Introduction

In central India, wheat is sown during October-November, and is harvested in March-April. Leaf rust of wheat caused by Puccinia recondita Rob. Ex Desm. f.sp. tritici normally does not appear in the region before the end of January (Bahadur et al., 1982; authors' unpublished observations). However, severe incidence of leaf rust occurred on susceptible wheat varieties during November-December 1997, and posed a serious threat of an epidemic build-up during the 1997-98 crop season. Causes and consequences of this early disease outbreak have been analysed and strategies for long-term management of wheat leaf rust are discussed.

Materials and Methods

Extensive crop surveys were conducted during 1997-98 to monitor wheat diseases in the region. Two wheat disease trap nurseries planted at Indore were also under regular observation throughout the crop season. In addition, information regarding the disease situation, and wheat leaf rust samples, was received through field workers and farmers.

Meteorological data of the 1997-98 season, and of the preceding 10 crop seasons (from 1987-88 to 1996-97) were analysed for the critical months of November and December, to establish the disease-weather relationship.

Wheat leaf rust samples collected from farmers' fields and the trap nurseries were analysed for race determination using appropriate host differentials. A number of durum and bread wheat cultivars released for central India were seedling tested with leaf rust isolates derived from representative leaf rust samples, following standard glasshouse procedures (Stakman et al., 1962).

Results

Leaf rust infection during 1997-98 crop season was first noticed in December 1997 on a highly susceptible bread wheat land race 'Dhar local'. The disease was subsequently observed on other wheat land races like Agra local, Pissi local, and unimproved durums, and on the old cultivars 'C 306' and 'Sujata' in farmers' fields and in the trap nurseries. High rust severity observed during December in several wheat fields indicated that the infection became established on the crop during November. While improved durum wheats like HI 8381 and Raj 1555 remained free from infection, improved bread wheat cultivars like WH 147, Lok-1 and Mangla started showing disease symptoms only later in the season, from January-February onwards.

During the 1997-98 crop season, rain occurred every week during November and the first half of December, resulting in high humidity (Table 1). In contrast, none of the preceding 10 crop seasons (1987-88 to 1996-97) received such a regular rainfall and hence humidity remained low during this period (Table 2).

The weekly mean maximum temperatures during December 1997 (22-25°C, Table 1), remained 4-6 degrees lower than the 'average' ones (the monthly mean maximum temperature for December ranged from 27-29°C during 1987-1996, Table 2). In contrast, the weekly mean minimum temperatures during November 1997 (17-18°C, Table 1) were 4-6 degrees higher than the 'average' ones (the monthly mean minimum temperatures of November ranged from 11-14°C during 1987-1996, Table 2).

Of the leaf rust races (Sawhney and Goel, 1983; Nayar et al., 1997) identified, race 104 was the most frequent (7/11 samples or 64%) during 1997-98, followed by the race 12 (3/11 samples or 27%). Leaf rust pathotype 77-5 appeared later and remained low (1/11 or 9%) in frequency (Table 3). The improved bread wheat as well as durum wheat cultivars were generally resistant to leaf rust isolates belonging to the races 12 and 104 (Table 4). In contrast, most of the bread wheat cultivars were susceptible to the pathotype 77-5, while durum wheats were resistant (Table 4).

Discussion

Frequent rains during November/December 1997 led to an early establishment of leaf rust infection on the October/November 1997-sown wheat crop in central India, since leaf rust inoculum is known to arrive in the region from the northern or southern hills through cyclonic winds and settle down on the wheat crop with rain showers (Nagarajan and Singh, 1974). Following infection, leaf rust developed rapidly on susceptible wheat cultivars because of the prevailing high humidity and optimum temperatures. Early establishment of infection foci and continued favourable weather conditions for disease development posed serious threat of an epidemic build-up in the region. However, the disease did not assume epidemic proportions due to a number of factors, as discussed below.

