Phytosanitary behaviour of waxy and high quality protein maize hybrids developed in Argentina --Corcuera, VR, Naranjo, CA The phytosanitary behaviour to Maize Common Leaf Rust (Puccinia sorghi Schw.) and Ear Smut (Ustilago maydis DC Corda) of several waxy, high quality protein and double recessive hybrids evaluated during the growing season 2001/02 are summarized in Tables 1 to 3. The trial was sown in Llavallol (35° S, 58° W) according to a 3 replicate complete randomized block design. The response to the attack of Ustilago maydis under field conditions was evaluated through % frequency of galls in the leaves (A), galls in the ears (B) and male flowers in the ear apex (C). Response to Puccinia sorghi was measured through 1) % frequency of weak plants, 2) attack severity or % of leaf area with pustules (based on the visual scale developed by Peterson et al., 1948 for Wheat Orange Rust) and 3) type or degree of infection based on a modification of Cobb´s scale as follows:
 
No. Pustules/leaf
Severity %
Degree of Infection
Response to Pathogen
< 11
< 5
I
VERY RESISTANT
12 to 19
6 to 10
II
RESISTANT
20 to 38
11 to 25
III
MID-RESISTANT
39 to 67
26 to 40
IV
MID-SUSCEPTIBLE
68 to 83
41 to 65
V
SUSCEPTIBLE
> 84
66 to 100
VI
VERY SUSCEPTIBLE

Measurements in the field were taken over 30 days after anthesis, and 3 leaves/plant (those of the upper ear and one leaf up and down) were considered to evaluate response to Leaf Common Rust under natural field conditions. The data shown in the Tables let us conclude that eleven hybrids were VERY RESISTANT to P. sorghi, as no symptoms were detected in any plant. All of these 11 hybrids include at least one parental also classified as resistant to the pathogen in previous years, and in some of them the Oh43 genetic background was included during inbreeding and ear-to-row selection process. On the other hand, nine hybrids behaved as SUSCEPTIBLE or VERY SUSCEPTIBLE to P. sorghi. Twenty-three hybrids were VERY RESISTANT to U. maydis, as no symptoms were detected in the plants under field conditions. These single-crosses were developed using at least one of the eleven inbreds reported as resistant to ear smut in previous issues. Nevertheless, more than 13% of the plants of the hybrids 3185, 3186 and 3197 showed symptoms and exceed the historic value of 13% historically registered at the Experimental Station. Seven hybrids were VERY RESISTANT both to P. sorghi and U. maydis, and their potential yield exceeds 10 tons/ha, yielding up to 18.4 tons/ha. In the case of Maize Leaf Common Rust, the lineal regression coefficient (byx) for yield over degree of infection was calculated.The value was negative for all hybrids: -0.20 for waxy hybrids, -0.29 for high quality protein hybrids, and -0.46 for modified starch and high quality protein (double recessive) hybrids. According to the values found for byx, yields decrease in average 200 to 460 kg/ha for each grade of infection. Considering an average yield of 1.7 tons/ha for all hybrids evaluated, it can be stated that yield decreases caused by attack of P. sorghi may range from 1.6% to 3.6% for each grade of infection. This means that a hybrid showing an infection grade III (25% of foliage wounded) may yield 5 to 11% less than if no infection were present. Although new multilocation trials must be run to complete the evaluation of the performance of these hybrids, the preliminary results obtained during 2001/02 are really very promising.

Table 2. Phytosanitary behaviour of high quality protein maize hybrids
 
 
Frequency%
Maize Common Rust
Hybrid No. (A) (B) (C) Frequency% Degree Severity %
3126 0 0 0 100 II-III 6 to 25
3145 0 6.7 0 100 I-III < 5 to 25
3146 0 0 0 26.7 I-III < 5 to 25
3147 0 0 0 100 I-III < 5 to 25
3148 0 0 0 66.7 I-III < 5 to 25
3150 0 0 0 80 I-III < 5 to 25
3151 0 6.7 0 100 III-V 11 to 65

Table 3. Phytosanitary behaviour of double recessive maize hybrids.
 
 
Frequency %
Maize Common Rust
Hybrid No. (A) (B) (C) Frequency % Degree Severity %
3159 0 0 0 0 0 0
3160 0 0 0 86.7 I-III < 5 to 25
3161 0 0 0 53.4 II-III 6 to 25
3163 0 6.7 0 86.7 II-IV 6 to 40
3164 0 0 0 33.4 II 6 to 10
3165 13.4 0 0 0 0 0
3198 0 11.2 0 11.2 I < 5
3199 0 0 0 77.8 II-VI 6 to 100

 
 
 
 
 

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