S.C.A.-AGRICULTURAL RESEARCH STATION
Turda, 3350, str. Agriculturii 27 Jud Cluj, Romania.
V. Moldovan, Maria Moldovan, and Rozalia Kadar.
Fusarium head blight or scab is a fungus disease that has become of increasing concern in most of wheat-growing areas. Yield losses from FHB are due to sterility of the florets and formation of shriveled kernels with light test-weight. FHB-infected grains may be downgraded by the market. The disease not only causes yield and quality losses, but also may be associated with mycotoxins that are dangerous for human or animals health.
Disease incidence is related to wet and warm weather at flowering.
Loose smut is seedborne and, therefore, widely distributed.
Seed treatment with fungicide and certification of subsequent
seed generations should ensure low incidence of this disease.
However, FHB occurs frequently in humid wheat-growing regions
such as Transylvania in Romania. Damaging levels of disease occur
about one year in ten. Although, a certain degree of control
can be achieved by cultural practice and using fungicides, growing
resistant cultivars is the most practicable and effective means
of reducing disease incidence. Evidence suggests that the current
cultivars available to growers are more or less susceptible.
FHB resistance is a trait that is subject to a large 'genotype
x environment' interaction. Screening data based on naturally
infected nurseries is notoriously unreliable because of spatial
and temporal heterogeneity leading to substantial variation in
the relative resistance status of genotypes.
We wanted to assess the FHB-resistance status of adapted cultivars
currently grown in the Transylvanian region (Table 1). These
cultivars were tested at ARDS-Turda under artificial inoculation
in a field nursery over two growing season (2001 and 2003). The
cultivars were included in the Fusarium-screening nursery, where
each entry consists of 2 rows, 1-m rows with 25 cm between rows.
Inoculation was made with local ecotypes of F. graminearum.
Isolates were obtained by culturing naturally infected wheat grains.
Following purification and assessment for aggressiveness, highly
aggressive isolates were increased on liquid Czapek-Dox medium.
The isolates were cultured for 7 days under continuous aeration
from an aquarium pump through a sterile filter. After 7 days,
the liquid media containing a mixture of conidia and mycelia was
homogenized by a mixer and adjusted with distilled water to approximately
50,000 spores/ml. For point inoculation, ten heads/plot (cultivar)
were chosen at anthesis and 0.5 ml of inoculum was injected by
a medical syringe directly through the glumes into a central floret
of two spikelets at the middle of the spike (one on each side).
The inoculated spikes were marked with colored clips. Disease
progresses up and down from the central infected spikelets and
may infect up to all spikelets in a spike when weather conditions
are favorable and the genotype is susceptible to FHB.
| Cultivar | Mean severity (%) 20 days after inoculation | AUDPC | ||
|---|---|---|---|---|
| Mean | Range | |||
| Minimum | Maximum | |||
| Ariesan | 100 | 1,301 | 1,291 | 1,310 |
| Fundulea 4 | 97 | 1,234 | 995 | 1,472 |
| Turda 2000 | 99 | 1,226 | 1,145 | 1,306 |
| Transilvania | 92 | 1,150 | 1,045 | 1,255 |
| Apullum | 100 | 1,097 | 980 | 1,214 |
| Ardeal | 100 | 1,095 | 1,090 | 1,099 |
| Turda 95 | 86 | 884 | 695 | 1,073 |
| Dumbrava | 67 | 430 | 405 | 454 |
Disease severity was assessed in the point-inoculation treatment (marked heads) by counting the number of visually diseased spikelets 10 and 20 days after inoculation in relation to the total number of spikelets of the respective head. The result was percent infected spikelets. The results of observations were used to compute AUDPC for each cultivar in each of the 2 years. AUDPC was calculated according to the function:
n-1
AUDPC = S [(yi + yi+1)/2](ti+1 - ti);
i
in which n is the number of assessment times (minimum 2), y is the disease measurement, and t is the time (days) from inoculation.
The level of disease recorded in this experiment as measured by FHB severity and AUDPC for the eight currently-grown adapted cultivars showed a moderate to highly susceptible status being conferred on them (Table 1).
FHB severity was recorded 20 days post inoculation and represents the mean scoring of ten inoculated heads/cultivar in 2001 and 2003. Despite the fact that no highly resistant cultivars were found, there are large differences in reaction to FHB of the studied cultivars. Three of the analyzed cultivars (Ariesan, Apullum, and Ardeal) reached the highest level of severity (100 %). However, Turda 95 and especially Dumbrava (see Ann Wheat Newslet 50:102-103) scored lower than the mean FHB severity (86 % and 67 %, respectively).
Mean AUDPC, which reflects both severity and duration of disease, showed the same situation in the response to FHB inoculation of the studied genotypes. In this respect, all of recommended cultivars seem to be highly susceptible to FHB, except Turda 95 and Dumbrava, with lower means for AUDPC values (884 and 430, respectively). The mean level of disease expression shown by a genotype can be considered as a reflection of the effectiveness of the resistance genes carried by that genotype. From the standpoint of wheat breeders or wheat producers, cultivars with lower mean level of disease are of greatest interest, because they possess great capacity to limit disease expression.
The stability of a cultivar's response to FHB inoculation also is important. Therefore, in the search for recommended cultivars approximately equal importance is given to the magnitude and the stability of reaction to the disease. In this study, the stability of performance response to FHB was measured by ranking the cultivar on the basis of the distance between the minimum and maximum limits of AUDPC (Table 1). According to this criterion, the most stable cultivars in their reaction to FHB appear to be Ardeal, Ariesan, and Dumbrava, which had a smaller distance between the range limits of AUDPC. Ariesan and Ardeal are stable in their susceptibility; Dumbrava is stable in that it is consistently moderately resistant to FHB. The other analyzed cultivars are more or less variable in their reaction. We thought it interesting that the level of disease infection and the stability of Turda 95, which has a reduced mean of AUDPC, was highly variable in its reaction. However, the maximum range value of AUDPC is slightly smaller than the minimum range value of Ardeal, which was the most stable.
The fact that no current commercial wheat grown in Transylvania
is resistant and only few (Dumbrava and Turda 95) have been found
to have acceptable tolerance reaction to FHB is a compelling reason
to intensify the breeding search for wheat resistance to this
disease. Thus, a Fusarium-screening nursery has become an integral
part of our wheat-breeding program, in which a lot of other cultivars
and advanced breeding lines are annually tested for FHB resistance
in the same way. Some of the previously tested genotypes with
low disease expression have been identified as sources of resistant
genes to FHB. These have potential as parental material in a
crossing scheme aimed at the introgression of resistance genes
into agronomically acceptable and highly productive winter wheat
genotypes, allowing opportunity for recombinants with even higher
FBH resistance than that of currently grown cultivars.