Alternaria infectoria species group associated with black point of wheat in Argentina

Regional surveys are being conducted in Argentina to assess the presence of wheat (Triticum aestivum) pathogens on grains across the main cropping area. During 2001 and 2002, grain samples with a dark brown or blackish discoloration around the embryo end, known as black point, were observed on several cultivars across the wheat region of Buenos Aires Province (Fig. 1a).

Seed analysis by blotter and agar tests (Neergaard, 1979) showed up to 55% of prevalence (number of samples infected over the total) of Alternaria spp. and a mean natural infection of 37%. Morpho-cultural studies were carried out on potato carrot agar (PCA). The isolates were typified as belonging to the A. infectoria species complex according to their sporulation pattern and the production of long secondary conidiophores (Fig. 2) (Andersen et al., 2002).

Figure 2. Conidia of Alternaria infectoria species group isolates from Triticum aestivum (400x)

Pathogenicity tests were carried out in Petri dishes following two standard methods: blotter test and agar test (PDA 2%), inoculating wheat seeds with a conidial suspension (2×10⁵ conidia/ml) of 20 selected isolates. Seeds were incubated for seven days at 20±2ºC in a growth chamber under 12 h fluorescent plus near ultraviolet (UNV) photoperiod. No disease symptoms were observed in the controls. Infected seeds became totally or partially black and seedlings from them appeared weak with reduced vigor (Fig. 2). Black point is known to affect grain quality adversely, impairing flour, semolina and their products (Mathur and Cunfer, 1993).

Simmons (1992) organized the genus Alternaria into 14 species-groups based on characteristics of conidia and chain formation. The A. infectoria species-group comprises nine known species and an unknown number of distinct taxa yet to be described. In Europe, North America and Australia studies have shown that grain samples are infected with A. alternata and A. infectoria species-groups ranging from low levels to 100%.

In Argentina, previous records of Alternaria spp. refer to A. alternata associated with black point in wheat. However, in this study the vast majority of Alternaria strains conformed to the A. infectoria complex. The incidence levels of this group are gaining importance and have increased in recent years probably due to changes in cropping systems in most of the different agroclimatic zones of Argentina.

This is the first published report of Alternaria infectoria species complex causing black point in wheat grains in Argentina. This highlights the necessity to better understand the relationship of this group with the deterioration of wheat sub-products and the risk of harmful mycotoxins production.