Leaf rust pathotypes that appeared early in the season and were most frequent, belonged to the race 104 or 12 (Table 3), and were avirulent to the improved wheat cultivars including WH 147 and Lok-1 (Table 4), the two most popular ones in central India. Leaf rust pathotype 77-5, having a broad spectrum of virulence to bread wheats (Table 4), appeared relatively late in the season and remained low in frequency (Table 3). Virulence analysis by the Directorate of Wheat Research-Shimla of the 212 wheat leaf rust samples collected from Madhya Pradesh (largest wheat growing state in central India) during 1997-98, revealed that 68% of isolates belonged to the race 104-pathotypes, 24% to the race 12-pathotypes, and only 6% to the pathotype 77-5 (Directorate of Wheat Research, 1998), corroborating the results of our virulence analysis. Hence, improved bread wheat cultivars like WH 147, Lok 1 and Mangla had little leaf rust infection until January/February 1998. Improved durum wheat cultivars like HI 8381 and Raj 1555 remained free from leaf rust infection throughout the crop season. Cultivation of these highly resistant wheats curtailed the inoculum load, checking the disease-spread.

Despite high terminal leaf rust severities, 'C 306' and 'Sujata' (a selection from C 306), the two most widely grown wheat cultivars in central India under rainfed/limited irrigation, contributed toward slowing down the epidemic and did not suffer much loss due to their known slow rusting and tolerance to leaf rust (Gupta and Singh, 1981). Timely warning to the field workers and farmers on controlling irrigation and avoiding excessive nitrogenous fertilizer application to wheat crop, helped in containment of the disease.

Thus, a number of measures need to be adopted as a planned strategy toward the long-term disease management in light of the present analysis. Highly susceptible wheat land races are still being grown in central India due to lack of information or local preferences. Their cultivation should be abandoned since they serve to establish early foci of rust infection, as was observed during 1997-98. Use of pure seed of improved wheat cultivars needs to be widely propagated. Most of the wheat fields surveyed, had mixtures of locals and outcrossed plants, which were observed to be severely rusted, thus serving as 'trap plants' for the rust inoculum.

There is also an urgent need to replace the currently grown wheat cultivars Sujata, WH 147 and Lok-1, as these are susceptible to several leaf rust pathotypes including 77-5 (Table 4). Recently evolved highly rust resistant bread wheat cultivars like 'Amar' (HW 2004), and 'Vidisha' (DL 788-2), both carrying Lr24 resistance, and 'Abha' (HI 1454) with apparently novel leaf rust resistance gene(s), should be encouraged in cultivation.

Cultivation of improved durum wheats like HI 8381, HI 8498 and HD 4672 needs to be popularised in central India. As observed during the survey, and in the glasshouse tests, durums have high levels of resistance to leaf rust including the pathotype 77-5, and the other race 77-pathotypes (Nayar et al., 1996). Generally, durums show resistance to the bread wheat-virulent leaf rust pathotypes, whereas bread wheats are more resistant to stem rust pathotypes with broad virulence to durums (authors' unpublished observations). Hence, cultivation of both durums and bread wheats in central India can help in keeping both leaf and stem rusts in check in the entire wheat belt, since central India serves as the secondary focus of infection for the later sown crop of northwestern India. In addition, durums have better field tolerance to loose smut and Karnal bunt diseases, compared to bread wheat, and can help in their management too.

It was observed during our surveys that poor cultivation practices and incorrect or unbalanced inputs aggravated the disease situation in many wheat fields. Hence, farmers need to be educated about the 'wholistic wheat cultivation technology' including use of recommended cultivars, appropriate seed rate, balanced fertilizer dosages, and correct irrigation schedule, which can contribute immensely toward the long-term disease management.

Acknowledgements

We thank Mr Jagdish for his help with the glasshouse studies. Receipt of the meteorological data from the College of Agriculture, JNKVV, Indore is gratefully acknowledged.

References

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