Manifestation of “black stem” by Phoma macdonaldii and its relationship with assimilates reserves in stem and the source/sink ratio in sunflower.

NUÑEZ BORDOY, E.; QUIROZ, F.J.; DOSIO, G.A.A.

Mar del Plata/Balcarce

Conferencia; 18th International Sunflower Conference; 2012

Asociación Argentina de Girasol

ABSTRACT Black stem (BS) by Phoma macdonaldii Boer of sunflower (Helianthus annuus L.) is an endemic disease in Argentina. The manifestation of this disease is affected by assimilate availability and requirements during the grain filling period (Nuñez Bordoy et al, 2011). High demand of assimilates during grain filling and leaf senescence cause a decrease in the source-sink ratio (SSR) which could favor the manifestation of this disease. This response, in turn, may be associated with remobilization of stem reserves, as was reported for Fusarium spp. in corn (Eslava et al, 2007). The aim of this work was to study the manifestation of BS by Phoma macdonaldii to changes in the SSR, and its association with the remobilization of stem reserves during grain filling in sunflower. Two field experiments were performed at Balcarce INTA Station (37° 45´S, 58° 18´O). Hybrids VDH 487 (Advanta) and Baqueano (KWS) with different health behavior were grown under good water and nutrients conditions. Treatments imposed to modify the SSR were the extraction of (i) 0% (T), (ii) 25 % (R1) or (iii) 75 % (R2) of total grains, or (iv) 50% shading (S) during the filling period. Stubble of sunflower crop with symptoms of BS was distributed previously in the experimental field. BS incidence (nodes with symptoms / total nodes) and severity (on leaves 8, 12 and 20) were evaluated during grain filling, as well as dry matter per plant. SSR (at physiological maturity) was calculated as the ratio of total dry matter per plant and number of grains per head corrected by the weight of the grains of treatment R2, considered close to the potential weight. Remobilization of stem reserves was calculated as the difference in dry weight of this organ between end of flowering and physiological maturity. The severity of BS varied between treatments for VDH 487 (range 2.8 – 94.4%, average= 49%) and Baqueano (range 5.6- 69.4%, average=23.8%). The incidence of BS was associated with severity. The range of SSR explored was 1.9 to 10.8 (g.g-1). The remobilization of reserves accumulated in the stems was associated with the SSR (P = 0.0008), and was higher for hybrid VDH 487 (average= 20%) than for Baqueano (average= 5%). At physiological maturity, BS severity of leaves 8, 12 and 20 and incidence were inversely related to SSR, adjusting both hybrids and years of experimentation to an unique relationship (P=0.0008, P=0.0022, P=0.0023 and P = 0.0145, respectively). Both severity and incidence of BS were explained by the variation in the remobilization of stem reserves, adjusting the results to an exponential model (P= 0.0001). These results strongly suggest that a simple model based on SSR can predict the BS of sunflower in the range of hybrid and environments used. The stem reserves could be an important factor in modulating this relationship. The knowledge achieved in this work could help in better understanding how crop structure and reserve management contribute to tolerance of sunflower genotypes to BS.