Timing of Infection and Development of Alternaria Diseases in the Canopy of Apple Trees.

Alternaria leaf blotch and fruit spot of apple caused by Alternaria spp. cause annual losses to the Australian apple industry. Erratic control using protectant fungicides is often experienced and may be due to the lack of understanding of the timing of infection and epidemiology of the diseases. We found that Alternaria leaf blotch infection began about 20 days after bloom (DAB) and the highest disease incidence occurred from 70 to 110 DAB. Alternaria fruit spot infection occurred about 100 DAB in the orchard. Fruit inoculations in planta showed that there was no specific susceptible stage of fruit. Leaves and fruit in the lower canopy of trees showed higher levels of leaf blotch and fruit spot incidence than those in the upper canopy and the incidence of leaf blotch in shoot leaves was higher than in spur leaves. Temperature, relative humidity, and rainfall affected leaf blotch and fruit spot incidence. The gained knowledge on the timing of infection and development of disease may aid in the development of more effective disease management strategies.

Timing of Fungicide Applications for Control of Husk Spot Caused by Pseudocercospora macadamiae in Macadamia.

Pseudocercospora macadamiae is an important pathogen of macadamia in Australia, causing a disease known as husk spot. Growers strive to control the disease with a number of carbendazim and copper treatments. The aim of this study was to consider the macadamia fruit developmental stage at which fungicide application is most effective against husk spot, and whether application of copper-only applications at full-size fruit developmental stage toward the end of the season contributed to effective disease control. Fungicides were applied to macadamia trees at four developmental stages in three orchards in two subsequent production seasons. The effects of the treatments on disease incidence and severity were quantified using area under disease progress curve (AUDPC) and logistic regression models. Although disease incidence varied between cultivars, incidence and severity on cv. A16 showed consistent differences between the treatments. Most significant reduction in husk spot incidence occurred when spraying commenced at match-head sized-fruit developmental stage. All treatments significantly reduced husk spot incidence and severity compared with the untreated controls, and a significant positive linear relationship (R2 = 73%) between AUDPC and severity showed that timing of the first fungicide application is important for effective disease control. Application of fungicide at full-size fruit stage reduced disease incidence but had no impact on premature fruit drop.

First Report of Soybean Rust (Phakopsora pachyrhizi) in Nigeria.

Rust caused by Phakopsora pachyrhizi H. & P. Sydow was observed for the first time on soybean (Glycine max [L.] Merr.) in Nigeria during the 1999 main soybean growing season. The disease was first noticed on soybean farms in Oniyo and neighboring villages located about 20 km west of Ogbomoso (Long. 4° 02' E and Lat. 8° 06' N), Oyo State, Nigeria. The area falls under the moist savanna vegetation with a bimodal rainfall pattern (from April to July, September to November) and an annual mean rainfall of 1,370 mm. Commercial cultivation of soybean by farmers in the area started in 1986. A similar disease outbreak in 1999 on soybean in Benue state, the major area of soybean production in Nigeria, could not be confirmed to be rust. In the Oniyo area, soybean is commonly grown as an early season crop usually planted between March and April in small plots to produce seeds for the main crop, normally planted in July. Rust was not observed on the early season soybean crop grown in the area in 1999. Symptoms were first observed on leaves close to the ground in September 1999 as chlorotic spots, which progressed to reddish brown lesions, then uredia. The symptoms appeared further up the plant until all the leaves were infected. Pimple-like uredia observed in the lesions were more pronounced and numerous on the lower surface of the leaves than on the upper leaf surface. Uredia were noted to cluster together in clumps (1). The urediospores released from a central ostiole in the uredia were hyaline and oblong, 17 µm wide (15 to 18 µm) and 22 µm long (19 to 25 µm). Premature defoliation occurred on infected plants. Seed weight of TGX 1485-1D, TGX 1448-2E, and TGX 1440-1E was reduced by 28, 52, and 49%, respectively. Disease severity was higher on the medium maturing cultivars and all those that were planted late. Reference: (1) J. B. Sinclair. 1983. Compendium of Soybean Diseases, 2nd ed. The American Phytopathological Society, St. Paul, MN.