarium spp.can also be isolated from symptomless roots (White, 2000).F. graminearum has been isolated from maize root rots by some authors (Hulea et al., 1968;Mohamed et al., 1968;Palmer and Kommedahl, 1969;Kommedahl et al., 1987) and from maize root and stem rots by others (Ramsey, 1990).This fungus has also been reported as pathogenic on maize in inoculation tests (Palmer and Kommedahl, 1969;Kommedahl et al., 1987); and has also been referenced as a collar rot pathogen in Andalucía (south of Spain) and Cataluña (northeastern Spain) (De Andrés et al., 1998), and as a stem rot pathogen in Cataluña (Marín et al., 1992).
To identify the main root rot pathogens related to lodging, 328 lodged plants were sampled from 23 cornfields in Abegondo (A Coruña) one month after pollination in 2001.Fragments of the stem and root basis in the affected plants were prepared for fungi isolation.The surface of these fragments was disinfected with 0.6% sodium hypochlorite for 4 min and plated on PDA (potato dextrose agar) (Rapilly, 1968).The fungi were grown under laboratory conditions and microscope observations were carried out every 24 h during one week.Fusaria isolates were classif ied according to Nelson et al. (1983).
The maize inbreds EC23D and EC136, susceptible and resistant to lodging respectively, were used in the experiment.The inoculation test was run in a greenhouse at 16-22ºC.Plants were grown in plastic trays containing a mixture of peat and sand previously sterilised at 120ºC for 45 min (1:1, w:w).A randomized complete block design with three replications per isolate-variety combination and 10 plants per plot were used.Non inoculated controls of each inbred were also included.The isolates specified on Table 1 were grown on PDA medium (Rapilly, 1968) at 24ºC for 7 days.The inoculum was prepared by shaking 100 ml of sterile water per petri dish (18 ml of medium) with the grown isolate.The suspension was shaken for 1 min and adjusted to 10 5 macroconidia -for F. graminearum, F. culmorum and F. semitectum-or microconidia -for F. moniliforme-per ml using a Burker camera.At the stage of four leaves, each plant was inoculated by placing 5 ml of the solution into the seedling collar with a sterile micropipette.Disease readings were registered 21 days after the inoculation.Roots of each plant were washed and the percentage of the root rotted area was assessed (Turner and van Alfen, 1983).The square root transformation was applied to the raw data, since it gave the best transformation for normality.Individual plants were also weighted after washing and drying the roots with a precision scale (0.01 g).The Waller-Duncan's multiple range test (at P = 0.05) was performed on the transformed disease severity and on the plant weight variable, using SAS 8.2, Cary, NC software.
The potential maize pathogens isolated from the lodged plants are listed on Table 1 most frequently isolated species, followed by F. graminearum and F. semitectum.
The pathogenicity of nine selected potential pathogenic isolates on each of the two maize inbreds is shown on Table 2. F. graminearum (241 Fr1 and 221 Fr1) and F. moniliforme (221 Fm) were the two fungi that induced signif icant disease symptoms on the susceptible inbreds, 241 Fr1 induced significant disease symptoms on the resistant inbred, but the disease level was lower than that registered on the susceptible line.Symptoms induced by the other strains did not differ significantly from the controls.
Growth reduction induced by the disease is presented in Table 3.Four of the nine isolates produced sig-nificant growth reduction on the susceptible maize seedlings.Two of them were F. graminearum isolates, which had also induced severe root rot symptoms (241 Fr1 and 221 Fr1); the other two were F. moniliforme and F. culmorum isolates, which did not induce severe root rots symptoms.The most pathogenic isolate, 241 Fr1-F.graminearum-, also produced the highest seedling growth reduction.Plant weights of the resistant inbred, after inoculation of all the strains, did not differ from the control.
Though root rot diseases,much like stalk rot diseases, result from the activities of numerous organisms including different fungi, bacteria, nematodes and root-feeding insects, in this study root rot was being F. oxysporum Schlectend and F. solani Sacc. the most commonly reported.Other pathogens including F. graminearum Schwabe, F. acuminatum Ellis & Everh.and F. equiseti (Corda) Sacc.F. moniliforme J. Sheld.,

Table 1 .
. F. moniliforme was the Potential maize pathogens isolated from collar and roots of maize lodged plants

Table 2 .
Pathogenicity of nine strains isolated from collar and roots of maize lodged plants, on two maize inbreds with different resistance to root lodging, 21 days after inoculation Inbreds EC23D (susceptible to lodging) and EC136 (resistant).* Means of the same column followed by the same letter are not significantly different based on Waller-Duncan's multiple range test at P = 0.05.

Table 3 .
Plant weight average of two maize inbreds with different resistance to lodging after being inoculated with nine strains of different potential pathogens isolated from collar and roots of maize lodging plants