This is a recently identified rust problem on eggplants. Pearl Millet Rust (Puccinia substriata var. substriata), the same fungus that infects Pearl Millet (Pennisetum glaucum), has been recorded infecting eggplant (since 2006). Here I provide you with a complete profile on what is currently known about this disease.
The family Pucciniaceae contains several pest fungi of grain crops, which belong to the Poaceae family (the grass family). The Puccinia genus contains a very large number of species and ‘variants’. ‘Variant’ being a rust of the same species but might differ in spore morphology or ecology. I will be discussing Puccinia substriata var substriata, Puccinia substriata var penicillariae and Puccinia substriata var indica.
Puccinia substriata var substriata infects the ‘common’ eggplant (Solanum melongena)and its horticultural rootstock, the turkey berry (Solanum torvum). This species was often confused with Aecidium tubulosum, but has now been confirmed to be the same species through inoculation experiments (Ref 1).
Puccinia substriata var penicillariae infects the ‘common’ eggplant from Brazil. Again confusion resulted in the same fungus been given two species names. Therefore, Puccinia substriata var indica is the same species as Puccinia substriata var penicillariae. No symptoms of Puccinia substriata var penicillariae infection was noticed on Solanum americanum (American nightshade), Solanum sisymbriifolium (Sticky Nightshade), Solanum tubersonum (Potato) or Solanum paniculatum (Jurubeba, a nightshade) after inoculation and therefore these species of Solanum are not considered host plants (Ref 2).
Puccinia substriata var substriata andPuccinia substriata var penicillariae infect the same host plants and have similar life cycles, they only differ by geographical dispersal. Puccinia substriata species occur in Africa, North America, Central America, USA, Mexico, Caribbean and South America (Ref 2 & 3).
Aecidium cantanese (potato deforming rust) is another similar rust species that has been recorded on the African eggplant (Solanum macrocarpon) in Benin, Africa during 2011 (Ref 4).
It has also been shown that the Puccinia grass-infecting (or Poaceae-infecting) rusts are evolutionarily distinct from the Puccinia hardwood-infecting (or Junaceae & Cyperaceae-infecting) rusts (Ref 5 & 6).
Life Cycle (Ref 2)
Rusts are ‘simple’ fungi with very complicated life cycles. They can reproduce asexually (without a compatible partner) and sexually (with the fusion of gametes). The mechanism of reproduction, their morphology and cytological structures define their growth/life stages. Typically rusts have five distinct life stages, but I am just going to discuss three.
Eggplant Rust, Topside of leaf, Puccinia substriata
Eggplant rust underside of leaf, orange aeciospores of Puccinia substriata, Pearl Millet Rust
Pearl millet rusts are heteroecious, meaning they require two host species to complete their life cycle.
The initial host, AKA alternate host, is infected early in the growing season, usually during spring. This will be the eggplant (Solanaceae) host. During this stage mature teliospores from the previous seasons’ rust growth infects the eggplant leaves. These germinate and grow on the eggplant host, in what is known as the Aecial stage (asexual). Aecia are cup shaped and grow into the leaf tissue. They cause local necrosis (cell death) and stunt the host plant. As they mature they will burst open and release aeciospores.
Aecioposres disperse with the wind and fall upon the next host, AKA definitive host, such as Pearl Millet where sexual reproduction takes place. Aeciospores germinate and grow as Uredinia. The Uredinial stage remains asexual and bolsters the rust population by infecting new pearl millet hosts with Urediniospores. Towards the end of the growing season, in late summer, Uredinia mature and become Telia. Telia cause the most damage to the host plant. Necrotic lesions increase in size, connect and coalesce. At this stage the rust causes large scale necrosis and leaf blight (dropping of dead leaves). Telia also burst open to disperse teliospores. Teliospores are thick, black structures that are able to overwinter and are almost impossible to kill. Teliospores will then infect the alternate host come spring.
The rust infection is more abundant, grows more aggressively and inflicts more damage to crops as each stage of its life cycle progresses, from the alternate host towards the definitive host. Therefore, effective control is applied during the correct stage of the life cycle and when both the alternate and definitive hosts are treated. All stages are similar in terms of their growth, eruption of mature structures and the release of wind dispersed spores.
Re-infection and the amount of spores that survive to the next season is reduced by controlling the fungus early in the season when symptoms are noticed (spring). You will never be able to eradicate rusts completely from an area as other hosts plants may distribute wind blown spores into your garden from a distance and spores will always be present in the soil. This control is for seasonal use to decrease the total damage caused to your crops and food production.
Avoid overwatering of plants
Avoid wetting leaves
Decrease crop planting density
Increase air circulation between plants
Something Interesting: Saint Anthony’s Fire
Rusts are related to another crop pest known as the Smuts, such as Claviceps purpurea. Claviceps purpurea is an ergot fungus that grows on rye and other cereals. The fungus infects the ears of rye and produces a black growth filled with toxic alkaloids. When you eat grains infected with ergot fungus you become intoxicated with the alkaloids and will suffer neurological symptoms. During the Middle Ages and medieval times people suffering from ergotism believed themselves to be on fire; they will strip, run down the walkways, yelling as they ran and other people would merely think them bewitched. The monks belonging to the Order of St. Anthony were very adept at treating the poisoning and soon people identified the source. Thus, the disease caused by ergot fungi became known as Saint Anthony’s Fire.
It seems that the Puccinia species are increasing in their geographical spread and that their distribution is no longer restricted to Africa and the Western Hemisphere, but they have now been recorded in Europe as well (Ref 3).
Please note that any herbs or vegetable crops that are grass-species (Poaceae family) can become infected with rusts and their control will be similar to that of eggplant (sulphur-based treatments). Rust infection can occur on lemongrass and any cereal crop (maize, rye, barley, rice…).
Ref 2: De O. de Carvalho, A. et al (2006) Description of the life-cycle of the pearl millet rust fungus– Puccinia substriata var. penicillariae with a proposal of reducing var. indica to a synonym. Mycopathologia 161: 331-336